Concave Welding Symbol: What It Means and How to Read It

A fabricator once signed off on a stack of brackets because every weld “looked smooth and clean”—only to have the inspector reject the lot for undersize throat on a symbol nobody had actually read. The little curved mark above the fillet triangle, the concave contour symbol, was the entire story, and it took ten seconds to understand once someone pointed at it.

The Concave Welding Symbol, Explained in Plain English

A fabricator once signed off on a stack of brackets because every weld “looked smooth and clean” — only to have the inspector reject the lot for undersize throat on a symbol nobody had actually read. The little curved mark above the fillet triangle, the concave contour symbol, was the whole story. It takes ten seconds to understand once someone points it out. By then the welds were already deposited, inspected, and sent back for rework.

If you have landed here asking what that curved mark above the weld symbol means, you are in the right place. The short answer: the concave symbol is a shape modifier. It tells the welder that the finished face must curve inward, sinking from toe to toe, rather than bulging out or lying flat. It is not a weld type. There is no “concave” glyph the way there is a fillet triangle or a groove V; the concave arc only ever rides on another symbol, modifying the face profile that symbol requires.

What “Contour” Means, and What a Concave Face Looks Like

Every weld has a contour: the shape of its face in cross-section, the surface you can actually see from outside the joint. There are three possibilities, and AWS drawings name all three:

• Convex — the face bulges outward, proud of the toes, like a shallow hill.
• Flush (flat) — the face runs straight from one toe to the other, no rise and no dip.
• Concave — the face sinks inward, forming a shallow dish or trough between the two toes.

A concave face is what you get when a fillet weld has long legs but a thin center — the classic spoon shape. The welder can build it in with technique, run it hot and fast, or add it afterward by grinding or machining. Either way, the drawing specifies the profile, and the concave symbol is how the engineer puts it on paper.

That matters to welders, fitters, and inspectors, because the symbol carries engineering consequences. A concave face on a load-carrying fillet weld changes the effective throat, and throat is what carries the load. It can also be specified for appearance, clearance, or fatigue life. Reading the contour correctly is not academic — it decides whether a weld passes inspection.

Who Governs All of This

In the United States, welding symbol language is governed by AWS A2.4, the American Welding Society’s Standard Symbols for Welding, Brazing, and Nondestructive Examination. AWS created the standard in 1976 by merging two earlier documents, A2.0 (first published in 1947) and A2.2. The current edition is AWS A2.4:2020, the eighth edition, which superseded A2.4:2012 and A2.4:2007. It made several notable changes, including updates to how contour symbols are used — covered next. If your drawing is dated or came from overseas, the conventions may differ slightly, another reason to check the standard rather than trust memory.

Weld Symbol vs. Welding Symbol: A Small Distinction That Causes Big Confusion

Two terms sound interchangeable but are not:

• A weld symbol is the small geometric glyph itself — the fillet triangle, the groove V, the plug circle. It tells you the type of weld.
• A welding symbol is the complete package on the drawing: the arrow, the horizontal reference line, the weld symbol above or below that line, contour and finishing marks, size and length notes, supplementary symbols, and the optional tail calling out a specification or process.

The concave mark is one element inside a welding symbol. You cannot read it in isolation; you read it against the weld symbol it sits on and the reference line that tells you which side of the joint is which. The next section covers that full anatomy.

What This Article Will Do

Over this guide we will place the concave mark precisely on the welding symbol, compare it with the flush and convex contour marks, and explain why a designer specifies it. We will work through the leg-size-versus-throat math — the exact trap that rejected those brackets — plus an inspection checklist for the shop floor. You will also get shop technique for producing a controlled concave profile, a troubleshooting table of the usual failure modes, and a rundown of how AWS A2.4 and ISO 2553 differ so international drawings do not catch you out.

By the end, you will look at any concave symbol on any drawing and know what face profile the engineer demanded, whether that profile costs you throat and strength, and how to verify the finished weld against it. No more signing off on welds that merely look clean.

Anatomy of a Welding Symbol: Where the Concave Mark Lives

Reading a welding symbol with confidence starts with knowing its anatomy. Every element has a fixed place and a fixed meaning, so the same drawing reads the same way in any shop. The structure is built around a few core parts: the arrow, the reference line, placement above or below that line, and supplementary elements in the tail. Once those are clear, the contour mark fits naturally into position. This layout holds across many welding processes, from Aluminum MIG Welding to Aerospace TIG Welding.

At the center of every welding symbol is the horizontal reference line, a straight baseline that carries most of the data. Attached to one end is the arrow, which points to the joint where the weld is to be made. The AWS A2.4 standard defines the side the arrow touches as the arrow side, while the opposite surface is the other side. This simple rule decides which symbols apply to which face of the joint. According to Pearson’s welding textbook, that division between arrow side and other side is the key to interpreting exactly where the weld metal goes.

