Shimming and Squaring a Window

Two window frames that look the same from the street can sit very differently in their openings, and the figures kept at window installation carlsbad madame and me show how shims decide the difference. A window installation in Carlsbad, CA depends on getting the frame plumb, level and square before a single fastener is driven.

Why shims come first

A frame fastened against an uneven opening takes on the shape of that opening. The sash then binds, the lock does not line up, and the seal against the weatherstrip leaks air. Shims fill the gap between the frame and the framing so the frame can stay true while the opening around it stays imperfect.

Where the shims go

Pairs of tapered shims are set at the bottom corners, at each fastener location and at the center of any long span. On a wide slider or a picture window, shims also support the head so the frame does not bow in the middle. The sill is shimmed level first because everything else is referenced from it.

Plumb, then square

With the sill level, the jambs are brought plumb using a long level held against them, and the diagonals of the frame are measured. Equal diagonals mean a square frame. A double hung with a leaning jamb will let the sashes drift, so the plumb check is repeated after fastening to catch any movement.

Fastening without distorting

Screws are driven only until snug. Overtightening a fiberglass window or composite frame can pull a flange tight enough to bend the jamb, and a bent jamb shows up later as a hard-to-open sash. After each fastener, the operating sash is cycled to confirm it still glides.

Final checks

A casement is opened and closed to see that the sash seats evenly all around, and a slider is rolled through its full travel. If anything drags, the nearest shim is adjusted before foam or sealant goes in. Houses near Oceanside and along the coast move slightly with seasonal moisture, so a frame set true on a dry day should still be set with room to move. The small gap left around the frame is what lets the foam and sealant absorb that movement without tearing or pushing the sash out of true during the first wet season.