How to flash a deck ledger
Water behind a ledger rots the band joist, and a rotted band joist cannot hold the ledger. The flashing is not a detail on this connection — it is the connection.
The layer order, from the wall out
- Flashing tape on the wall, applied over the sheathing and the top plate of the foundation, lapping down over the sill area.
- Z-flashing (or ledger flashing) with its upper leg behind the siding and its lower leg over the top edge of the ledger.
- The ledger, fastened through the flashing tape into the framing.
- Joist tape over the top of the ledger and over the tops of the joists, before the decking.
- A cap or drip edge over the top of the ledger flashing, so water cannot sit on the horizontal leg.
The governing principle: every layer laps over the one below it, and water always has a path off the front edge. Anything that creates a horizontal ledge where water can pool is wrong.
The three mistakes
- Flashing over the ledger only. If the Z-flashing sits on top of the ledger and against the siding but is not tucked behind it, water runs down the wall and into the joint between siding and flashing.
- Fastening through the flashing without sealing. Every fastener that penetrates the flashing is a hole. Seal it, or run the fastener below the flashing's coverage.
- No cap. A horizontal flashing leg exposed to the sky holds water and debris. A cap or a slight slope on the top of the ledger keeps it clear.
Standoffs are for siding, not for waterproofing
A ledger standoff holds the ledger away from the wall so it can bear on the sheathing rather than crushing vinyl or fibre-cement siding. It does not make the connection waterproof — the flashing still has to be there, and the standoff has to be flashed too.
On a wall with a rain screen or a drainage plane, the ledger interrupts the drainage. The flashing has to redirect water out and over the ledger, not just keep it off the top edge.
When the wall cannot take a ledger
- Brick or stone veneer. The veneer is not structural. Either remove it at the ledger, or build a freestanding deck.
- Cantilevered floor framing. The band joist at a cantilever is not designed for the added load; a freestanding deck is the usual answer.
- Stucco. The ledger has to reach the framing behind it, and the stucco has to be cut and re-flashed, which is a specialist job.
- A wall with no rim joist, such as some older construction, needs a different detail entirely.
Inspecting an existing deck
You cannot see behind a ledger, but you can look for the signs:
- Staining or softness in the siding directly above the ledger.
- A ledger that has pulled away from the wall — check with a putty knife at the top edge.
- Sagging at the house end of the deck, which means the ledger is moving.
- Rust streaks from fasteners, which means water is reaching them.
Any of these is a reason to open the joint and look, not to repaint over it.
What this guide will not specify
- It does not specify a fastener — see ledger attachment.
- It does not cover wall assemblies with exterior insulation, which need a longer fastener and a different flashing detail.
- It does not cover decks below a roof, where the ledger also takes roof load.
- It does not replace a manufacturer's flashing system, which may specify its own layer order.
Questions people actually ask
Does a deck ledger need flashing?
Yes, and it is the most commonly omitted detail on a deck. Without it, water reaches the band joist, the wood rots, and the ledger loses its anchorage.
What order does ledger flashing go in?
Flashing tape on the wall, then Z-flashing tucked behind the siding and over the ledger, then the ledger, then a cap over the top. Each layer laps over the one below.
Can I attach a ledger over vinyl siding?
Not directly. The siding has to be cut back or a standoff used so the ledger bears on the sheathing, and the flashing has to be integrated with the siding's drainage plane.
How do I know if my deck ledger is rotten?
Look for staining or softness in the siding above the ledger, a gap at the top edge that a putty knife fits into, sagging at the house end, or rust streaks from the fasteners.