How Would I Build a Support Under a Deck?

Mark out the area designed for the deck by setting stakes and string around the perimeter of the deck area. Measure the distances to ensure the deck size will fit within the requirements of the deck joists. Hammer stakes into the ground and attach the string from stake to stake. The distance a deck joist can span without additional support depends on the type of wood used and how far apart the joists are set from each other.

Designate the areas for deck supports based on the timber used, the distance between joists and the span distance. For instance, a 12-by-12 deck built 16 inches on center can use 2-by-8-inch joists made from pine or Douglas fir, with the support at the outside edges of the deck. For additional support, add a deck support halfway between the end of the joist and the wall where the joist attaches to the house.

Mark planed deck support locations with a stake.

Dig down below the frost level with a posthole digger to create footings at the position marked. At the bottom of the hole add gravel for drainage. Insert an 8-inch cardboard footing form, available from any home improvement store, into the hole so that the footing will sit above the ground the required distance to meet building codes. This is to ensure the wood is the correct distance above the ground to avoid termite infestation.

Mix up the concrete needed by adding water to the concrete in a wheelbarrow and mixing with a hoe until it reaches the correct consistency as per the manufacturer's instructions on the bag. Add the concrete to the footing form and tamp it down. Pierce the concrete periodically with a rod to remove the air as you add the concrete. For extra strength, add rebar to the concrete footing at a length shorter than the footing. Alternatively, dig all the footings and have them ready to accept concrete from a concrete truck delivery. This can save time on mixing the concrete for the footings. Some companies have smaller concrete delivery trucks for these types of jobs.

Add a post anchor or bracket to the concrete once it starts setting. This will secure the bracket to the concrete and allow the timber support to be attached to the footing with bolts.

Remove the cardboard footing after the footing has completely set. Cut through the cardboard footing with a utility knife and rip the form from the footing.

Set the support timber in the bracket and mark the location of the bolt's holes through the bracket onto the wood from both sides of the bracket. Remove the wood.

Drill the bolt holes with the appropriately sized drill bit all the way through the support timber at the marked locations.

Set the timber support back in place, insert the bolts through each of the bracket holes, through the timber and attach the nut on the other side. Use the wrench to hold the bolt head still and the socket and ratchet to turn the nut onto the bolt. Tighten until the nut is snug against the bracket and timber.

Repeat this procedure for each deck footing and support.