Deck Ledger Flashing: Why It Matters and How to Do It

The ledger board attaches the deck to the house structure. Without proper flashing, water gets behind the ledger, soaks the house rim joist and band joist, and rots the framing that holds the deck up. This failure mode is responsible for the majority of deck collapses.

What the IRC requires

IRC Section R507.2 requires that the ledger be flashed to prevent water from entering the wall and that the flashing direct water to the exterior. The code does not specify a single approved method, but most jurisdictions accept the detail in the AWC Wood Frame Construction Manual (WFCM) as the prescriptive standard.

The correct flashing sequence

• 1. Cut a horizontal slot in the house cladding above where the ledger will sit.
• 2. Install a piece of flashing (Z-flashing or self-adhesive membrane) that tucks into the cut and laps over the top of the ledger.
• 3. Install the ledger and fasten it through the flashing into the rim joist.
• 4. Install a second piece of flashing (cap flashing) that overlaps the first from above.
• 5. Install housewrap lapping over the top flashing and sealed at all edges.

Material options

Self-adhesive membrane flashing (Grace Ice & Water, Vycor) is the most reliable option — it bonds to the sheathing and wraps around fastener shanks to seal them. Metal Z-flashing is the traditional alternative and works well when properly installed, but it cannot seal around bolt shanks the way membrane can.

What not to do

Never caulk the top of the ledger and call it flashing. Caulk cracks, shrinks and allows water intrusion. Never install a ledger flat against vinyl siding without removing the siding and installing flashing behind it. Siding is a rain screen — it is not a waterproof barrier.