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 as: Arrow 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
Four slots along a lap joint
“Cut four 1/2 wide slots two inches long on six-inch centres and fill them.”
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.
Partially filled slots on thick plate
“Fill each 5/8 wide slot only 3/8 deep rather than level with the surface.”
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.
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.
Typical joints
Related symbols
Practise reading welds, not just symbols
The drawing tells you what the weld should be. The gauge trainer is where you find out whether you can tell what it actually is.