Placement relative to the reference line is strictly prescribed. A symbol below the line refers to the arrow side of the joint; a symbol above the line refers to the other side. This convention is shown clearly in the AWS weld symbol chart used by drafters and inspectors. A designer may also attach a tail to the end of the reference line opposite the arrow. The tail carries extra information such as process, filler metal, procedure specification, or notes that do not fit in the standard locations.

Several supplementary symbols may also appear. The weld-all-around symbol (a circle) at the junction of arrow and reference line means the weld is required around the entire joint, not just one segment. The field weld symbol (a flag) pointing toward the tail indicates the weld is made in the field rather than in the shop. Dimensions, angles, root openings, pitch, and length all occupy fixed positions on the reference line as specified by AWS A2.4. These location rules ensure a symbol drawn in one facility means the same thing in another, whether the work involves Alloy Steel Welding Electrodes or specialized alloys like those used in Alloy 20 Welding Procedures.

Where the Contour Symbol Sits and How It’s Oriented

The contour symbol is a shape modifier that tells the welder what face profile to produce on the finished weld. It is not a weld type by itself; it modifies the weld symbol already shown. The AWS standard location chart places it in a fixed position relative to the reference line, typically on the same side as the weld symbol it modifies, and it may carry a finishing designator when mechanical finishing is required. Orientation of that curved mark is critical: the arc is drawn to mimic the required contour of the weld face. As noted in the Pearson textbook, the contour symbol represents the shape the deposited face should take after welding or finishing.

For example, when the fillet weld symbol appears below the reference line (arrow side), the concave contour symbol sits below the line next to that fillet symbol, indicating a concave face on the arrow side. If it appears above the line, it applies to the other side and its curve matches that face. The arc opens in the direction of the concavity so the symbol visually suggests the hollow profile of the finished face. When no contour symbol is added, standard welding and shop practice governs the resulting profile.

Finishing methods such as grinding, chipping, or machining may be indicated by a letter (G, C, M) placed inside the contour symbol or adjacent to it, depending on the drawing convention in use. This tells the welder whether the final shape comes from welding technique alone or from post-weld mechanical work. The ANSI summary of AWS A2.4:2020 changes also highlights clarifications to contour and finishing designators that affect how these elements are applied. Correct placement matters in complex assemblies, from Aluminum Boat Welding to Aerospace Welding.

In short, the anatomy is fixed: the arrow points to the joint, the reference line divides arrow side from other side, symbols above and below follow that division, the tail adds process notes, and supplementary marks such as weld-all-around and field weld sit at the junction. The contour symbol lives on the appropriate side of the reference line, oriented so its arc mimics the required face profile. These rules, defined in AWS A2.4, keep symbol reading consistent across industries.

Flush, Flat, Convex, and Concave: The Three Contour Marks Compared

Every weld symbol can carry a small curve or line directly above or below it, and that little mark changes the entire job. The weld symbol tells you what kind of joint you are making; the contour mark tells you what the finished face must look like. Misread it and you either grind away metal nobody paid for, or you leave a proud bead that will not clear a cover plate. Here is the full family of contour marks side by side, plus the 2020 revision that changed how two of them are used.

What a Contour Symbol Actually Is

A contour symbol is a shape modifier. It sits on the weld symbol itself, drawn as a continuation of the weld symbol’s top edge on the side where the finished face is expected. For a fillet triangle, the mark sits above the triangle when the contour applies to the arrow side face drawn above the reference line, and below when the symbol is below the line. The curve is oriented so it mimics the required profile: a convex mark bulges outward like the crown of a healthy bead, and a concave mark curves inward like a scooped-out face. The straight-line marks say the face should be level with the surrounding plate.

Under the AWS weld symbol chart, there are only three graphic forms: a straight line, an outward-bulging arc, and an inward-curving arc. The naming gets confusing because two of the four common terms share the same straight-line glyph.

Contour mark	How it is drawn	Face it demands	Where it typically appears
Flat	Straight line across the top of the weld symbol	Fillet face ground or welded level with the triangle’s theoretical surface	Fillet welds on machine frames, brackets, and any fillet marked for a smooth transition
Flush	Straight line (same glyph as flat)	Groove, plug, or slot face level with the plate surface	Butt joints, pressure vessels, skin panels, plug-filled holes
Convex	Arc bulging outward, away from the weld symbol	Crowned face with extra reinforcement left standing	Static structural fillets where throat area matters more than appearance
Concave	Arc curving inward, toward the weld symbol	Scooped or dished face	Fatigue-loaded joints, cosmetic corners, root side of single-sided butt welds

Why Two Names Now Share One Glyph

This is where the ANSI summary of AWS A2.4:2020 matters. Older drawings and older textbooks teach “flat” and “flush” as interchangeable words for the same straight-line mark, and for years the standard did not draw a hard line between them. The 2020 edition does: the flat contour symbol is now limited to fillet welds, while the flush symbol is used for groove, plug, and slot welds. If you are reading a drawing issued after 2020, a straight line over a fillet triangle is a flat contour, and the same line over a groove symbol is a flush contour. On older drawings the distinction was looser, which is exactly why welders trained in the 1990s and inspectors trained in the 2020s occasionally argue over the same print.

