Refurbish Deteriorating Deck Lumber

James Dulley
Wed, 25 Mar 2026 21:00:00 -0700
Lifestyle

Dear James: The surface and some of the support lumber is deteriorating on my raised deck. How can I test the lumber integrity and repair the rotten areas? — Anthony D.

Dear Anthony: Even when it is constructed properly using pressure-treated lumber and decking, it still requires regular maintenance and care. The pressure-treated lumber will resist rot and insect infestation, but the effects of the sun, weather, dirt, mold, etc., can deteriorate the wood.

The first step is to clean the deck surface with a pressure washer or some deck cleaner. The actual damage to the wood surface may not be as severe as it first appears. Dark and discolored areas may just be mold or mildew growth, and this can easily be cleaned off. Such a mildew problem may have been accelerated if you ever used a nonsynthetic-based sealer on the deck.
Updated: Thu Mar 26, 2026

Dear James: The surface and some of the support lumber is deteriorating on my raised deck. How can I test the lumber integrity and repair the rotten areas? — Anthony D.

Dear Anthony: Even when it is constructed properly using pressure-treated lumber and decking, it still requires regular maintenance and care. The pressure-treated lumber will resist rot and insect infestation, but the effects of the sun, weather, dirt, mold, etc., can deteriorate the wood.

The first step is to clean the deck surface with a pressure washer or some deck cleaner. The actual damage to the wood surface may not be as severe as it first appears. Dark and discolored areas may just be mold or mildew growth, and this can easily be cleaned off. Such a mildew problem may have been accelerated if you ever used a nonsynthetic-based sealer on the deck.

When you use a pressure washer on a deck, read the instructions carefully for the proper settings. A tremendous amount of pressure is not necessary to accomplish your inspection and cleaning goals. If the pressure is set too high and the nozzle held too close to the decking, the wood surface can be eaten away by the water stream.

With your deck 4 feet above the ground, you can easily get beneath it to check the condition of the joists without having to tear off excessive decking. Use an awl to probe any suspected rotten areas on the joists and the decking. If the joists are rotting and uneven or weak, they may not provide proper support for the decking and may cause premature failure.

If several pieces of the decking seem to be only slightly rotten or have only surface damage, pry them up very carefully with a pry bar. If the underside is in good condition, flip it over and use it in the same spot. Since the underside is not exposed to the sun or a layer of dirt, foot travel, etc., it is often still in good condition.

Also, pry off decking over any locations where the joists appeared to be damaged from your under-deck inspection. With the joists removed, you will be able to do a more thorough inspection of the suspected joists. Don't be afraid to probe aggressively with the awl. If you can damage it with just an awl in your hand, the wood has already suffered rot.

Take a hammer and a wood chisel and remove any of the rotten areas on the joists. Make sure to remove all the bad wood. You will be able to feel when you are getting down to the solid wood. Apply a heavy coating of preservative sealer to the area you just chiseled. Let it dry and apply a second coat.

Cut a reinforcing joist for the same size pressure-treated lumber. Make it long enough to reach to the header or ledger. Nail or screw it against the damaged joist and toenail it into the header or ledger. Reattach the old decking or new decking. Drive the nails or screws into the new reinforcing joists.