Planning and building a deck

A deck looks simple, but its cost and its safety are decided early: how big it is, how high it stands off the ground, and what it sits on. The boards are the part you see, yet the footings, the framing and the fasteners decide whether the deck is still level and solid in ten years. This planner takes you through the job in the order you will do it.

Start with the permit and the footings, then the frame, then the surface. Each step points to the calculator that works out the quantity and the guide that explains the choices.

Step 1: Decide the size, the height and the permit

Mark the deck on the ground with string or spray paint and put your furniture inside it. A table for four fits a deck of about 10 × 10 feet; a table, a grill and a seating group want more like 12 × 16. Then talk to your building department before you buy anything. Most places require a permit for an attached deck or one that stands more than a foot or two above the ground, and they set the footing depth, the railing height and the framing you must use. Call 811 a few days before you dig.

Step 2: Square the layout and set the footings

Mark the footing positions and check the layout is square by measuring both diagonals: they must match. Footings go below the local frost depth and rest on undisturbed soil. Their size depends on the loads and on the soil, so use the size on your plan or the one the building department gives you, and let the concrete cure before you load it.

Step 3: Frame the deck

The frame is the ledger (if the deck is attached to the house), the beams and the joists. The joist size and span come from a span table or your plan: they cannot be guessed, and our calculators give counts rather than sizes. Joists at 16 inches on center suit most decking laid straight, and closer spacing is common for diagonal patterns and some composites. The ledger flashing and the bolts or structural screws that fix it to the house are the most important details in the whole job.

Step 4: Order the boards and fasteners

Decking comes in lengths from 8 to 20 feet. Choose lengths so that the butt joints land over joists and the cutoffs are short, and add 5 to 10% for waste on a straight lay, more for diagonals or a border. Use the fasteners the board maker specifies, and with treated lumber only hot-dipped galvanized or stainless steel.

Step 5: Add the railings and stairs

A guard is generally required when the deck surface is more than about 30 inches above the ground. The usual minimum height is 36 inches, and no opening may let a 4 inch sphere pass. Stairs have their own limits on the height of each riser and the depth of each tread, and the stringers need solid fixings at the top and bottom.

Step 6: Finish and protect it

New pressure-treated lumber is wet, and it needs to dry for weeks (sometimes months) before you stain it; sprinkle water on a board and if it soaks in, it is ready. Composite decking needs only cleaning. Stain and sealer are sold by the gallon and cover less on rough or dry wood, so check the label.

Before you start

• The permit, setbacks and inspection schedule from your building department.
• Call 811 several days before you dig the footings.
• The frost depth and the soil, which set the footing depth.
• The decking material (treated lumber, cedar, composite), since it affects the joist spacing and the fasteners.
• A dry place to store the lumber, flat and covered, before it goes in.

Mistakes that cost people money

• Fixing the ledger with nails, or without flashing, instead of the bolts or structural screws and flashing the code requires.
• Footings that are too shallow or too small for the frost depth and the loads.
• Plain screws or bright nails in treated lumber, which corrode and stain the wood.
• Building flat. The surface should fall slightly (about 1/8 inch per foot) away from the house.
• Ordering boards by area alone. The board lengths and the joint positions decide how much goes to waste.

Frequently asked questions