What Each Mark Looks Like in the Field

The practical difference shows up at the joint. A convex contour is the easiest to produce naturally: a bead with proper heat and a slow travel speed stands slightly proud, and no postwork is needed. A flush contour is the most demanding to achieve by welding alone, because the face must land exactly level with the base metal. A concave contour falls in between, and it is the one most often overdone, leaving a face that dips below the required throat. On aluminum work in particular, where thermal distortion and soft filler metals make profile control tricky, shops lean on process selection and heat control first; the aluminum MIG welding guide covers how wire feed and travel speed dictate the resulting face shape before any grinding starts.

You will see straight-line flush marks on vessel and pressure-part drawings where a level surface is a sealing or fit-up requirement, convex marks on structural fillets where extra throat buys strength, and concave marks on base plates, crane lugs, and any joint called out for fatigue service. Cosmetic work adds another layer: handrails, alloy wheel repairs, and visible corner welds frequently carry a concave or flush callout purely so the finished piece looks intentional.

Read the contour mark in context, with the weld type, the side, and any finishing letter beside it. AWS A2.4:2020 is the definitive reference when a print and a memory disagree.

When a Designer Specifies a Concave Contour (and Why)

That little curve above the fillet triangle is easy to dismiss as a stylistic preference. It is not. When an engineer puts a concave contour symbol on a drawing, they are buying something measurable: a smoother load path, a cleared envelope, a better-covered surface. They expect the shop to deliver that profile deliberately, and a weld that merely “looks clean” may not be the weld that was specified.

Fatigue Service: Taking the Sharpness Out of the Weld Toe

Most fatigue cracks in welded structures do not start inside the weld metal. They start at the toe, where the weld face meets the base metal and the load path takes a hard turn. A convex bead standing proud of the parent surface makes that corner tighter and the stress concentration worse. A concave face spreads the same transition over a longer distance, lowering peak stress right where cracks initiate.

This is codified, not folklore. AWS D1.1 includes prequalified tubular joint details that call for a concave “improved” profile on heavy sections in fatigue service. In those details, concavity is the required geometry, and running the weld flat over it is a deviation. D1.1’s fatigue tables go further: for the relevant branch connection categories, a concave, smooth, fully ground profile is listed as permitting unlimited branch member thickness. The code effectively trades the lost throat for a better toe shape and lets the designer stop size-limiting the member. You can review the current D1.1/D1.1M preview for those tubular details and fatigue tables.

Aerospace work tells the same story with different paperwork, since toe blending is routine on fatigue-critical hardware there, as discussed in our guide to aerospace welding practices. And when a grinding or chipping designator sits beside the curve, keep in mind that under AWS A2.4:2020 contour may be produced by welding or by mechanical finishing. A ground concave toe satisfies the symbol just as completely as a welder-shaped one.

Clearance for Covers, Seals, and Moving Parts

The second most common reason is pure envelope control. Sliding covers, bearing bores, seal faces, rotating assemblies and shrouds are drawn to a theoretical surface, and a bead standing a millimetre proud of that surface is interference. Here the concave mark tells the welder to keep the reinforcement inside the design envelope, sometimes with no post-weld machining allowed afterward.

Appearance, Flow, and Coating Behavior

Visible joints do get specified for looks, but “looks better” usually has a functional root. On a boat hull, a concave blended weld keeps the running surface clean and free of drag-inducing ridges, which is why aluminum boat welding guides spend so much time on finish work. On wheels and architectural panels, the same profile is the difference between a repair that disappears and one that announces itself, as covered in alloy wheel welding repair.

Coatings behave better too. A convex bead forces paint to break sharply over its crown and thin out at the toe, exactly where corrosion starts first. A concave transition wets and covers far more evenly, and a weld face matched to a machined or cast parent surface reads as continuous instead of patched. Matching that transition also makes corner and lap joints look intentional: instead of a caterpillar bead crawling along a visible edge, the weld sits down into the joint line and the eye reads one surface.

Root-Side Concavity on Single-Sided Butt Joints

Here the symbol appears on the back side of a butt weld deposited from one side only. Some root concavity is unavoidable, but it must not eat the joint. Los Alamos National Laboratory weld acceptance criteria state plainly that concavity of the root surface on single-welded butt joints must not reduce the total joint thickness below the nominal thickness of the thinner component (LANL welding standards, WFP 2-01 Attachment 2).

