Introduction

With proper flashing, a wood deck will last many years; without it, the deck and the ledger it's attached to will quickly rot. Detailed photos in this article show the difference.

Rotted deck? Here’s the right way to install the new one

After you demolish the old deck but before you start work on the new one, you’ll need to make sure the wood on and in the house is still in good shape. Water leaking around an old deck ledger can cause significant damage to the house framing. The existing house rim and the lower support walls need to be solid enough to support the multi-ton weight of a deck filled with people. After the old deck is removed, you’ll be able to tell if you can still use the house rim. If the wall sheathing behind the rim looks good, you’re okay. But if the sheathing is rotten, investigate further by removing the rotted areas, and check the house’s rim joist and wall framing to see if they’re rotten too. Don’t be too eager to rip apart and replace moist or discolored wood; it may still be intact below the surface. Jam a screwdriver into the wood in several places. If the screwdriver penetrates more than 1/4 in. or so, it’s replacement time.

Replacing rotted-out rim joists and lower wooden support walls can be a huge job. You may want to have an experienced carpenter on hand to help walk you through that gauntlet. To properly attach a deck ledger, follow these guidelines:

1. Drop the level of the new ledger enough to allow at least a 2-in. gap between the bottom of any doors and the top of the deck boards. That way, water and snow melt won’t be able to accumulate and seep under doorsills to ruin interior floors, and storm doors will swing past ice, snow and leaf deposits.
2. Install metal flashing (drip cap) over the entire length of the top of the ledger. Custom cut and fit another strip of flashing over this drip cap and under doors to cover and protect remaining unflashed areas under doorsills. Caulk the gap between the flashing and the bottom of door thresholds. Install Z-flashing behind the bottom of the ledger and over the siding below. This keeps water from wicking along the bottom of the ledger and getting behind the siding. Leave a 1/8-in. gap between the end of the ledger and the ends of siding and fill it with high-quality exterior caulk to keep water from leaking behind the ends of the ledger.
3. Anchor deck joists to the ledger with joist hangers, filling each hole with galvanized joist hanger nails rated for pressure-treated wood.
4. Predrill and sink 1/2-in. dia., galvanized or stainless steel lag screws in every other joist space, alternating up and down positions to keep the ledger from splitting. Select lag screws specifically rated for treated wood, in lengths that will penetrate the ledger, exterior sheathing and the 1-1/2 in. thickness of the house rim joist (4 in. is usually the right length).

Use only treated wood intended for soil contact for all the structural members of the deck, including the ledger, joists, beams and posts.