End Gap Specs for a Solid-Surface Windowsill

An unusual application for solid surfacing leads to a discussion about movement joints and tolerances. May 23, 2007

Please settle a debate among our fabricators. We are making 1/2" solid surface window sills. Let's say the "tight" wall to wall window opening is 72". Fabrication manuals say 1/8" gap at all walls in a 144" run. That would be a total of a 1/4" deduct in length on 144" sill. Now if the sill is 72", would the total deduct be the same 1/4" overall, or would you cut that dimension in half because the sill is half as long? Just as an added twist, because it is a window sill, should we be increasing the gaps because of the fact that the window sill may be subjected to extra heat from sunshine?

Forum Responses
(Laminate and Solid Surface Forum)
From contributor B:
If it were a window sill and the ends were visible, then I would cut it tight. At most 1/16" at each end, then caulk. Sometimes the rules don't work for everyday applications.

From contributor J:
This may or may not help, depending on your trim work. Usually when I install window trim, the sill is the first piece in. The rest of the molding is built up off of the sill. In that situation you can have at least an eighth inch on either end, as it will never be seen. As for expansion and contraction, your best bet would be a call to the tech guys at the manufacturer. They are the ones who will be able to give you factual info.

From contributor B:
The techs can give information but it is by the book, not thinking out of the box. You can bet their product has not been tested as a window sill, so there will be no specific information.

From contributor R:
Best way to go is to call the manufacturer... If your gap is off, that voids the warranty, then the trouble really begins. We do Wilsonart Solid Surface and have yet to come under a situation the fab couldn't help us out with. Great group of guys.

From contributor U:
I worked for one of the solid surface companies. One of the area managers made a jig to hold a 4" x 48" (approximately) strip of solid surface. One end was fixed and the other had a thumb screw to apply a little pressure. He'd do this for architects, designers or fabricators. Apply a little pressure - 1/8" or so will do. Now measure the height of the buckled material in the center. Try this at home. Leave the gaps as specified by the manufacturer.

From contributor T:
Expansion gaps should be proportionate to the length of the sill; a 72" sill would require half that of a 144" sill. If the window faces south, it would be best to err on the side of caution. Of course if the window will be trimmed out later, larger gaps are appropriate.

From contributor O:
The more tolerance you provide, the less possibility for problems. The most important concept in all of solid surface installation is to use the proper adhesives. 100% silicone is the only adhesive that will give solid surface the flexibility it needs.