How Can I Seal the Under Side of a Deck?

Lay plastic sheeting underneath the deck to protect the ground underneath it. Also cover any nearby plants that might be coated in overspray.

Clean the underside of the deck with a commercial deck cleaner and a stiff scrub brush. Follow the deck cleaner's instructions for application methods and amounts. Rinse the surface of the deck with water to remove all traces of the deck cleaner. Allow the underside of the deck to dry for at least 48 hours or until the wood is no longer damp.

Apply the sealer to the underside of the deck with a garden pump-style sprayer. A sprayer allows you to apply a thin, no-drip coat to the underside of the deck. Follow both the sealer's and sprayer's instructions to apply one thin coat to the underside of the deck. Allow the sealer to cure for twice the maximum amount of time listed on the sealer's label. The underside of the deck receives little sunlight and takes longer to dry than the top side of the deck.

Apply a second thin coat of sealer to the underside of the deck.

Remove the plastic sheeting.