How to Build a 9' by 12' Deck

Check the building code in your town before you begin. If you plan to attach the deck to your house or build it more than 30 inches off the ground, you'll generally need a building permit. Obtain a permit from the building code officials in your community or plan a low deck as a workaround.

Measure off the 9-foot-by-12-foot area for your deck using a measuring tape. Sink wooden stakes at each corner to mark the perimeter.

Remove the sod at the corners and at the midway point along each side, using a shovel. Tamp down the earth to compact it, then spread a thin layer of gravel on the soil. Nestle a concrete block at each corner and the midpoint, just on top of the gravel. If your yard slopes, bury the bricks at varying heights to produce a level base of concrete. Fill in the exposed soil around the brick with gravel. Check that the bricks are level before you continue, using a spirit level.

Lay out your two 12-foot beams on the blocks; they will be 9 feet apart. Screw temporary planks of wood, called stretchers, onto the ends of the beams to create a grid that forms the template for your 9-foot by 12-foot deck.

Mark the 12-foot beans every 11 to every 16 inches. As you build, you'll create a joist or support beam along these points so the deck doesn't sag over time. The smaller measurement gives more support, but the wood can bear joists very 16 inches if you need to save money on a few support boards.

Drill an angle bracket into each joist, which will hold the joists when you attach them. Use an electric drill and screws to attach the brackets.

Set a joist against an angle bracket, starting with the two ends. Screw the joist to the angle bracket. Let the joist run over the side for now; you'll trim the board when you're almost done. Attach the center joist next. Fill in the middle joists last, screwing each one to the angle brackets.

Reinforce the center of the deck by screwing wooden blocks down the center of your deck, which is 4 1/2 feet from each end. Set the blocking just 1/2 inch to the left or right of the center line, switching from left side to right side with each block. This creates a zigzagging support block down the center of the deck base.

Add decking to the top of the deck, using deck screws or hidden fasteners. Deck screws let you add the planks quickly but create visible holes. Hidden fasteners increase the cost and time of the deck venture, since they take longer to apply. Beginning on one end, add your first plank. Work across the deck until the surface is covered.

Trim the boards to the 9-foot length once they're attached. Also trim the ends of the joists to the requisite 9 feet.

Add decking to the bottom to hide the joists, using one board for each of the four sides. Attach it with deck screws or hidden fasteners.