A Guide To Correctly Attaching a New Deck to A House

Out here on California’s Central Coast, it’s rare to find a house without a deck. If you don’t see one in the front of the house, it’s probably in the back, and many of them are attached to the house. That means that builders like my former employer, Are Dann, retired owner of Dann Construction, must be adept at connecting the deck to the building.

It is possible to make a freestanding deck that abuts the building and to run decking boards right up the siding, but that’s something you only need to do if the siding is faux brick, faux or real stone or some other material that doesn’t lend itself to making the connection. Most houses in Dann’s service area have wood, fiber cement or vinyl siding, though, which he says are easier to work with. “You can cut back the siding far enough to allow you to fasten a ledger board to the sheathing and to the building’s rim joist,” says Dann. “Then install flashing, repair the siding and get on with building the deck.”

Dann and I have worked together on several deck projects. Using the techniques I learned while working with him, I’ve built a few on my own and with the help of other workers. Ahead, you’ll find our pointers for attaching a deck to a house the right way.

Preparing a House for a New Deck

In some cases, you might be replacing an old deck with a new one, and if so, it’s important to remove the old ledger board when you demolish the deck. After removing the old deck, you can determine if you can still use the house rim to support the new ledger board.

If the wall sheathing behind the rim looks good, you’re okay. But if the sheathing is rotten, it’s important to investigate further by removing the rotted areas and checking the house’s rim joist and wall framing to see if they’re rotten, too. Dann cautions not to be too eager to rip apart and replace moist or discolored wood because it may still be intact below the surface. Jam a screwdriver into the wood in several places, and if the screwdriver penetrates more than 1/4 in. or so, it’s replacement time. That can be a huge job, so you may want an experienced carpenter to help walk you through that process.

If the sheathing is intact, or there is no existing deck to worry about, you only need to map out the deck’s perimeter on the ground and mark the positions of the postholes for the deck supports. Before digging, don’t forget to call 811 to get a map of underground utilities on your property you don’t want to place a post directly over a sewer or cut through a pipe or wire when digging. Dann also recommends laying a plastic weed barrier, which also helps improve drainage.

How Decks Connect to Homes

Installing a ledger board is the most secure way to connect a deck to a house. That way, you’ll be using the solid structure of the house to support your deck. The ledger has the same dimensions as the rest of the deck joists, and it serves as a rim joist to support the joists running perpendicular to the wall.

Like the joists, the ledger should be made from pressure-treated wood to prevent rot. “A rotted ledger board is a disaster,” says Dann. “It compromises the stability of the entire deck, the rot often spreads to the building’s structure, and it’s difficult to replace without demolishing a significant portion of the deck.”

Attaching a Deck Ledger

Rather than attaching the ledger board to the siding, Dann typically cuts the siding back so he can attach the ledger to the sheathing. After cutting the ledger to the proper length, he attaches it with long fasteners that will go all the way through the sheathing and penetrate the building’s band joist.

“Don’t use nails,” he cautions. “They’ll pull out.” The old-school fastening method uses 1/2-in. galvanized lag screws, but most builders these days opt for 1/4-in. structural screws they can drive with an impact driver. Despite the smaller diameter, they are stronger and speed up the project. Local building codes usually regulate how many screws you need and where you should place them, so do your research before you start the project.

Proper Deck Height

The ledger board’s height determines the deck’s height, and to prevent water pooling and ice build-up, the deck surface should be an inch or two below the thresholds of the doors that open onto the deck. To determine the height of the ledger, add the thickness of the decking to this gap. For example, if you set the deck two inches below the door thresholds and you use 1 1/2-in. decking, place the ledger board 3-1/2 inches below the thresholds.

The height of the ledger board determines the lengths of the deck posts, which is why Dann always sets the ledger before cutting the posts. Once the ledger is in place, it’s easy to attach a line to the top, stretch it out to each post, level it with a line level and make marks. All he has to do is lower these marks by the width of the beams the posts support, and he can mark the cut lines on the posts.

Why Flashing is Important

Even if you leave a gap between the top of the deck and the door thresholds, water will still run off the ends of the decking and seep down along the siding. You must install flashing to prevent the ledger and the siding from rotting.

Dann installs metal Z-flashing (drip cap) over the entire length of the top of the ledger and tucks the vertical portion under the siding. When working with horizontal cladding, he removes cladding boards to expose the sheathing and installs new cladding directly over the flashing. He then custom-cuts and fits another strip of flashing to put over this drip cap and under doors to cover and protect remaining unflashed areas under door sills. Finally, he caulks the gap between the flashing and the bottom of door thresholds.

It’s good practice to also 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. To keep water from leaking behind the ends of the ledger, Dann leaves a 1/8-in. gap between the end of the ledger and the ends of the siding and fills it with high-quality exterior caulk.

FAQs

Do I need a permit to build a deck attached to my house?

Yes. Most decks require permits. The International Residential Code provides an exemption for a deck built less than 30 inches above the surrounding grade and with a total area of less than 200 square feet, but not if the deck is attached to the house. Be sure to obtain this permit before you start, or you might have to tear it down after completing all the work.

Will adding a deck increase the resale value of my home?

Generally, yes. The amount varies, but it’s generally in the neighborhood of 60 to 75 percent of the cost of building the deck. The actual bump in your home’s resale value depends on a number of factors, including the materials you use and the design of the deck. Climate is also a factor: A deck adds more value if the climate is warm enough for people to spend time outside using it.