Overview
The deck you look at - the boards and the railing - sits on a structure that does the real work. Underneath the decking is a system: footings in the ground, posts standing on those footings, beams carrying the load across the posts, joists running between the beams, and a ledger board tying the whole thing to your house. That system holds people up off the ground, which makes a deck an engineering and code problem, not a carpentry project. I have walked decks that looked fine on top and were one party away from dropping their occupants into the yard. Two things cause the catastrophic failures that put people in the hospital: the ledger connection to the house and the footings in the ground. Get those wrong and the deck comes down with people standing on it. Get them right and the rest is straightforward. This is also why a permit and an inspection are normal and smart here. An inspector who makes you dig footings deeper or add a few more lag bolts is doing you a favor. The decking - covered in our decking materials comparison - is the easy part. The structure under it is where the stakes live.
Footings
Footings are where the deck meets the dirt, and their whole job is to transfer the load down to soil that can carry it. Two rules govern them. First, they have to bear on stable, undisturbed soil that will not settle. Second, in any climate that freezes, they have to extend below the frost line so frost heave does not lift the deck through a freeze-thaw cycle and rack the structure out of square. Your local code sets that depth, and it is not a number to guess at. Poured concrete piers are the standard: a hole, often a cardboard form tube, sometimes a wider footing pad at the base to spread the load, and a post base bracket cast into the top to anchor the post. For bad soil - fill, clay, anything that will not hold - helical or screw piles get torqued down to load-bearing strata and skip the digging. Precast pier blocks and surface blocks sitting on grade are the exception: they work only for small, ground-level, floating decks that code treats differently, and they do nothing for frost depth. Size the footing to the load it carries and the soil it sits on. This is the base of everything above it, and it is code-driven for a reason.
Posts, Beams, and Joists
Above the footings, the load travels through a chain, and every link has to be sized and connected right. Posts come first: pressure-treated, sitting on the footing through a post base that holds the wood up off the concrete so it does not wick moisture and rot from the bottom. Posts carry the beam. The beam - built up from multiple plies of dimensional lumber or a solid timber - runs across the post tops and carries the joists, and its size comes straight off a span table based on how far it reaches between posts. Joists are the repetitive members the decking screws to. They get sized and spaced by their span and by the decking you are running on top: 16 inches on center is typical, and some decking products want them closer, 12 inches, especially on a diagonal. Joists tie into the ledger and the beam with joist hangers, not toenails. The connections matter as much as the lumber: post-to-beam and beam-to-post connectors, hangers rated for the load, all of it tied together so the chain holds. Use ground-contact-rated pressure-treated lumber and hot-dip-galvanized or stainless hardware, because the chemicals in treated wood eat cheap fasteners. Our pressure-treated lumber guide covers that corrosion issue in depth.
The Ledger: The Connection That Kills
This is the section that matters most. The ledger is the board that bolts the deck to the house, and a failed ledger is the number one cause of catastrophic deck collapses in this country. People die when it lets go. The rules are not subtle. The ledger must be lag-bolted or through-bolted into the house rim joist - the solid framing band behind the wall - never just nailed, and never fastened to siding or sheathing alone, because siding and sheathing have no structural pull-out strength and will tear away under load. The bolts go in at a code-specified spacing and pattern, staggered, sized to the load. And the ledger has to be flashed: metal flashing tucked behind the siding and over the top of the board so water sheds away instead of running down behind it and rotting the rim joist out of sight. Most collapses trace to one of two failures - a ledger that was nailed instead of bolted and pulled away, or one that was never flashed and rotted the framing behind it until it gave. If you have any doubt about the wall you are attaching to, build a free-standing deck on its own posts and skip the house connection entirely. This is life-safety, not a cosmetic detail.
FAQ
Do I really need a permit just to build a deck? Yes, in nearly every jurisdiction, and you want one. A deck is a structure that holds people off the ground, and the inspection catches the things that kill people - an undersized ledger connection, footings that stop above the frost line, missing hangers. The inspector is a free second set of eyes on the two details most likely to put someone in the hospital. Skipping the permit also creates problems when you sell the house.
Can I attach the ledger to my siding or just nail it on? No. The ledger has to be lag-bolted or through-bolted into the house rim joist - the solid framing behind the wall - never to siding or sheathing, and never just nailed. Siding has no structural pull-out strength. A nailed or siding-mounted ledger is the leading cause of deck collapses. If you cannot reach solid framing, build a free-standing deck on its own posts.
How deep do footings need to go? Below your local frost line, on undisturbed bearing soil. The exact depth is set by code and varies by climate - shallow in warm regions, several feet in cold ones. If a footing stops above the frost line, freeze-thaw cycles heave it up and rack the deck out of square over a few winters. Check the number with your building department before you dig.
Why do I need special hardware instead of regular nails and screws? Modern pressure-treated lumber is treated with chemicals that corrode ordinary steel fast. Use hot-dip-galvanized or stainless joist hangers, post bases, and fasteners rated for contact with treated wood. A corroded connector is a hidden structural failure - it looks fine from the deck surface right up until it lets go under load.