When installing a deck over a high open space, undermount deck brackets provide the most secure support. Use galvanized fasteners throughout to help your deck withstand years of changeable weather without rotting, splitting or sinking. Follow these steps to install your deck with undermount deck brackets.
Step 1: When to Use Undermount Deck Brackets
Undermount deck brackets work best on high decks that adjoin a staircase. You can then stand beneath the deck to attach surface planks to the brackets.
Step 2: Install the Deck Joists
Measure, frame and install the undersurface deck joists for your deck at a distance of 12, 16, or 24 inches from the centers, depending on the total square footage of your deck. Use galvanized screws at corners and galvanized nails along the frame to attach the joists. Check each joist with the carpenter's level as you fasten it to ensure it is level.
Step 3: Purchase the Correct Number of Undermount Brackets
The galvanized aluminum undermount brackets prevent direct wood-to-wood contact around fasteners, which can cause your deck joists and surface planks to rot. The brackets are 22-1/2 inches long, so measure the deck joists once installed to calculate how many you will need. For joists with 12 inch centers, you will use 100 brackets for every 150 square feet of deck. For 16 inch centered joists, 100 brackets will support up to 250 square feet, and for 24 inch joist spacing, 100 brackets will be enough for 350 square feet of deck surface.
Step 4: Attach the Undermount Brackets
Attach the undermount brackets on alternating sides of each deck joist. Fasten them with galvanized 2-inch screws in every other hole of the bracket. For joists around the perimeter of your deck, attach the brackets in line on the inner edge of the perimeter joist only. Use tin snips or a power band saw to cut brackets to fit as needed.
Step 5: Fasten the Surface Deck Planks
When you fasten the surface deck planks to the undermount brackets, you will drill the screws up into the planks from below. As you walk on the deck surface, pressure from above will help the screws remain fastened tightly into the wood and the bracket. Fasten deck planks to the brackets 1/8 inch apart for water drainage. Lay them so they span the entire width of the deck at 90 degrees to the undermount brackets and joists. If you have to abut 2 planks, align them snugly. Stagger the plank ends as needed to avoid creating a continuous seam where planks are joined.
Step 6: Attach the Last Surface Plank
Where space permits under the deck, attach the last surface plank in the same way as all the others. If there is no room under the deck, use longer galvanized screws to go all the way through the plank into the undermount bracket. Countersink the screw heads for a neat surface finish.