A deck is where pressure washers do their most visible damage, and the damage is permanent: too much pressure raises the wood grain and leaves a washboard pattern that only sanding removes. The way to avoid it is to let the cleaner do the work and treat the machine as a rinse tool.
Timber or composite?
Timber has two problems. The gray is oxidized surface fiber; the black and green is mildew and algae. A deck cleaner addresses both. Keep pressure low and the fan wide.
Composite does not weather gray the way timber does, but it spots and develops a surface film, and it damages in ways pressure washing makes permanent — a fuzzed surface that then holds dirt worse than before. Most composite manufacturers publish a maximum pressure in their care guidance. Find yours and treat it as the limit. A brush and a composite-safe cleaner is the safer route.
The pressure rule for wood
Use the white 40-degree tip, hold a consistent distance of a foot or more, keep it moving, and work along the boards.
Those four together prevent nearly all deck damage. The washboard pattern comes from working across the grain or from hesitating in one spot, and once it is there sanding is the only fix.
A surface cleaner at modest pressure is actually safer than a wand on decking, because it spreads the force across a wide disc and you physically cannot hold it in one place.
Cleaner, brightener or stripper
A cleaner removes growth and surface gray. This is what almost everyone needs.
A brightener (usually oxalic acid) restores weathered timber color by reversing tannin darkening. A second step after cleaning, and what makes a gray deck look genuinely new.
A stripper removes a failed existing coating. Only if you are going to bare wood.
If a stain or sealer is going on
Two things decide whether it takes. A clean rinse — cleaner residue in the grain blocks absorption, which is why a colorless, unscented cleaner that rinses completely is worth choosing. And drying time.
Two dry days for softwood is a reasonable rule, longer for dense timber or after heavy rain. A cheap moisture meter removes the guesswork, and the full sequence is in pressure washing before staining a deck.
Things to check before you start
Loose nails and screws. Pressure lifts them further and they catch the wand.
Soft or rotten boards. Pressure will go straight through them. Replace first.
What is stored underneath. Run-off goes through the gaps onto it.
Whether a railing is painted. Painted wood lifts far more readily than bare timber. Hand-wash it, or use a brush attachment.
The machines suited to this are in best pressure washers for decks and siding.
