Deck Rim Joist Flashing Detail at Door Threshold

Why this matters

Almost every rotted-rim-joist callback traces to one detail: the flashing at the door threshold above the deck ledger. Water leaving the door pan has to clear the ledger flashing and exit to the deck surface without ever touching the house rim. Get the laps wrong and you trap water behind the ledger; the rim rots, the lags lose pullout capacity, and the deck pulls away from the house in 8 to 15 years.

The water path

Read this from top to bottom; this is the order water travels:

1. Door threshold (and door pan flashing under it)
2. Wall WRB (housewrap or membrane) lapped over the door pan
3. Top of ledger flashing tucked under the WRB
4. Ledger board nailed to the rim of the house
5. Bottom-leg of ledger flashing or shedding angle
6. Open air gap between ledger and decking board
7. Deck surface, sloped away from the house

Any reversed lap in that sequence creates a dam. Water always finds the lowest break.

Flashing material selection

The flashing in contact with ACQ or CA pressure-treated lumber must be compatible:

• Copper: compatible with treated lumber, very long-lived, expensive
• Stainless steel (304 or 316): compatible, long-lived, mid-cost
• Powder-coated or PVC-coated aluminum (G90 minimum): compatible if the coating is intact; bare aluminum will corrode against treated lumber within 2 to 5 years
• Bare aluminum: not acceptable
• Galvanized steel (G90 minimum, G185 preferred): acceptable, with the understanding that nicks in the coating will start corrosion
• Self-adhered membrane (Grace Vycor Plus, Henry Blueskin, MFM Peel and Seal): acceptable as the top-leg under-WRB layer, not durable as the exposed shedding leg

A two-piece system is the standard: a self-adhered membrane top piece going up the wall under the WRB, and a metal shedding angle (Z-flashing or drip-cap) over the top of the ledger.

Step-by-step at the door

Before the ledger goes on:

1. Cut the WRB back at the ledger location, exposing sheathing 6 to 8 inches above the planned ledger top.
2. Install a door pan flashing under the door threshold (Jamsill, EnduraPan, or site-built). The pan front edge should overhang the eventual ledger flashing by 1/4 inch.
3. Apply a 9-inch wide self-adhered membrane to the wall sheathing at the ledger location, extending 4 inches above the planned ledger top and 5 inches below the ledger top. This is the back-up moisture barrier behind the ledger.
4. Install the ledger and lag it per IRC Table R507.9.1.3(1), using 1/2-inch DTI washers under the lag heads.
5. Install a metal Z-flashing (or PVC-coated aluminum drip-cap) over the top of the ledger, with the vertical leg going up the wall a minimum of 4 inches and the horizontal leg covering the ledger top, with a hemmed drip edge clearing the ledger face by 1/4 inch.
6. Reinstall the WRB from above, lapping it over the vertical leg of the Z-flashing by at least 4 inches.
7. Reinstall the door pan front leg over the top of the WRB-over-flashing assembly, so the pan sheds water onto the front face of the metal flashing.

The 1/2-inch gap

Leave a 1/2-inch gap between the back edge of the first deck board and the ledger flashing face. This is a code-recognized practice that lets debris wash through and prevents the back board from trapping water against the flashing. Some hidden-fastener systems set this gap automatically; if the system does not, set it manually with a scrap of 1/2-inch PVC.

What goes wrong on remodels

On a remodel where the wall finish is already installed and the housewrap is captured under siding, the correct sequence requires removing siding above the ledger. Crews who skip that step end up with the ledger flashing trapped behind the housewrap (water dam) or no flashing at all (rim rots in 5 years). The fix is to cut the siding back at least 12 inches above the planned ledger top and rebuild the WRB lap correctly.

Door threshold transition options

The deck board adjacent to the door should be at least 1.5 inches below the door threshold to prevent water and snow from running into the house. Where the customer wants a flush transition (no step down), use:

• A door with a higher threshold (look for ADA-rated thresholds with a 1/2-inch maximum rise)
• A drainage scupper in the deck immediately outside the door
• A separate trench drain at the door
• A canopy or roof over the door reducing rain exposure at the threshold

A flush transition with no drainage management leaks. Document the customer's choice in writing.

References

• 2021 International Residential Code, Section R703.4 (flashing)
• 2021 IRC R507.9.1 and Table R507.9.1.3(1) (deck ledger attachment and lag schedule)
• 2021 IRC R311.3.1 (floor elevation at door thresholds)
• ASTM E2112 Standard Practice for Installation of Exterior Windows, Doors and Skylights
• American Wood Council DCA 6-2015, Prescriptive Residential Wood Deck Construction Guide, ledger flashing detail
• Building America Solution Center, Deck Ledger to Rim Joist Connection, flashing detail drawings