Where Concavity Is the Wrong Call

The same D1.1 guidance that blesses concave toes warns about concavity in the wrong place. Excessive concavity of a crater or of initial passes is called out as something to avoid precisely because it becomes a crack origin; those depressions should be filled and blended, never accepted as profile. Concavity is also the wrong choice when throat area governs the design and no mechanical finishing credit exists, in corrosion or sanitary service where a concave pocket traps moisture, debris or product residue, and in impact-loaded joints where reduced throat and a notch-like geometry compound each other. When the symbol and the service seem to conflict, the engineer of record decides, not the welder on the floor.

Leg Size, Throat, and What Concavity Really Costs You

The curve in the concave welding symbol is not a style preference; it is a warning about geometry. A fillet weld can measure out perfectly on both legs and still be short on strength, because strength lives in the throat, and that is what a concave face eats into first.

Theoretical Throat vs. Effective Throat

Two throat terms get used constantly, and mixing them up is the fastest route to a rejected weld:

• Theoretical throat is a design value. For an equal-leg fillet weld it is the leg size multiplied by 0.707 (the sine of 45°). Los Alamos National Laboratory’s acceptance attachment states it plainly: theoretical throat equals 0.7 times the weld size (LANL WFP 2-01 Attachment 2). A 6 mm leg gives roughly 4.2 mm of throat; a 1/4 in (6.4 mm) leg gives about 0.178 in (4.5 mm).
• Effective throat is what actually exists in the finished joint. AWS D1.1 defines it as the shortest distance from the joint root to the weld face of the diagrammatic weld. Shortest is the operative word: a concave curve can make that path much smaller than the drawing assumed.

D1.1 also treats stress on the effective throat of a fillet weld as shear stress, regardless of load direction. The throat is the only line the code lets you count on for capacity.

Why a Concave Weld Can Fail While the Legs Pass

Here is the trap inspectors see on the shop floor. The gauge reads 6.4 mm on both legs, so the box gets ticked, but the face dips inward and the shortest root-to-face distance sits well below the 4.5 mm the designer calculated. The weld is short in throat even though every leg measurement passed.

Specified leg size	Theoretical throat (0.707 × leg)	Measured actual throat on a concave face	Shortfall
6 mm (≈1/4 in)	4.2 mm	3.1 mm	~26% under
10 mm (≈3/8 in)	7.1 mm	5.7 mm	~20% under
1/4 in (6.4 mm)	0.178 in (4.5 mm)	0.135 in (3.4 mm)	~24% under

Those are worked examples, not code limits. A concave weld can be a quarter short in load-carrying area while looking tidy. Aggressive weave or fast travel on thin material makes it worse, compounding lack of fusion at the root. The same throat logic governs aluminum work, where heat input is tightly controlled, as covered in our Aluminum MIG Welding: The Complete Practical Guide.

How to Measure Throat on a Concave Fillet

A standard fillet weld gauge set carries two scales per leaf: leg length and throat. Use both, every time.

1. Read the legs first, with the blades seated flat against both fusion faces.
2. Read the throat blade at the root, perpendicular to the face; that is your actual effective throat candidate.
3. Use the concave radius leaves, or a straightedge across the toes with a depth rule, to measure the dip.
4. For code-style checking, roll a small disk along the weld face as a consistent reference for convexity and concavity depth.
5. Check the toes for undercut at the same time, since undercut steals throat exactly where stress concentrates.

Throat measurement is built into the early modules of hands-on programs like our Aluminium Welding Course: What You Learn, Costs & Careers, rather than left to the certification exam.

Groove Welds and Root Concavity

Groove welds follow the same definition from a different starting point. For a fully penetrated groove in a butt joint, effective throat runs through the joint thickness, and reinforcement above the surface adds nothing to design strength. On a single-sided butt weld, the contour curve on the root side flags a concave root bead. Excessive root concavity thins the joint where bending stress peaks, making that mark a genuine throat callout.

The AWS A2.4 welding symbol standard places that contour curve relative to the weld symbol so it mirrors the required face profile, showing which surface must be dished.

The Crater Rule Nobody Reads

One more D1.1 line matters for sizing: no reduction in effective length is assumed in calculations to allow for the start or stop crater. The full specified length counts as structural length, so filling that crater is your job, not a licensed dead zone. A concave, unfilled crater is a throat deficiency at the end of the weld, and inspection reads it that way.

The practical takeaway is simple: when you see the concave contour mark, verify throat, not just legs. Measure it, compare it against the required effective throat, and only then call the weld good.

Inspecting a Concave Weld Against the Symbol and the Code

When a drawing calls for a concave contour, inspection stops being a matter of “that looks smooth.” The inspector has to prove three separate things: that the finished face is actually concave, that the effective throat still meets the specified weld size, and that the toes and crater are free of defects the code will not accept. Miss one of those and a clean-looking weld is still a reject.

The Tools That Settle the Argument

Visual inspection of a concave fillet starts with a fillet weld gauge set. A standard kit holds several blades per size, and the concave blade is the one with the arc cut into it. Lay the gauge so the blade contacts both legs and the arc sits against the weld face. If daylight shows between the gauge and the metal, the profile does not match. Gauges report leg length, but the number that matters is throat, so many inspectors read the gauge at its shortest point rather than at the toes.

