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Indoor airPLUS

Technical Guidance to the Indoor airPLUS Construction Specifications

1. Moisture Control

Sections 1.5 - 1.6: Water-Managed Wall Assemblies

Best Practice: Deck Ledger Attachments

Many houses have decks attached to them. They are usually attached by first installing a ledger board to the side of a house. The ledger board carries the load for one end of the floor joists of the deck. The ledger and its attachment points penetrate the siding and must be flashed to protect the wall from rain water running down the siding and running down the drainage plane. This area of the wall experiences more rain exposure than most others because rain hitting the deck next to the wall splashes against the wall. "Free-standing" decks avoid this problem because they do not penetrate the siding and drainage plane.

Detailed Illustrations: Best Practice Techniques

Click on the images for a full-page version

Best Practice (1 of 2) - Flashing diverting water from deck ledger

Flashing diverting water from deck ledger

Best Practice (2 of 2) - Detail - Flashing diverting water from deck ledger

Flashing diverting water from deck ledger

Flashing a ledger board with membrane material

flashing a ledger board with membrane material


Decks and porches attached to the exterior walls of houses create long horizontal joints in the cladding that must be properly flashed to avoid rain leaks. As with window and door installations in walls, paying careful attention to flashing details is critical to avoid potential rot and mold. Water from direct rainfall, splash from decks and runoff from incorrectly sloped deck surfaces can leak into the exterior wall where the deck attaches to the house. Several conditions contribute to the water problem:

  • The ledger board is simply attached to the house with numerous lag screws or other hardware that penetrate the wall’s cladding and drainage plane, but no flashing has been installed to protect these areas.
  • Water is often trapped behind the ledger board.
  • Upward splashing of rain from the deck adds significant wetting to the siding, and inadequate flashing results in wetting and rot in the wall’s sheathing and framing.

Integrating the attachment of the ledger board with the drainage plane behind the wall’s cladding and adding proper flashing will maintain the integrity of the drainage plane and channel water away from the wall’s surface.

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Install a capillary break in the form of a self-adhesive membrane on the wall before the ledger board is applied. This break provides moisture protection between the ledger board and the sheathing. If the cladding continues down below the deck, lap the self-adhesive strip down over the drainage plane material below the deck. Apply an L-shaped flashing to the top of the ledger board. The vertical leg of this flashing extends along the wall (above the ledger) and the horizontal leg extends out and over the top of the ledger. The drainage plane material laps the vertical leg of the flashing so any water that runs down the drainage plane is diverted out of the wall by the flashing.
Note: if the ledger is ACQ (arsenic- and chromium-free) preservative-treated lumber, flashing material should be ACQ-resistant to prevent corrosion. For a discussion of the water-based wood preservative ACQ , see www.epa.gov/oppad001/reregistration/cca/acq.htm.

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References/Additional Information

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