Siding Butt-Joint Flashing
Slipping a flashing piece behind each siding butt joint instead of relying on caulk.
Don't Caulk the Siding Joint Above Doors and Windows
Rain will get behind siding — the wall assembly is designed to drain it back out over the head flashing above your doors and windows. Caulk that joint shut and you've dammed the escape route: the trapped water rots the wall from inside.
water leaking behind siding, gap above window trim, house wrap drainage, should you seal every gap outside
The Problem
A tube of caulk and a free afternoon have ruined a lot of walls. The gap where siding passes over a door or window head looks like an oversight — a crack begging to be sealed against drafts and rain. It isn't. It's a drain.
The Technique
This one is a don't: leave the joint between the siding and the head flashing above doors and windows uncaulked.
The reasoning is the whole tip:
- Rain water can and will get behind your siding — no cladding is a perfect barrier, and the wall is designed with that assumption.
- The water that gets behind runs down the drainage plane and back out over the head flashing above your openings. That "gap" is the exit.
- Caulk it shut and the water still gets in — it just can't get out. It sits against sheathing and trim, and the result is rot, hidden until it's expensive.
Sealing that joint does more harm than good. Caulk belongs where it keeps water out of a joint that can't drain — not across the mouth of a joint that exists to let water leave.
Pro tip: Before caulking anything on an exterior, ask which way the water is supposed to move at that joint. Vertical seams that admit water, seal; horizontal laps and flashing edges that shed or drain water, leave alone. When a house has weep gaps — above openings, under siding courses, at brick weep holes — the maintenance job is keeping them clear, not filling them.
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