How do I install flashing between my deck ledger and house wall in Riverview NB?

Install a combination of self-adhering waterproof membrane behind the ledger and metal Z-flashing above it to create a continuous water management system that directs moisture away from the house wall. This is the single most important detail in any attached deck, and getting it wrong is the leading cause of structural rot at the ledger connection, a problem that is especially common in Riverview given the area's significant rainfall and heavy snow loads.

Start by preparing the wall surface where the ledger will be mounted. If your home has vinyl or aluminum siding, remove the siding in the area where the ledger will sit, plus about 6 inches above and to each side. You need to work directly against the wall sheathing and house wrap. If the house has no house wrap, which is the case with some older homes in the Riverview and Greater Moncton area, apply a layer of self-adhering waterproof membrane directly to the sheathing. Products like Grace Vycor Plus, Blueskin, or similar peel-and-stick membranes work well for this application. The membrane should cover the full area behind the ledger board and extend at least 4 to 6 inches above the top edge of where the ledger will sit. Wrap it down below the ledger position as well so that any moisture that reaches the sheathing behind the ledger has a waterproof barrier preventing it from soaking into the wall framing.

With the membrane in place, position your ledger board and bolt it to the rim joist of the house. Use 1/2-inch diameter hot-dipped galvanized or stainless steel lag screws or through-bolts spaced no more than 16 inches on centre in a staggered high-low pattern. Between the ledger and the membrane-covered sheathing, install stainless steel standoff washers or purpose-made spacers. These create a small gap, typically 1/2 inch, between the back of the ledger and the wall. This gap is critical because it allows any water that gets behind the ledger to drain downward and out rather than sitting trapped against the wood and rotting both the ledger and the rim joist behind it.

After the ledger is bolted in place, install the metal Z-flashing. This is a bent piece of corrosion-resistant metal, either galvanized steel or aluminum, shaped like the letter Z when viewed from the side. The top leg of the Z tucks up behind the house wrap or building paper above the ledger. The middle section bends out over the top edge of the ledger board. The bottom leg kicks outward, directing water away from the face of the ledger and onto the top of the deck joists or decking. Secure the top leg with a few roofing nails or screws through the house wrap, and then lap the siding back down over it so that the siding covers the top leg entirely. Water running down the siding hits the siding, passes behind it to the house wrap, runs down the house wrap, hits the Z-flashing, and gets directed out and away from the ledger. At no point does water get behind the flashing and against unprotected wood.

In Riverview, where the Petitcodiac River valley creates a microclimate with frequent fog, heavy dew, and significant precipitation, this flashing system is not a nicety but a necessity. Without it, moisture infiltration at the ledger connection can compromise the rim joist in as few as 5 to 8 years, leading to a structural repair that runs $2,000 to $5,000 depending on the extent of the damage. Many older decks in the Moncton-Riverview-Dieppe tri-community area were built without proper flashing, and if you are replacing or reattaching a ledger, take the opportunity to do it correctly this time.

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