Q:

My house has a second-story deck whose joists cantilever through the brick-veneer wall. Whenever it rains a lot, water flows into the house through the cantilever openings in the wall and cascades from the lower-floor ceiling. How can I prevent the flow of water into the house?

J.H. Marshall, Great Falls, VA

A:

Contributing editor Scott McBride replies: First off, if the joists are in bad condition and need replacing, I would consider filling in the openings and lag-bolting a ledger board to the brick wall instead of running new cantilevered joists. The holes for the lag shields (anchors) will be much smaller than the existing pockets and can be sealed under compression as the bolts are tightened. A sealant or, better yet, some type of gasket can be used to make the seal. I would use standoffs between the ledger and the masonry so that water can pass down freely between the ledger and the wall. These standoffs could be pressure-treated wood blocks or metal spacers.

If the cantilevered joists are still sound, here’s a way to flash the job. First, cut shallow kerfs into the top and bottom edges of the joists to prevent water from migrating back toward the house. The top kerf should be pitched to allow for drainage. Then, run a continuous L-shaped base flashing down the wall and over the joists before the decking is applied.

Run the decking across the joists diagonally, and install blocking between joists to support the ends of the deck boards. Diagonal decking takes a little longer to install and requires extra material, but the ends of the diagonal deck boards that abut the house can cantilever over the base flashing without being nailed through the flashing. Running the decking parallel to the house requires nailing through the flashing to attach the innermost course of decking, thereby compromising the flashing’s effectiveness. As an added bonus, diagonal decking will stiffen the deck considerably.

To prevent water from running down behind the base flashing, you’ll need to install counterflashing as found on brick chimneys. You can rake out a mortar joint to receive the counterflashing and then tuck-point the joint with mortar. An easier and neater alternative involves cutting a kerf in the masonry with an abrasive blade. For a neat cut, prop a board on edge against the house along the deck, and let the shoe of the circular saw ride the board like a straightedge. Then, insert the counter-flashing into the kerf and wedge it in place. I pin flashing in kerfs with some little lead plugs. I make the plugs by rolling up 1-in. wide strips of the lead sheet that is used for shower pans.

With the flashing secure, you can seal the joint with high-performance caulk. For added weather resistance, seal around the cantilever openings.