Weld symbol
Double U-groove weld symbol
U-shaped preparations on both faces of a butt joint, drawn as a U above and below the reference line. It is the lowest-volume preparation available for very thick, two-sided access.
Reads asArrow side: U-groove weld, 3/4 size, 20° groove angle, 1/8 root opening. Other side: U-groove weld, 3/4 size, 20° groove angle.
How to read it
Two U symbols on one reference line mean the joint is prepared from both faces. In cross section it resembles two shallow bowls meeting back to back with a land between them. Each U carries its own included angle beyond the open end and its own depth to the left, and the root opening is written once for the joint as a whole.
This is the preparation reached for when the plate is thick enough that even a double-V would take too much weld metal. Because a U's cross-sectional area grows slowly with depth, splitting it across two faces keeps the total volume remarkably low on joints several inches through. Less deposited metal means less shrinkage, less residual stress, and far less arc time.
The trade is preparation cost and access. Both faces have to be machined or gouged to a radiused profile, and the joint has to be turned partway through so the second side can be reached. Where the part cannot be rolled, a designer will usually choose a single-sided U with a backing arrangement instead, so seeing a double U tells you the assembly is meant to be turned.
The two preparations need not be identical. Read the depth and angle on each symbol before setting up the cut, and expect the deeper preparation on whichever face has better access or better welding position. As with any two-sided joint, the root is normally cleaned back to sound metal before the second side is filled.
Where each dimension goes
- Included angle on each face
- Angle between the two prepared faces on that side of the joint, typically small on a U.
- Depth on each face
- How far that face's preparation is cut and filled with weld metal.
- Root opening
- Gap between the root faces where the two preparations meet in the middle of the thickness.
- Root face or land
- Thickness left unprepared between the two Us, given on the joint detail.
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: Symmetric double U on thick plate
TitleSymmetric double U on thick plateRefDouble U-grooveEx1 / 2Reads as
Machine 20 degree U grooves 1-1/2 deep into both faces with a 3/32 root gap.
Which mark says so
- U-groove symbolBelow the reference lineA U-groove weld on the arrow side of the joint
- 1-1/2Left of the arrow-side symbolGroove weld size - how deep the weld itself goes
- 20°Beyond the outer face of the arrow-side symbolIncluded angle the joint is prepared to
- 3/32Inside the arrow-side symbolRoot opening - the gap left between the members before welding
- U-groove symbolAbove the reference lineA U-groove weld on the far side of the joint
- 1-1/2Left of the other-side symbolGroove weld size - how deep the weld itself goes
- 20°Beyond the outer face of the other-side symbolIncluded angle the joint is prepared to
Matching U symbols above and below the line describe equal preparations from each face, so passes can be alternated to keep angular shrinkage balanced across the thickness.
And what it does not say
- No length is given, so each weld runs the full length of the joint.
- There is no circle at the elbow, so the weld does not continue around the joint.
Example 2: Unequal double U for restricted access
TitleUnequal double U for restricted accessRefDouble U-grooveEx2 / 2Reads as
Cut a 2 inch deep U on the arrow side and a 1/2 inch deep U opposite.
Which mark says so
- U-groove symbolBelow the reference lineA U-groove weld on the arrow side of the joint
- 2Left of the arrow-side symbolGroove weld size - how deep the weld itself goes
- 20°Beyond the outer face of the arrow-side symbolIncluded angle the joint is prepared to
- 1/8Inside the arrow-side symbolRoot opening - the gap left between the members before welding
- U-groove symbolAbove the reference lineA U-groove weld on the far side of the joint
- 1/2Left of the other-side symbolGroove weld size - how deep the weld itself goes
- 25°Beyond the outer face of the other-side symbolIncluded angle the joint is prepared to
The differing depths put most of the filling on the accessible face and leave a shallow preparation on the far side, just enough to gouge out and cap the root.
And what it does not say
- No length is given, so each weld runs the full length of the joint.
- There is no circle at the elbow, so the weld does not continue around the joint.
Common mistakes
- Assuming both preparations are equal without reading the depth on each symbol.
- Specifying a double U where the part cannot be turned to reach the second face.
- Skipping the back gouge before filling the second side and trapping root defects in the middle of the thickness.
- Treating the sum of the two depths as a single groove weld size.