Flashing details
Flashing is not a product, it is a sequence. Water has to be handed from one layer to the next, each lapping over the one below, until it reaches the ground or a drain. Most deck leaks are a sequence error, not a missing part.
The water path, from the top down
- Rain lands on the siding above the deck and runs down the wall.
- It reaches the top of the ledger — a horizontal shelf pressed to the wall — and stops.
- It either runs over the front of the ledger (fine) or into the joint behind it (rot).
- Anything that gets behind the ledger runs down the sheathing to the bottom of the wall, where it either exits or accumulates.
The whole detail exists to make step 3 go the first way. That is what a cap flashing over the ledger does: it becomes the new surface the water runs on, and it laps over the ledger face and under the wall's own flashing above.
The sequence that works
| Layer | Material | Laps |
|---|---|---|
| Wall weather barrier | House wrap or felt | Over the flashing below, under the siding above |
| Wall flashing | Metal or self-adhesive membrane | Up the wall behind the siding, out over the ledger top |
| Ledger | Treated, fastened to framing | — |
| Cap flashing | Metal Z or L | Over the ledger top, up under the wall flashing |
| Siding / trim | — | Over the flashing, held 1–2″ above it |
Each layer laps over the one below and under the one above. If you can trace a continuous downhill path from the siding to the deck surface without hitting an upward-facing seam, the water will follow it.
Material compatibility is not a detail
Copper-based pressure treatment attacks aluminium. An aluminium flashing laid on treated lumber will corrode through, usually at the fastener holes, in a few years. The compatible choices are copper, stainless, lead-coated copper, or a coated self-adhesive membrane — and any metal must be isolated from the treatment by a membrane if the two touch over a large area.
Fasteners follow the same logic: galvanised or stainless in treated wood, stainless in cedar, and never aluminium against either.
Joist tape, and what it does
A butyl or acrylic flashing tape over the top of joists has become standard practice under decking. It does two useful things: it keeps water out of the fastener holes, which is where rot starts, and it isolates the deck board from the joist so the two can move independently as they take on moisture. It does not fix a frame that holds water, and it is not a substitute for a ledger flashing.
Places decks leak that are not the ledger
- Post bases. A post sitting in a hole with soil at the bottom becomes a straw for water. A gravel base and a crowned concrete surface fix it; a standoff base avoids it.
- Stair stringers at grade. The bottom of a stringer in soil rots first. Cut it to bear on a concrete pad, not on the ground.
- Butt joints in decking over a joist. Two board ends meeting over a joist create a drainage path into the fastener holes. Stagger them, and tape the joist.
- Guard post connections. Any penetration through the deck surface is a water entry point. Seal it, and detail the post so water cannot stand against the wood.
Where flashing fails anyway
- It does not give a wall assembly. How the house's weather barrier, sheathing and siding are layered is a separate detail, and getting it wrong at the deck is often just exposing an existing error.
- It does not cover roof-to-deck junctions. A deck under a roof has its own flashing problem at the roof-wall intersection.
- It does not replace the manufacturer's detail. Flashing profiles, membranes and tapes all have installation instructions that govern their laps and adhesives.
Questions people actually ask
Can I just caulk the ledger instead of flashing?
No. Caulk is a sealant, not a drainage layer: it fails within a season or two on a horizontal joint that takes sun and water, and when it fails the water goes behind the ledger where you cannot see it.
What kind of flashing goes over a deck ledger?
A metal Z-flashing or a self-adhesive membrane that laps up the wall and over the ledger top. Metal must be compatible with the treatment — copper or stainless rather than aluminium.
Do I need joist tape?
It is not code-required in most places, but it is cheap and it addresses the most common rot location: the fastener hole in the top of the joist. It also reduces squeaking as the boards move.
Why does aluminium flashing corrode on a deck?
Because the copper in modern pressure treatment is cathodic to aluminium, and the aluminium becomes the sacrificial metal. The corrosion concentrates at cut edges and fastener holes.
How high should siding sit above a deck?
At least 1–2″ above the deck surface or the flashing, so water can drain and the siding is not wicking from the deck. Some manufacturers specify more.