Slope a Window Sill With Bevel Siding and a Scrap-Wood Back Dam

The Problem

Every window lets some water past eventually — a failed sealant joint, wind-driven rain, condensation. If the rough sill under it is dead flat, that water sits there and soaks into the framing, and the first sign of trouble is a rotted sill years later. Good practice is to build the sill so water that gets in has somewhere to go: sloped toward the exterior, with a dam at the interior edge. Manufactured sill pans do this, but they're one more line item on every opening.

The Technique

Two pieces of material you probably already have on site do the same job:

1. Cut a length of bevel siding to the width of the rough opening and lay it flat on the rough sill, thick edge toward the interior. The siding's factory taper pitches the whole sill surface toward the outside.
2. Rip a 3/8-inch strip from scrap wood and fasten it along the interior edge of the sill. This is the back dam — a raised lip that stops any water on the sill from migrating over the edge into the wall and floor below.
3. Flash the sill assembly as you normally would — flexible flashing over the slope and back dam and up the jack studs — then set the window. Anything that gets past the window now lands on a surface that drains it back out.

Because bevel siding comes with a consistent factory taper, the detail is repeatable on every opening — no custom bevel rips, no fussing with shims. It works in new construction or a renovation any time there's room in the opening, or room can be made.