Cill, sill and sill plate

Frequently asked questions

If a drawing says sill, which component is it?

In North American practice, unqualified, assume the SILL PLATE — the treated timber bolted to the top of the foundation that the wall framing sits on — because that is the member most drawings are discussing when they say sill without a qualifier. The piece under a window is then usually the window sill on the outside and the STOOL on the inside, with the apron below it. In British practice, unqualified, assume the window component, because the framing equivalent is the sole plate or the wall plate and has its own name. The consequence of getting it wrong is not cosmetic: one is a weathering detail measured in linear metres of stone, cast or uPVC, the other is preservative-treated structural timber with a fixing schedule and a gasket.

What does the drip groove under a cill do, and why does blocking it matter?

It stops water tracking back along the underside and reaching the wall. Water running off the face of a window reaches the front of the cill and, left alone, surface tension will carry it around the bottom edge and back inwards along the soffit of the cill until it meets the wall — where it soaks in, exactly where the cill was meant to protect. A groove, or a sharp projecting edge, breaks that path: the water cannot follow the corner and drops clear. It is why the cill projects beyond the face of the wall at all, and why a render or a paint build-up that fills the groove, or a replacement cill fitted flush, reproduces the damp patch the original detail existed to prevent. The same logic governs the stop ends at each side of the cill.

Why does the cill need a DPC or flashing behind it?

Because the cill is a joint as well as a surface, and joints leak eventually. Water that gets past the frame, around a failed sealant line or through a porous cill unit arrives on top of the wall beneath the opening, which is a horizontal surface inside the construction — the one place it can sit. A cill DPC or a flashing tray beneath the cill catches that water and returns it outwards, usually with stop ends so it cannot escape sideways into the reveals. This is why the flashing membrane at a window opening is detailed before the window goes in and cannot sensibly be added afterwards, and why replacement windows fitted into an old opening frequently produce damp at the reveals: the new frame is sound and the tray was never there.