How often should you seal a deck?

Short answer

Clean once a year, reseal every 1 to 3 years. A wood deck needs the yearly wash to strip the gray film and mildew, and a coat of stain or sealer every 1 to 3 years to keep water out of the boards. A composite deck gets the yearly cleaning and skips the sealing. Cleaning and sealing together take 240 to 480 minutes spread over a couple of days, and the cleaner and sealer cost $50 to $150. Hiring it out runs $500 to $1,500.

Why it matters

Wood outdoors is in a losing battle with water. Rain soaks into the boards, the sun dries them, and the cycle opens the grain, lifts the fibers into splinters, and turns the surface gray. Water that sits in the open grain feeds mildew and rot, and rot starts at the ends of boards and in the joints where it can't be seen. A sealer keeps the water on the surface so the boards stay tight.

The structure underneath matters more than the surface. The joists and the ledger board that holds the deck to the house rot the same way, slowly and out of sight, and a deck that fails does so with people on it. The yearly cleaning is when you see the loose board, the popped nail and the soft spot, while they're a repair and not a rebuild of something that costs $4,000 to $15,000 to replace.

When to do it more often

The index gives one modifier, and it removes a job rather than adding one: composite decking is cleaned once a year and doesn't need sealing, because it isn't wood. Everything else is about where the deck lands in the 1 to 3 year sealing range:

Full sun all day: the short end. Sun breaks down sealer faster than anything else.
Heavy foot traffic, or furniture dragged across it: the short end.
A covered deck in shade: the long end for sealing, but watch for mildew, which likes shade.

What you need

A deck brush on a long handle, for scrubbing the cleaner in
A pressure washer on a wide tip, or a garden hose, for rinsing
A roller or a pump sprayer, for applying the sealer
A screwdriver, to tighten loose screws and reset popped nails
Deck cleaner, a wood cleaner or brightener that lifts the gray and kills mildew
Stain or sealer, with a water test to decide whether it's needed this year

Steps at a glance

Clear the deck, sweep it, and tighten every loose screw and nail you find.
Wet the deck and the plants around it, apply the cleaner, and scrub it in with the brush.
Rinse with the hose or the washer on a wide tip, following the grain and keeping the pressure low enough not to fuzz the wood.
Let the deck dry for a couple of days, then sprinkle water on a few boards: if it beads, the old sealer is still working and you can stop here.
If it soaks in, apply stain or sealer in a thin coat with the roller or sprayer, a few boards at a time, back-brushing to even it out.
Keep everyone off the deck until the sealer has cured as its label says.

You're done when water beads on every part of the deck and no board is loose, soft or splintered.

Signs it's overdue

Water soaks into the boards instead of beading on top.
Boards that have gone gray.
Splinters, especially on the boards in full sun.
Loose boards, or nails that have popped up above the surface.

DIY or pro

This is a moderate job that's mostly patience: cleaning one day, drying for a couple more, and sealing on a dry, mild day after that. The mistakes to avoid are pressure washing too hard, which tears up the surface, and sealing a deck that isn't fully dry, which traps water under the coat and makes it peel.

A deck company charges $500 to $1,500 to clean and seal, depending on the size, and that's worth it for a large or multi-level deck, for one with railings and stairs that take as long as the floor, or for a deck that's been let go and needs stripping before it takes a new coat. If the cleaning turns up a soft joist or a ledger board pulling away from the house, that's a repair to get a professional opinion on before anyone uses the deck.

Safety

Chemicals: deck cleaners and brighteners are harsh, and oil-based sealers give off fumes. Wear gloves and eye protection, wet the plants before and after, and keep pets off the wet deck. Rags soaked in oil-based sealer can heat up and catch fire on their own, so spread them flat to dry outdoors before throwing them away.
Structural: a deck with a rotten joist, a soft ledger board or loose post connections can collapse under a crowd. If the cleaning turns up soft wood in the framing, keep people off until it's been looked at.

FAQ

How do I know if my deck needs to be sealed?

Sprinkle water on it. If the water beads up, the sealer is still working. If it soaks in and darkens the wood, it's time.

Should you pressure wash a deck before sealing?

Clean it, yes. A pressure washer on a wide tip at low pressure is fine, but too much pressure tears up the wood fibers. A deck cleaner and a brush, rinsed with a hose, work too.

How long should a deck dry before sealing?

A couple of dry days after cleaning, until the wood is dry through. Sealer over damp wood peels.

Does a composite deck need sealing?

No. Clean it once a year and leave it at that. Sealer has nothing to soak into and leaves a film.