Both are plastic, both are colour coded by gauge, both have an open slot so they fit around a fastener that is already in place. Then they stop being alike.
Quick answers
What is the difference between a horseshoe shim and a U shim?
Size and slot. A horseshoe shim is 3" x 4" with a 7/8" bolt hole; a U shim is 1-1/2" x 2" with a 1/2" bolt hole. A 1/2" slot will not fit round a 7/8" anchor.
When should I use a horseshoe shim instead of a U shim?
Under heavy units, long-life sills and soft substrates. Its 12 sq in footprint spreads load that a U shim's 3 sq in would concentrate.
Can a shim be used as a setting block?
No. Setting blocks carry the weight of the glass in the glazing pocket; a shim only levels and spaces the frame.
Can I order U shims from you?
They are not listed on the site at the moment while we settle the line, but we can still supply them. Tell us the gauge and the quantity and we will price it.
The difference that decides it
| Horseshoe shim | U shim | |
|---|---|---|
| Footprint | 3" x 4" | 1-1/2" x 2" |
| Bolt hole | 7/8" | 1/2" |
| Gauges | 1/16, 1/8, 1/4, 1/2" | 1/16, 1/8, 1/4" |
| Per box | 1,000 | 1,000 |
A 1/2" slot will not go round a 7/8" anchor. That is the whole decision. Measure the anchor or bolt you are shimming against and pick the shim whose slot clears it.
When the bigger footprint matters
The horseshoe is 3" x 4" — twelve square inches against three. That spreads load, which is what you want under a heavy unit, under a sill that will carry weight for thirty years, or anywhere the substrate is soft enough to let a small shim print into it.
The U shim's smaller body is an advantage where there is no room: tight pockets, narrow jambs, and anywhere a 4" shim would foul the frame or show in the joint. The rounded shape leaves no corners sticking out and gives more room to move while you are aligning.
Stacking
Both stack. Build the thickness you need from the gauges you have rather than hunting for one shim of the exact dimension — three 1/16" shims and one 1/8" gets you 5/16". Keep the stack tidy and square; a fanned stack carries load on one edge.
What they will not do
Neither is a setting block. Setting blocks carry the weight of the glass in the glazing pocket and are specified by durometer and material for that job. A shim levels and spaces the frame. Do not substitute one for the other.
Fitting
A permanent shim sits entirely within the joint, behind the finish materials, clear of the sealant. Anything proud into the joint becomes a hard spot in the bead, and that is where it will split when the building moves.
Horseshoe shims, box of 1,000 · The shim range · The colour and gauge chart
U shims are off the site for now — ask us and we will quote them.