For groove welds and root-side concavity, inspectors use calibrated templates, a straightedge, or a depth micrometer to check reinforcement and root depression against the drawing. For tubular connections detailed as fatigue-sensitive, AWS D1.1 references a disk test: a disk of the specified diameter is rolled along the weld, and the profile must allow it to pass without catching, which confirms the transition is smooth enough for the detail class. The acceptance tables themselves come straight from the visual inspection criteria in AWS D1.1/D1.1M, supplemented by whatever the project specification adds.

Acceptable Concavity vs. Excessive Concavity

Here is the distinction most welders get wrong: the concave contour on the symbol is a requirement, not a defect. The rejectable condition in the D1.1 visual inspection tables is listed specifically as excessive concavity of the weld or crater, alongside undersize welds and undercutting. Concavity becomes excessive when it eats into the required effective throat.

The math is unforgiving. A 1/4 in (6 mm) equal-leg fillet has a theoretical throat of about 0.177 in (4.5 mm). If the concave face dips far enough that the shortest distance from the joint root to the weld surface measures 0.14 in (3.6 mm), the weld is undersize even when both legs measure a perfect 1/4 in. This is why inspectors measure throat, not appearance. Craters get the same treatment: a concave crater with a pipe or a reduced cross-section at the stop point is flagged as excessive concavity of the crater, and the fix is to fill the crater properly, not to grind it pretty.

Undercut Limits at the Toe

A concave face is only beneficial if the toe transition is smooth. Undercut at that toe undermines the whole point. D1.1 limits undercut to 1/32 in (1 mm) where the base material is less than 1 in (25 mm) thick, and allows 1/16 in (2 mm) on heavier material. Project specifications often tighten that further. The Los Alamos acceptance criteria, for example, cap undercut at 1/32 in and explicitly prohibit it from encroaching on the minimum required section thickness of the part, no matter how thick the material is.

A Short Inspection Checklist

1. Pull the drawing and confirm the contour arc, side, size, and any finishing designator against the standard weld symbol chart.
2. Confirm the weld is on the correct side of the joint before measuring anything.
3. Check the profile with the concave gauge blade or a calibrated template, reading throat at the shortest point.
4. Compare measured throat to the required size, not to the nominal leg.
5. Inspect both toes and the crater for undercut, cracks, or a crater pipe.
6. Run the disk test where the specification calls for it on fatigue details.
7. Record measurements and the acceptance criteria used, and flag any weld that cannot be verified as conforming.

If a weld cannot meet the concave profile as welded, the correct answer is to raise it with the engineer of record and agree on mechanical finishing, not to sign off because the surface looks clean. A symbol that nobody read is exactly how a lot of brackets end up rejected after the fact.

Fatigue, Stress Concentration, and When Concave Beats Convex

A fillet weld can measure out perfectly on the gauges and still crack first in service. Legs correct, length correct, size correct on the drawing, and the crack still starts at the toe. That happens because fatigue does not care how much metal you deposited; it cares about how abruptly the load has to change direction to get through the joint. The concave contour symbol is, at its core, a designer’s instruction about that transition.

Why the Toe Is the Weak Point

As a load path moves from the base metal into the weld and back out again, the stress flows like traffic. A smooth, shallow curve lets it merge gradually. A sharp step forces the flow lines to crowd together at one small point, and that crowding is stress concentration. The sharper the toe angle and the smaller the toe radius, the higher the local stress, and fatigue cracks nucleate exactly there.

This is why contour can matter more than it looks on paper. Research literature compiled through the US DOT National Transportation Library has reported cases where weld shape at the toe influenced fatigue life more than the measured depth of the undercut beside it. The geometry of the transition, not just the flaw size, was driving the result. AWS supports the same logic in design terms: the fatigue detail categories in D1.1 Section 2 distinguish joints by profile, attachment geometry and sharpness rather than by weld size alone.

When a Proud Convex Bead Works Against You

Nothing is automatically wrong with a convex face. On static, lightly loaded brackets, a slightly crowned bead is strong, fast to run and tolerant of field conditions. Trouble starts on cyclic service. A proud convex face creates a steeper toe angle, and the added height sits where it does the least good structurally while amplifying the stress concentration at the edges. It also fights you downstream: coatings build unevenly over a crown, and the crest hides toe defects from visual inspection.

Face profile	Toe transition	Typical service
Flush	Aligned with the base metal, no step	Appearance joints, groove welds, machined surfaces
Convex	Steeper angle, higher local stress	Static loads, root passes, undersized fills being built up
Concave	Gradual, large toe radius	Fatigue, cyclic and fracture-critical service, visible seams

The aerospace sector pushes this hardest, because airframe structures spend their lives in cycles and every gram matters: see Aerospace Welding: Processes, Standards, and Careers Guide for how tightly profile control is written into that work.

