How a Deck Works

A typical rectangular deck is not a complicated structure. It’s simply a reinforced box that rests on a few vertical supports, is attached to the house on one side, and is covered on top with boards. Its purpose is to provide a flat living space outside the house. For a long time, a patio made from brick or stone fit the bill. But as the price of traditional patios went up, so did the popularity of wood decks.

A foundation is a stable surface, usually starting below ground level, that supports a structure. In states with generally warm climates, such as most of Florida and Texas, a foundation can be installed above ground. In the rest of the country, winter frost penetrates the ground and makes the soil, as well as the foundation components in it, move.

To create a stable base for a deck, the foundation has to begin below the frost line, the point of maximum frost penetration in your area. This can be as deep as 6 feet in northern Minnesota or as shallow as 6" in most of Alabama. Your local building department will stipulate the code-enforced frost line where you live.

To get below this point requires excavation. In areas with deep frost lines, this is a job best left to an auger mounted on the back of a tractor or the front of a Bobcat. Shallow holes can be dug by hand using a posthole digger, especially if the soil is mostly sand.

You can get from the bottom of these holes to the bottom of your deck in two ways: by installing either pressure-treated wood posts or solid concrete footings and piers. The concrete approach is better, but wood posts are much easier to handle. Again, your local building department will stipulate which type you should use.

A good foundation will keep a deck from moving up and down, but it won’t necessarily keep it from moving sideways. To prevent this, the deck should be attached to the house with a ledger board.

Ledger boards are usually the same width as the deck joists—2 × 8s or 2 × 10s. The boards are bolted to the first-floor rim joists. To prevent water leaks, aluminum flashing is installed behind the siding and over the top of these boards, and galvanized joist hangers are nailed to the outside face to receive the ends of the deck joists. Once these joists are nailed to the hangers on one end and the beams on the other, the deck structure can’t move.

Posts and beams work together to create an efficient system for adequately supporting joists:

Without the beams, a post would have to be installed under each joist.

Beams are generally made of three joists that are glued and nailed together in lengths that match the center-to-center span between the posts. Once the beams are installed, the top of each should be level with all other beams and with the bottom edge of the ledger board. This way, when the joists rest in the hangers and on the beams, they will be level.

When the beams are constructed, look down the top of both edges to see which side has a crown, a convex curve along the length of the board. Then place each beam so the crown points up. Once the joists and decking boards are in place, their weight will push the beams down and flatten them. If you place the crowned edge down, this weight will create a sag in the deck.

Joists both tie together the beams and the ledger and support the deck boards to form a solid, flat surface for comfortable living. Joists are usually spaced 16" apart, from the center of one to the center of the next. The joists are kept upright at one side of the deck by the joist hangers on the ledger, and on the other side of the deck with the rim joists that go around the deck perimeter. If two rows of joists meet over a beam in the middle of the deck, the ends of each row are nailed together so both stay upright.

As with beams, look down both edges of each joist to find out which one is crowned. The joists should be installed so the crowned edges point up. When the deck settles over time, it will tend to flatten out, not sag.

Deck boards span the joists and create a floor for people, furniture, plants, and pets. The deck boards should be flat and free of obvious splinters, cracks, and loose knots. If any of these defects are present, they should be cut off. Only sound board sections should be used.

You can install deck boards in several ways, the easiest and cheapest being to nail them to the joists with rust- resistant, hot-dipped galvanized nails. A step up is to use Phillips-head galvanized deck screws, which hold better than nails but take longer to drive. The best—but most expensive—option is to use stainless steel screws, either with a Phillips or square-drive head.