What happens to the stucco around a nailing fin window

A nailing fin window needs a clean strip of sheathing to nail to, and the way riverside window replacement in spiraling describes it (the order of the cuts is the whole job) starts with the stucco. In a Riverside, CA house, the old window sits behind a lip of stucco, so a new window with a fin means cutting that lip back before any install can begin.

How far the cut goes

The cut runs about two inches past the edge of the old frame on all four sides, measured to leave room for the full fin and the flashing tape that laps over it. A diamond blade on a grinder makes a straight line with a vacuum on the housing, because stucco dust travels. The line is scored first, shallow, so the lath behind it is not struck and the paper beneath stays whole.

What is under it

Older walls have a layer of felt paper over wire lath, and the paper has usually dried and torn near the openings. A tear is not a problem when it is found, only when it is covered. The paper is cut back to sound material and new building paper is tucked under the upper edge, so water running down the wall passes over the new layer and never behind it.

Setting the unit

A vinyl window or fiberglass window with a fin goes in over a sill pan and a bead of sealant, shimmed level, and nailed through the fin. A double pane unit with insulated glass is heavy, so a picture window or slider is propped on blocks while the fin is fastened. Flashing tape goes on in order: sill first, then sides, then head, each piece lapping the one below it.

The stucco return

The patch that closes the gap is called the return. It is built in three coats, the scratch coat pressed into the lath, a brown coat leveled flat, and a finish coat floated to match the texture. A crew working near the Santa Ana River mixes the stucco fresh because the old wall carries a sand colour that a bagged repair rarely matches. The head of the return is finished with a weep screed or a gap, so moisture has a way out.

Curing and trim

The patch needs a few days before paint, and in Inland Empire summer heat it must be misted so it does not dry too fast and crack at the fin line. Cracks at a new fin edge nearly always come from a mix that dried too quickly or a patch that was thicker than the wall around it.

When the cut is skipped

A retrofit insert keeps the old frame and leaves the stucco alone, which is common on walls that are in good shape. When the frame itself is poor, there is no way around the cut.