Toe Grinding and Dressing

When a weld comes out convex or merely acceptable, the fix is post-weld treatment. AWS D1.1’s commentary describes effective toe grinding and toe dressing as post-weld treatments for fatigue-critical welds. Grinding removes the sharp notch and blends the toe into a generous radius; dressing, typically peening or controlled burping of the toe, works by introducing compressive residual stress and smoothing the transition. Both improve the fatigue detail without adding a single pound of weld metal.

Grinding is not free. It removes base and weld metal at the very location you still need for load path. That is where the trade-off lives, and it is the reason AWS D1.1 carries a prequalified detail called concave improved profile for heavy sections and tubular connections subject to fatigue: the profile is deliberately produced or finished to a smooth concave shape so the improved geometry can be credited in design. Fracture-critical bridge work runs on a stricter book, the AASHTO/AWS D1.5 Bridge Welding Code, which sets its own profile and undercut limits for contract acceptance.

The Trade-Off: What Concavity Costs

Here is the arithmetic that catches people out. For an equal-leg fillet, the required size usually governs the theoretical throat at 0.707 times the leg. A concave face shortens the actual throat below that theoretical line, so a weld can hold correct leg length and still fall short at mid-face. Design a weld so the required throat is met after grinding or after a concave contour is applied, and the detail stays both fatigue-friendly and code-compliant.

That balance shows up everywhere cyclic loading is real, including marine hulls described in Aluminum Boat Welding: The Complete Guide for Strong Hull Joints, where wave loading repeats thousands of times and a smooth toe is worth more than an oversized bead.

The practical rule: specify concave when the detail is fatigue-critical and the throat has margin; keep throat priority when it does not. When the symbol, the minimum size and the service all point the same way, the concave contour is not a cosmetic preference. It is the design.

How to Actually Produce a Concave Face (and Fix Common Mistakes)

Reading the symbol is the easy half. The hard half is standing at the machine and making the weld freeze with exactly the dish the drawing asked for, then reproducing it tomorrow. A concave face is not a happy accident. It is a repeatable outcome of heat, speed, and manipulation, and when welding alone cannot get you there, the standard lets you dress your way to it.

Dialing In the Profile at the Machine

Face profile is a ratio: filler going in versus heat spreading it out. Everything below feeds that ratio.

• Travel speed. This is the biggest lever. Run too fast for the filler you feed and the face sinks below the toes: unplanned concavity and a short throat. Run slow with a wide weave and the bead stands proud. For a deliberate dish, keep deposition steady and let speed shape the face.
• Weave width and rhythm. Narrow, evenly paced weaves give a smooth, slightly dished face. Wide weaves let the center solidify while the edges stay fluid, producing a wavy contour and inviting toe undercut. Pause at each toe just long enough to wet in.
• Current and arc length. Excessive amperage washes the face out and deepens concavity; too little leaves a cold, high, lumpy bead. A tight arc keeps the profile where you put it.
• Position. Flat-position fillets are easiest to shape. Vertical-up stacks more metal and runs convex naturally; overhead wants lower heat and faster travel to keep the face from sagging. Rod and parameter choices are position-specific, so read our all position welding rod guide before blaming technique. Aluminum narrows the window because heat builds fast; our aluminum MIG welding guide covers travel-speed sensitivity.
• Pass sequencing. Large fillets and grooves are built in layers, and the layers are not interchangeable. Keep early passes flat or slightly reinforced and save the concavity for the cover pass. AWS D1.1’s commentary is blunt: excessive concavity of initial passes should be avoided to prevent cracking, since a dished early pass leaves a sharp toe notch that the next pass must fill and shrinkage stresses act on.
• Crater control. Taper the current or back-step over the stop so the crater fills instead of sinking. A crater pipe at the end of an otherwise perfect concave bead is a rejection waiting to happen.

Groove welds get their own version: root concavity on the far side of a single-sided butt joint, usually from a root pass run too cold or too fast, or from grinding afterward. Bring the root up hot, not with a disc.

When You Finish It Mechanically

Sometimes geometry fights you: restricted access, an out-of-position tie-in, capped heat input on thin sheet. Weld close, then dress to the curve. AWS A2.4:2020 makes that explicit, allowing the required contour to be produced by any method, welding or mechanical, as seen fit at the work site.

That flexibility sits alongside supplementary designators from AWS A2.4, placed right after the contour symbol: G for grinding, C for chipping, M for machining, H for hammering, and R for rolling. So a fillet triangle with the concave curve and a G means “produce this concave contour by grinding.” Without a letter the method is yours, but the shape is not negotiable. The full arrangement is on the official AWS welding symbol chart.

