How do I fix a deck to a house wall?

Attaching a deck to the house solves two problems — it removes a row of posts and it gives you a level threshold at a door — and creates two more: the connection has to carry load reliably for decades, and it puts a horizontal timber against a wall that must stay dry. Both are solvable, and both are routinely got wrong. If any part of this reads as beyond what you want to take on, a free-standing deck with its own posts along that edge is a completely legitimate answer and avoids the whole problem.

Safety

What you'll need

• Ledger timber, same section as the joists, treated
• Resin anchors or structural through-bolts rated for the substrate
• Spacer washers or a proprietary ledger spacer system
• Joist hangers
• Flashing — lead, or a proprietary ledger flashing
• SDS drill and the correct bits
• Cable and pipe detector
• Spirit level and a long straightedge

Step by step

1. Establish what the wall is before anything else.Solid masonry, cavity masonry, timber frame with a brick outer leaf, or an insulated render system all take fixings differently, and one of them — an outer leaf of a cavity wall with nothing behind it — will not take a ledger at all without through-fixing to the inner leaf. If you cannot tell, find out.
2. Find the damp proof course and set your level from it.The finished deck surface must sit at least 150mm — two brick courses — below the damp proof course. This constrains the ledger height, which constrains the whole deck, so it is the first dimension you fix.
3. Scan for services along the whole fixing line.Cables to outside lights and sockets, the gas service, a boiler condensate, waste pipes. Detector along the full length before any drill goes in the wall.
4. Cut the ledger and set it dead level.One piece if possible. Prop it in position, check level along its whole length, and mark the fixing positions. Any error here is repeated in every joist.
5. Hold the ledger off the wall on spacers.Stainless washers or a proprietary spacer, giving roughly 10mm of clear air between ledger and wall at every fixing. Water that gets behind the ledger then drains and dries instead of sitting in a closed joint against the brickwork. A ledger bolted tight to a wall traps water on both faces.
6. Fix into structural material with the right anchor.Resin anchors into solid masonry, or through-bolts to a plate on the inside face where you have access and it is appropriate. Two fixings vertically staggered at each position, at the centres the anchor manufacturer specifies for the load. Not frame fixings, not screws into plugs, not fixings into mortar joints alone.
7. Flash over the top of the ledger.A flashing tucked into a mortar joint or under the cladding above, dressed down over the top edge of the ledger and out over its face, so water running down the wall is thrown clear of the joint rather than into it. This is the detail that decides whether the wall behind stays dry.
8. Hang the joists rather than skew-nailing them.Galvanised or stainless joist hangers, correctly sized for the joist, fixed with the specified hanger nails or screws — not ordinary screws, which are not rated for shear in a hanger.
9. Build the fall away from the house into the frame.About 1 in 100 running away from the wall, set by the joist heights or by packing at the outer bearer. Never by sloping the boards.
10. Keep the deck surface clear of any airbrick or vent.Boarding over an airbrick blocks underfloor ventilation and rots the floor joists inside the house. If a vent falls within the deck, a periscope vent or an open grille in the decking maintains the air path.

Tips

• A free-standing deck with a post row along the house wall avoids every one of these problems and is what many builders now do by preference. It costs a row of posts and pads.
• If the wall is timber frame with a brick outer leaf, the ledger has to reach the timber frame behind the cavity — never the brick alone. That is a job for someone who can confirm what is in the wall.
• Buy the anchors before you drill. The hole diameter, depth and edge distance are set by the anchor, not by what you have in the box.
• Take a photograph of the flashing and the spacers before the boards go on. Nobody will ever see this detail again and it is what the deck depends on.

Common mistakes

• Fixing the ledger through render or cladding — The fixing is only as good as the material it grips. Render, insulation and cladding have no useful pull-out strength and the ledger is effectively hanging on the render's bond to the wall.
• Bolting the ledger tight to the wall — It creates a permanently damp closed joint against the masonry. The ledger rots, the wall behind stays wet and both problems are invisible until the deck fails.
• Setting the deck level with the door threshold — It nearly always puts the deck above the damp proof course. If a level threshold is essential, the deck has to be built as a properly detailed and drained threshold, which is a different and more demanding piece of construction.
• Using ordinary screws in joist hangers — Hangers are designed around the shear strength of the specified nails or hanger screws. A general woodscrew is brittle in shear and can snap without warning.

If it doesn't work

Questions people ask