Flash Siding Butt Joints Instead of Caulking Them

The Problem

Every run of lap siding has butt joints, and every butt joint is a path for water. The common answer is a bead of sealant at each joint — which works until it doesn't. Caulk shrinks, cracks, and peels with the siding's movement, and from then on it's a recurring maintenance job the homeowner inherits, usually without knowing it.

The Technique

Skip the sealant and flash the joint instead.

1. At each butt joint, slip a small rectangle of rigid joint flashing behind the siding, centered on the joint. The pro in the video uses purpose-made plastic joint flashes about 4 1/2 by 8 inches (a pack of 100 ran him $32 — about 32 cents a joint).
2. The flashing tucks up behind the course above and hangs down behind the joint, so any water that gets through the gap drains down the face of the flashing and back out — the same shingle logic as the rest of the wall.
3. Leave the joint itself dry. No caulk, nothing to renew.

Because the pieces are rigid plastic, a stack rides in a pocket all day, and there are no sliced fingers — the traditional coil-metal version can't say either of those things.