Troubleshooting: Causes and Fixes

Symptom	Most likely cause	Fix on the next pass
Too concave, throat under minimum	Speed too fast, amperage high, too little filler	Slow down, feed more filler, ease current, add a cover pass
Uneven or wavy face	Inconsistent speed, wandering arc, erratic weave	Lock arc length, use a steady drag angle, pace the weave
Undercut at the toes	Current too high, arc too long, weave lingering past the toe	Reduce amperage, tighten the arc, wash back into the toe
Crater pipe at the stop	Abrupt arc break with no fill	Use the crater function, taper current, or back-step over the stop
Concave first pass, hard to tie in	Excessive concavity of initial passes	Run root and fill flush or reinforced; leave the dish for the cover

Whatever method you choose, verification is unchanged: check the leg with a fillet gauge against the specified size, then judge acceptance on the actual throat. The curve shows the shape; the gauge shows whether the metal is there.

AWS vs. ISO: Reading Concave Symbols on International Drawings

As fabrication goes global, it’s becoming more common to receive drawings from overseas vendors that use a completely different welding symbol system. While the concave contour curve may look familiar at first glance, assuming it follows American conventions can lead to an incorrect profile being applied to a weld. Knowing the split between AWS and ISO is the key to avoiding that mistake, especially when the face shape is being strictly controlled.

The Biggest Structural Difference: One Line vs. Two

The clearest divide between the two systems comes down to their reference layout. The AWS system uses a single horizontal reference line. Arrow side elements go below the line, and other side elements go above it.

By contrast, AWS A2.4:2020 points out that ISO 2553 uses an additional identification line alongside the reference line to distinguish arrow side from other side. Rather than relying solely on “above or below” a single line, the ISO layout uses this secondary line to label which side the symbol applies to. This means you cannot transpose AWS positioning rules directly onto an ISO drawing. Misreading which line is which is enough to apply a required concave face to the wrong side of the joint entirely.

Contour Placement, Weld-All-Around, and Field Welds

In both systems, the concave contour mark is still a face-shape modifier, not its own weld type. It will sit in contact with the specific weld symbol it’s modifying, mimicking the required profile. The difference is how the rest of the symbol is built around it.

ISO 2553 also has its own notation for the weld-all-around and field weld marks, which differ in shape from their AWS counterparts. On top of this, AWS A2.4:2020 includes an annex on ISO 2553 welding symbol comparison to help bridge these layout differences. Notably, the same revision also added detailed usage guidance for flare groove welds and a combination groove weld symbol for two different edge shapes, both of which can appear with a concave contour requirement on more complex joints. These updates reflect just how often shops cross between the two systems, from structural work to precision jobs such as aerospace welding.

Common Reading Mistakes and How To Avoid Them

With two standards in play, a few errors come up time and time again:

• Treating contour as a weld type: By far the most frequent misread is treating the contour curve as a weld type instead of a face-shape requirement applied to the weld symbol it sits with. Always identify the weld symbol first, then read the curve as a modifier.
• Assuming above/below equals side: On ISO drawings, do not default to AWS “arrow side below, other side above”. Trace to the identification line and follow ISO’s labeling before placing any concave face.
• Mixing finishing and contour: A contour symbol is a profile requirement. A separate finishing designator (such as grinding) is what dictates the method of achieving it. These are not interchangeable between AWS and ISO either, so check the exact symbol for each.
• Assuming full standard parity: Just because a drawing looks familiar doesn’t mean every element matches. When in doubt, reference the system noted on the drawing and cross-check with the applicable standard before fabrication.

At the end of the day, the goal is the same in both AWS A2.4 and ISO 2553: control the weld face profile to meet the design intent. The path to reading a required concave face is just different. Take a second to confirm which system you’re on, locate its reference/identification lines, and never read the curve alone. That small bit of diligence will prevent a very avoidable inspection rejection.

Quick-Reference Workflow, Final Checklist, and Conclusion

Reading a concave welding symbol is not a glance; it is a sequence. Skip a step and you land exactly where this article opened: a clean-looking weld, rejected on throat, over a mark nobody actually read. Here is the whole process compressed into a workflow you can run on any drawing, in any shop, in under a minute.

A Six-Step Method for Reading Any Concave Symbol

1. Find the arrow and sort the two sides. The arrow points at the joint. Elements below the reference line belong to the arrow side; elements above it belong to the other side. Decide which side you are welding before you interpret anything else.
2. Locate the basic weld symbol. A triangle means fillet; the V, bevel, U, and J variants mean groove. AWS A2.4 requires every element to sit within the length of the reference line and never at either end of it, so anything dangling off the tip of that line is nonstandard and worth a question (see the standard).
3. Read the contour curve. The concave mark is a shallow arc drawn directly against the weld symbol, curving inward like a crescent. Its shape mimics the finished face profile you must produce.
4. Read size, length, and pitch. On a fillet, the number left of the symbol is leg size, and the actual throat still governs strength. Check for length and pitch values after the size.
5. Check the finishing designator and the tail. A letter after the contour (G for grinding, C for chipping, M for machining) tells you how the profile is achieved. The tail carries specification, process, or other reference information, such as a WPS number or letter designation.
6. Verify in the field. Put a fillet gauge on the deposited weld, confirm the concave face, and measure actual throat rather than trusting the legs.

