Step 1
Measure the length of the rafters underneath your deck and the overall width of the deck. Include the width of the rafters at the end of each side of the deck in your measurements. Divide the length and width of the space beneath the deck into sections that are shorter than the length and width of your plywood panels.
Step 2
Cut the length and width of your panels according to the measurements you took to create identically sized panels that will cover the space below the deck without overlapping each other. Measure the distance between the rafters underneath your deck and mark each wood panel with parallel lines that match the spacing of your deck rafters. If your deck posts are inside the perimeter of your outer deck rafters, measure the width and depth of the deck's corner posts. Select two plywood boards and cut a square section out of one corner of the board that matches the measurements of the corner posts.
Step 3
Install narrow wooden firring strips over the narrow bottom end of the deck joists at the end of the deck furthest from your home. Attach the firring strips using wood glue so that successively thinner strips run down the length of each rafter. This will create a raised flat surface that gradually slopes up towards the original level of the rafters as it nears the exterior wall of your home. Sand the ends of the firring strips so that each strip seamlessly matches the shims on each side of it.
Step 4
Place the first section of plywood paneling against the joists and firring strips beneath the deck at one corner of the deck with the cut out section flush against the deck post. Use deck screws to mount the paneling to the rafters along the lines you marked. Add a deck screw every foot to ensure that the paneling stays flush against the underside of your deck.
Step 5
Apply a light bead of caulking to the side of the first panel that will rest adjacent to another panel. Mount the next panel so that its sides are flush with the first panel you installed, with the caulk forming a seal between them. Scrape off any caulk that pushes out from between the panels when they are pressed together. Drive deck screws through the plywood and into the rafters above to secure the second panel. Repeat this process until the underside of the deck is completely covered with plywood sheathing.