Most decks do not fail because of the weather. They fail because water sat on them through a winter and froze, and nobody noticed until the boards started to cup.

Why Fall Is the Right Time

Cleaning in fall gives the wood time to dry and take a fresh coat of sealer before the first freeze. Do it in spring instead and you are sealing in whatever the winter did to the boards.

The Routine

1. Clear everything off. Furniture, planters, the grill. You will miss half the deck otherwise.
2. Sweep, then wash. A stiff broom followed by a deck cleaner and a soft brush. Do not use a pressure washer on full blast. It furs up the grain and drives water into the seams.
3. Let it dry two or three days. Not one. Two or three, and only if there has been no rain.
4. Seal it. A penetrating sealer, not a film forming paint. Film coatings trap moisture and peel within a season or two.

Check the Fasteners While You Are Down There

Look for popped nails and any soft spots underfoot. A soft board means rot underneath. Better to find that in October than in April when it gives way under someone.

I skipped sealing my first deck for two years because I thought pressure treated wood did not need it. It does. It still dries out, checks, and greys. One weekend of work in the fall buys years.