Printable-Style Field Checklist

• Arrow side and other side identified before the first pass
• Weld type confirmed: fillet or groove
• Concave curve read as a shape requirement, not decoration
• Leg size, length, and pitch taken off the symbol
• Tail contents (WPS, process) pulled and followed
• Finishing method noted: grind, chip, machine, or as-welded
• Actual throat measured, not just the legs
• Toe undercut checked against acceptance limits
• Final profile matched to the symbol before sign-off

Five Mistakes That Still Trip Experienced Welders

• Reading the weld symbol and skipping the curve. The triangle says what to weld; the arc says what shape it must finish at.
• Measuring legs on a concave fillet. Correct legs with a hollowed face can still leave the actual throat under size.
• Ignoring the finishing designator. A “G” after the contour means the required face may depend on grinding, not deposition alone.
• Reading AWS symbols with ISO habits, or the reverse. The A2.4:2020 revisions shifted some conventions, so inherited shop charts can mislead.
• Guessing when a drawing is ambiguous. Welders should never guess at contour requirements.

When to Escalate to the Engineer of Record

Some decisions are not a welder’s to make. Raise a request for information when the contour mark is missing but notes call for a machined or ground face; when size, length, or side data conflicts with the detail drawing or the WPS; when a fatigue-critical joint shows a concave requirement with no size, or a proud convex bead where toe dressing was expected; or when the symbol and the project specification disagree on finishing or acceptance criteria.

The code and the responsible engineer govern, and the tail reference on the symbol usually points to the document that settles it. If you are still building reading speed, practicing on real joint details helps, and our Aluminum Boat Welding walkthrough is a good place to start, as is the Aluminium Welding Course guide for what formal training actually covers.

Conclusion: What the Concave Symbol Actually Means

Strip away the notation and the answer stays simple. The concave welding symbol is a shape modifier, not a weld type: it tells you that whatever weld the triangle or groove symbol describes must finish with a face that curves inward. That curve is not cosmetic. It fixes where the effective throat actually sits, and on a fillet weld it can quietly pull the real throat below the required size even when the legs measure perfectly.

Read it in sequence: identify the arrow side, find the weld symbol, read the contour curve, take the size and length, check the finishing designator and tail, then verify the deposited weld against the symbol with a gauge. When the drawing is clear, that workflow takes a minute. When it is not, the code and the engineer of record decide, not the welder’s best guess. That discipline is exactly what the bracket stack in our opening story was missing.

Frequently Asked Questions

What does the concave welding symbol mean?

It is a contour symbol—a small curved line placed with a weld symbol—that requires the finished weld face to sink inward between the toes instead of standing proud or lying flat. It modifies the shape of the weld face; it does not indicate a different weld type.

What is the difference between flush, flat, and concave contour symbols?

Flush and flat are drawn as a straight line and mean the face should run level from toe to toe. Concave is drawn as an arc and means the face curves inward. Under AWS A2.4:2020 the flat symbol is limited to fillet welds, while flush is used for groove, plug, and slot welds.

Does a concave fillet weld make the weld weaker?

It can, because the effective throat is measured from the root to the face, and a concave face reduces that distance even when the legs measure full size. That is why inspectors gauge the actual throat, not just the leg, when a minimum weld size is specified.

When is a concave profile actually required instead of being a defect?

It is commonly specified for fatigue-critical tubular and heavy-section joints, where AWS D1.1 lists concave improved and fully ground concave smooth profiles. Designers may also require it for appearance, clearance, or coating reasons.

How do I check a concave weld in the field?

Confirm the symbol, then measure the leg with a fillet weld gauge and calculate or gauge the actual throat against the required size. Check the toes for undercut and look for crater cracks or excessive concavity at stops, applying the acceptance limits in the governing code or specification.

Do finishing symbols change what the concave symbol means?

No. The contour symbol states the required face shape, while a finishing designator placed with it states how that shape must be produced, such as grinding or chipping. If a finishing symbol is shown, the drawing overrides shop practice.

Is the concave symbol the same in ISO 2553 drawings?

ISO 2553 also uses contour marks for concave faces, but its reference line and identification line conventions differ from AWS A2.4. Always identify which standard the drawing invokes before interpreting the symbol.

Where does the concave contour mark go on the welding symbol?

It sits with the weld symbol it applies to and is oriented so its curve mimics the required face profile, following the AWS standard location of elements. If no contour mark appears, normal shop welding practice determines the final profile.

Can I fix an overly concave weld without re-welding it?

If the throat still meets minimum size and no cracks or undercut exist, out-of-position corrections such as a small wash pass or permitted grinding can sometimes bring the profile into spec. If the throat is short or the toe is undercut, additional weld metal must be deposited under an approved procedure.