How to Build a Deck Substructure

Determine the number of piers needed. For every 4 feet of substructure width and depth you'll need one pier at that corner. For example, an 8-by-8-foot deck is divided into four, 4-feet square units. You will have nine piers arranged in a grid at each corner of the squares, which equals three piers wide by three piers long. An 8-by-12-foot deck would have six 4-square feet squares, which would require three more piers in a 3-by-4 grid.

Level the ground under each individual pier instead of leveling the entire area. When the piers are set at the planned grid points, drive a stake at each of the four outside corners. Attach a string to all four stakes at a height that is 3 inches lower than you want the top of the deck to be and going around the perimeter of the piers. Adjust the string so that it is level around the perimeter. Adjust the height of the outside piers so that they are level with the string. If you add or take away soil under the pier ensure that it is properly tamped down so that it will not shift later.

Place one of the cross-beams in the piers running left to right to check the level of the center piers. Adjust the level of the piers as necessary so that each pier is level with all others.

Place one cross-beam on the top of each row of piers so that the cross-beam is approximately 1 or 2 inches to the outside of the pier. These will look like the rungs on a unfinished ladder without side rails when you are complete.

Attach the side rails to each end of the cross-beam on both sides of the deck with deck nails or screws. The combination of the cross-beams and the concrete piers every 4 feet gives the deck a sturdy substructure on which you can mount a top surface of plywood or planks.