Windows in a small building
Every opening is a deliberate weakness in the wall, so the number and size of windows is worth deciding on use rather than on appearance. A workroom wants light on the bench. A store wants none at all, and a store with no window has one less detail to keep watertight and one less invitation to anyone passing.
The sill is the detail that decides whether the opening survives. It slopes away from the frame so water runs off rather than sitting on the joint, it projects past the cladding face, and it has a groove on its underside so that water running back along the soffit drops clear instead of tracking to the wall. A flat sill with no projection and no groove will wet the wall below it at every shower.
Above the opening, a head flashing or a projecting drip takes water over the frame. Down the sides, the membrane behind the cladding is dressed into the opening so that anything getting past the cladding is directed back out onto the sill rather than into the frame.
Fixing the frame with a small clearance all round and packing it plumb, rather than forcing it into a tight opening, leaves room for the frame to move with the seasons and for the gap to be sealed from the outside. Glass or sheet is bedded rather than dry-fitted, and beads go on the inside where they are out of the weather and can be removed without disturbing the seal.
900 mm opening width
Next note: Treating and painting softwood
Previous note: Hanging a door that stays square