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Attaching a New Deck to a House: The Correct Method

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.

By the DIY experts of The Family Handyman Magazine

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

Bad construction details

Bad construction details

A Poorly Built Deck

Poor construction details and materials result in a deck that rots quickly and can damage the structure of the house.

Good construction details

Good construction details

A Deck That'll Last

Good flashing details and the proper materials insure that a new deck will last for decades.

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Required Tools for this Project

Have the necessary tools for this DIY project lined up before you start—you’ll save time and frustration.

    • Hammer
    • Miter saw
    • Cordless drill
    • Circular saw
    • Socket/ratchet set
    • Caulk gun
    • Level
    • Drill bit set

Required Materials for this Project

Avoid last-minute shopping trips by having all your materials ready ahead of time. Here's a list.

    • Pressure-treated wood
    • Galvanized lag screws
    • Galvanized nails and screws
    • Drip cap
    • Z-flashing
    • Galvanized flashing
    • Galvanized joist hangers
    • High-quality exterior caulk

Comments from DIY Community Members

Share what's on your mind and see what other DIYers are thinking about.

1 - 6 of 6 comments
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June 12, 5:38 PM [GMT -5]

Like the article pointed out, I had water entering behind the siding and destroyed plywood sheathing (once wet, doesn't last). Water was entering on top of threshold, then draining to the sides under the door casing and behind the siding.

Repaired the sheathing. Ripped the old deck down. Removed the ledger and flashing and replaced with siding.

New freestanding deck, built by some pro's, is awesome. No regrets.

January 28, 3:15 PM [GMT -5]

I think when they said "Lag Screws" After looking at the Picture 'reeeeallly Close" they are actually Lag Bolts. I believe there are some building codes require them to be spaced at various intervals along the lengths of the Ledger. Depending on the Heigth (width) of the 2 x ? it may be required to put 2 lag bolts instead of just one. The really BIG deal is to ensure it gets through to the House Main Joist system.

As for the Floatintg decks some bldg codes require them to be attached to the house if they are with in so many feet/inches. Come to think of it some Insurance companies may not insure a deck unless it is connected to the hose.
Bottom line is to check with LOCAL Building code and just don't do the minimums.

November 17, 5:11 PM [GMT -5]

The picture shows alumunum flashing touching PT lumber but my understanding is that this causes corrosion because of copper in the lumber and should not be used. If someone has a different method of flashing, please submit. I have used plastic ledgers instead of the shown metal in the picture but the picture here still shows an aluminum drip cap right on the lumber. Any addtional advice would be appreciated.

April 27, 7:47 PM [GMT -5]

Why attach to house at all?

I simply build free-standing decks with about an 1 1/2" gap and eliminate a number of potential problems.

November 14, 3:28 PM [GMT -5]

I'm in the planning process for a new deck. From what I understand the new code in most parts of the country require "bolting" the ledger board to the house instead of using lag screws. The requirement is bolts with washers and nuts to secure the ledger board.

May 25, 9:25 PM [GMT -5]

A few months ago I built a deck using Maine Brackets. Not only are these aluminum "I" beams cool to look at, but when installed correctly they eliminate the need for flashing and will never trap water. It is another option.

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