A Weather-Proof Deck

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If central portions or ends of a few floor joists are badly deteriorated, it is easiest to set in new beams and hangers beside the damaged ones. Or, you can jack the beam, cut out the rotten center span, or cut off damaged beam ends and pull the hanger, then add "sisters"—lengths of new wood at one or both sides of the original beam. Fasten new ends to the ledger with new hangers.

It's best to sandwich the original beam between a pair of sisters, connecting them with through-bolts (carriage bolts with square heads that lodge in the wood), washers, and nuts.

If the front rim joist (the frame member that supports the front end of the decking and holds deck-front joist hangers) is shot, you may be able to sister a new length of PT outside of it and fasten new hangers between the old ones by running long bolts through thickness of both old and new wood.

If one end or another of a post is damaged, or if a knot has admitted significant rot, it is easiest to use floor jacks to support the deck and replace the entire post on all-new fittings. Prying off old nailed-on fittings takes a hammer and pry bar with a chisel end, plus a strong arm and patience. It'll make you wish they'd invented removable deck screws 25 years ago.

The most severe problem I've encountered is where the ledger board (the rear frame member) is rotted out—usually because the deck-to-house joint was not constructed so it excluded water. If major portions of the ledger are wet and soft, you can be fairly sure the decay extends into the house foundation—threatening to compromise the board (the sill or bottom plate) that supports the walls and roof. A rotten sill calls for major reconstruction that's beyond the competence of most amateurs. Don't put it off; once a sill begins to go, the whole house wall will begin to rot from the bottom up.

I wouldn't trust a deck with very many bad joists or post ends or a rotten ledger. I would pull the whole thing, saving what good wood I could, and rebuild, weatherproofing correctly and fastening with removable deck screws.

Good Joints

Nails are inadequate to hold a deck frame together. Fasten (or refasten—as above) posts to platform beams with 1/2" or 3/8 lag screws or carriage bolts. Similarly, fasten or refasten ledger boards to the house with lags. Use lag screws if the sill is a stout timber. If the deck butts up to less substantial siding or frame boards that you can get to from inside the house, tack the ledger on with nails, drill holes through ledger and house, and fasten with carriage bolts. Put washers behind the inside nuts. In thin walls use metal backing plates behind the nuts—or center-drill foot-squares of 1/2" plywood, butt them up to the inside surface of the wall, and fasten through the drill hole.

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