Deck cleaning, without shredding the boards

What is deck cleaning?

Deck cleaning removes the algae, mildew, ground-in dirt and greying film that build up on a wood or composite deck, usually as preparation for sealing or staining rather than as an end in itself. The defining constraint is that timber is soft and fibrous: enough force to lift dirt off the surface also tears the wood fibres beneath it. Competent deck cleaning is therefore mostly chemistry, gentle agitation and rinsing, with pressure used sparingly if at all.

Signs you need deck cleaning

• The boards are slippery underfoot after rain, particularly in the shaded run along a wall or under a tree canopy
• There is a dark green or grey-black film that dulls the whole surface rather than sitting in patches
• Water no longer beads on the boards but soaks straight in and darkens them
• You can see a distinct clean rectangle where a planter, a grill or a furniture leg has been sitting
• Moss or dark growth is packed into the gaps between boards and around the fasteners
• You are booked in for a stain or a sealer and the installer has said the deck must be cleaned and dry first

Seeing something not on this list? Browse pressure washing problems by symptom →

How deck cleaning actually works

1. 1

   Establish what the deck is made of, and how sound it is

   Pressure-treated pine, cedar, a tropical hardwood and a composite board all behave differently under both water and chemistry. Before anything is applied the crew should also be looking for loose boards, proud fasteners, soft spots at the joist line and rot at the posts.

   Watch forA composite deck treated as if it were timber. Composite carries its own manufacturer care instructions, and the wrong cleaner or too much force can permanently mark the cap layer. Hand those instructions over if you still have them.

2. 2

   Clear the deck and protect what surrounds it

   Furniture, grills, planters and rugs come off entirely rather than being shuffled around, because a half-cleared deck cleans in patches. Planting beneath and beside the deck is soaked first, and anything growing through the structure gets particular attention because runoff concentrates there.

   Watch forPlanters left in place "to save time". You will see their outlines in the finished surface for the life of the boards.

3. 3

   Apply the cleaner and let it lift the growth

   A deck cleaner is applied across the whole surface and given time to work on the biological film and embedded dirt. Where the growth is heavy, a stiff brush on a pole does the mechanical work that a pressure washer would otherwise do - with far less consequence for the fibre.

   Watch forCleaner allowed to dry on the boards, and spot-treating one bad area only. Both leave the deck two-tone once dry, and on wood a patchy clean shows up dramatically the moment stain goes on.

4. 4

   Rinse with the grain, at low pressure and constant distance

   Where pressure is used at all on timber it is a wide fan held well back and moved continuously along the length of the board, never held still and never worked across the grain. On composite it is gentler still. The rinse runs until the water coming off the boards is clear.

   Watch forA narrow tip, a close hold, or an operator stopping mid-board. Each gouges softwood in about a second, and the marks are in the timber permanently.

5. 5

   Let it dry fully, then decide about coating

   Wood needs to dry through, not just look dry, before a sealer or stain will bond. Between cleaning and coating is also when you find out whether the boards have raised grain that needs sanding back.

   Watch forA same-visit seal upsell. Sealing over a damp board traps moisture under the coating, and the coating fails early and blotchily - which then has to be stripped before you can try again.

What does deck cleaning cost?

Deck cleaning is priced by deck area after the boards are seen, with steps, railings, spindles and lattice adding hand-work rather than square footage. Heavy growth, stripping an old failed coating, and any sanding are separate items. The Cost Guide sets out how these stack.

See the full deck cleaning cost guide - by material, size and region →

If the number is the problem, here are the ways people pay for it →

Exterior cleaning tools → Measure the deck field, steps and railings before you take quotes.