Deck failures and resulting injuries are increasing (see REPLACEMENT CONTRACTOR, June). That trend will likely continue, due to the aging of existing deck structures and the fact that many decks continue to be installed with little regard for engineering essentials such as safe and secure attachment of the ledger board. Flashing is a big problem, perhaps the biggest one. Without proper flashing, water intrusion will rust non-treated fasteners and cause wood rot to the home's band joist — never mind creating an environment for mold.
Lack of Standardization Compounding the problem is a lack of code-sanctioned best practice. Flashing ledger boards for decks is covered in section 703.8 of the IRC, but the language is vague: “Approved corrosion-resistive flashing shall be provided in the exterior wall envelope in such a manner as to prevent entry of water into the wall cavity or penetration of water to the building structural framing components.” Lynn Underwood, code official for the City of Norfolk, Va., calls the IRC language a “performance-driven standard rather than a prescriptive standard.” On site, that means the standard comes down to whatever will keep the water out.
Approaches to the Problem Deck builder Pat Nicholson, of Pittsburgh, uses self-adhesive bitumen membrane to form a gasket. “We strip the siding, apply Grace Ice & Water Shield to the sheathing, then lag the ledger to the band joist behind it,” he says. In the past, Nicholson has tried using self-adhesive rubber membrane together with PVC-based Z-flashing to cap the ledger, but he has determined that perforating the PVC-based flashing material with the lags defeats the purpose.
The Engineered Wood Association suggests the following: Remove the siding at the rim joist. Then install a piece of flashing on the house at the bottom of the joist to act as a drip-edge. Flash the band joist itself, then tuck flashing up under the siding that laps over the band joist flashing. Finally, when you install the ledger, pre-caulk the lag holes to seal them, and pack the ledger out on four or five cut-washers, creating a drainage gap behind it.
This writer uses what I call a belt-and-suspenders approach. Strip the siding, then tuck Ice & Water Shield up under the siding and any housewrap to maintain the home's drainage plane. The rubber seals the bolt holes. Instead of using Z-flashing, tuck flexible copper flashing up under the siding and housewrap. Then lay it over the ledger as a drip-edge. I also caulk under any doors where the flashing can't ascend the wall.
The key to success with whatever method you settle on is that it must meet your local code, your production standards, and it must keep out the water. Until the code prescribes a best practice, deck builders are on their own. —Mark Clement is a freelance writer and former contractor based in Ambler, Pa.
