Weld symbol
Slot weld symbol
The same open rectangle as a plug weld, but the hole is elongated. Because a slot has length, width, orientation, and end radii, much of its geometry lives on the drawing.
Reads asArrow side: slot weld, 1/2 size, 3/8 depth of preparation, 2 long, 6 pitch, 4 welds.
How to read it
A slot weld is a plug weld stretched into an elongated hole. The near member is cut with a slot rather than a round hole, and that slot is filled with weld metal fusing into the member below. It gives more weld area per opening than a plug, and it lets the designer align the weld with the direction the load runs.
The symbol is the same three-sided rectangle used for a plug, which means the symbol alone cannot distinguish the two. What separates them is the accompanying information: a slot has a width and a length, and the drawing has to show the slot's orientation and the radius at each end. Expect a detail view or a dimensioned plan whenever you see a slot callout.
On the symbol itself, the width sits to the left, the length sits to the right, and the pitch follows the length as a centre-to-centre spacing along the joint. A count in parentheses says how many slots the callout covers, and a depth to the left of the width says how deep they are filled where that is less than the full thickness of the near member.
Orientation is the part people miss. A slot running across the load direction behaves quite differently from one running along it, and the symbol says nothing about which. Always take the slot's direction, position, and end radius from the drawing, and treat the symbol as the instruction for what to do once the slot exists.
Where each dimension goes
- Width (left of the symbol)
- The narrow dimension of the slot, measured at its base.
- Length (right of the symbol)
- How long each slot runs, end to end.
- Pitch (after the length)
- Centre-to-centre spacing between slots along the joint.
- Count (in parentheses)
- Number of slots the callout covers.
- Depth of fill
- How deep the slot is filled with weld metal. Omitted means filled to the near member's surface.
- Orientation and end radius
- Taken from the drawing, never from the symbol, because the glyph cannot express direction.
Worked examples
Each one is a drawing, the sentence it comes out as, and then the mark-by-mark account of why. Read the sheet first and say the sentence yourself before you look down.
Example 1: Four slots along a lap joint
TitleFour slots along a lap jointRefSlot weldEx1 / 2Reads as
Cut four 1/2 wide slots two inches long on six-inch centres and fill them.
Which mark says so
- slot symbolBelow the reference lineA slot weld on the arrow side of the joint
- 1/2Left of the symbolDiameter of the hole, or width of the slot
- 2Right of the symbolLength of each weld
- -6Right of the length on the symbolPitch - centre to centre spacing of an intermittent weld
- (4)In parentheses beside the symbolHow many welds are called for
The 1/2 to the left is the slot width, the 2 to the right is its length, the 6 after it is the centre-to-centre pitch, and the parenthesised 4 is how many slots there are.
And what it does not say
- Nothing sits above the reference line, so the far face of the joint stays bare.
- There is no circle at the elbow, so the weld does not continue around the joint.
Example 2: Partially filled slots on thick plate
TitlePartially filled slots on thick plateRefSlot weldEx2 / 2Reads as
Fill each 5/8 wide slot only 3/8 deep rather than level with the surface.
Which mark says so
- slot symbolBelow the reference lineA slot weld on the arrow side of the joint
- 3/8Left of the symbol, before the sizeDepth of preparation - how far the joint is cut back
- 5/8Left of the symbolDiameter of the hole, or width of the slot
- 3Right of the symbolLength of each weld
- (3)In parentheses beside the symbolHow many welds are called for
The depth written ahead of the width limits how much weld metal goes in, which keeps heat input down on thick material where a full fill would add nothing structurally.
And what it does not say
- No pitch follows the length, so the weld is continuous over that length.
- Nothing sits above the reference line, so the far face of the joint stays bare.
- There is no circle at the elbow, so the weld does not continue around the joint.
Common mistakes
- Reading a slot symbol as a plug weld because the glyph is identical and missing the length dimension.
- Cutting slots in the wrong orientation because the symbol does not carry direction.
- Leaving square corners at the slot ends where the drawing calls for a radius.
- Filling slots to the surface when a shallower fill depth is specified.