How to Attach a Deck to a House
Install a new deck, or reattach an existing one, that is set up to last
Whether you’re installing a brand new deck or shoring up an old one, the trick to attaching a deck that will last for decades is in making sure the wood you use is decay resistant, and the ledger board that carries the supporting floor joists is properly anchored to the house. Using nails instead of bolts is a frighteningly common mistake that can result in a deck collapse that causes serious injuries.
For the deck frame, including the ledger board, rim joists, and posts, make sure you use pressure-treated wood which is designed to withstand rot. The proper use of waterproofing flashing is also integral to the structural soundness of an attached deck. Without flashing, the wood is vulnerable to decay, rendering deck-to-house joints dangerously loose.
Here are the steps for attaching a deck, depending on whether you’re working with a new or old one.
If You’re Installing a New Deck:
- If your new deck will be replacing a set of outdoor stairs, remove the old flashing from behind the ledger board and inspect plywood sheathing for rot. If necessary, patch the sheathing before continuing.
- Cover the sheathing with flexible adhesive-backed flashing to prevent water penetration. Apply a second layer of sheathing over the first as a counterflashing.
- Fasten metal L-brackets to the end of the rim joists with hanger screws. Set the ends of the rim joists against the house and screw the L-brackets into the sheathing. If necessary, create spacers by attaching pieces of decking to the top side of the frame that goes against the house.
- Install the ledger between the rim joists and fasten it to the house with 3-inch decking screws.
- Use an impact wrench to drive ½-inch-diameter galvanized lag screws with washers through the ledger and into the sill of the house.
If You’re Reattaching an Existing Deck:
- Temporarily brace the deck against the house with 2x4 stakes and 2x6 diagonal braces. If necessary, raise the deck to its original height by temporarily installing 2x6 support posts beneath the deck. Tap the posts into place with a sledgehammer.
- Drill a ½-inch-diameter screw-shank clearance hole through the ledger board, but not into the sill plate. Position the hole 3 inches below the upper edge of the ledger and at a slightly upward angle.
- Place a ½-inch x 10-inch galvanized lag screw with washer into the hole, then use an impact wrench to drive the screw into the sill plate. Repeat to install one lag screw in each joist bay.
- If there's a gap between the ledger and the house foundation, use filler pieces of pressure-treated wood to fit snugly into the space. The filler pieces will prevent the ledger from splitting when it’s bolted into the foundation.
- Use a hammer drill fitted with a ½-inch-diameter masonry bit to bore holes through the ledger, in through the filler pieces and into the foundation; then use a hammer to tap ½-inch x 10-inch masonry wedge anchors into the holes.
- Tighten the hex nut on the wedge anchor with the impact wrench. Continue to install masonry anchors, spacing them 36 to 48 inches apart.
- If the existing joist hangers are undersized or corroded, reinforce each one with an engineered-lumber joist hanger. Slip the engineered-lumber joist hanger over the existing hanger and secure it with galvanized hanger nails.