Cleaning and Maintaining Your Deck: Costs and Techniques

Timber and composite decking are exposed to rain, UV light, algae, and leaf debris year-round, so even a well-built deck will deteriorate without regular attention. The question of how and when to clean typically arises each spring, when homeowners notice green algae or greyish weathering that accumulated over winter. Left untreated, slippery surfaces become a safety hazard and the underlying structure — joists, posts, fixings — can begin to rot well before the decking boards themselves show obvious wear.

Key points

• Timber decking should be cleaned and re-treated at least once a year; composite decking needs cleaning but generally does not require oiling or sealing.
• Pressure washing above 100–130 bar can raise the grain on softwood boards, causing fibres to lift and accelerating future decay.
• A biocidal treatment containing benzalkonium chloride or sodium hypochlorite kills algae and moss more effectively than pressure washing alone.
• Decking boards that are visibly splintering, splitting along the grain, or showing dark rot patches may be beyond cleaning — replacing individual boards is often more cost-effective than whole-deck renewal.
• Decks positioned beneath a roof overhang or flat-roof gutter line are especially prone to organic growth from water run-off; addressing roof drainage can reduce the frequency of deck cleaning needed.

How often should you clean your decking?

Most timber decking benefits from a full clean and re-oil or re-stain once a year, ideally in spring after frost risk has passed and before the main outdoor season. If the deck is heavily shaded, north-facing, or beneath overhanging trees, a mid-season algaecide treatment is also worthwhile.

Composite decking manufacturers typically recommend twice-yearly cleaning — a light brush-down in spring and a more thorough wash in autumn — but oiling or sealing is usually neither required nor recommended. Always check the manufacturer's maintenance guidance: some composite products void their warranty if treated with oil-based products not formulated for composites.

A simple test for timber decking: sprinkle a small amount of water on the surface. If it beads, the existing sealant is still active. If it soaks in within a few seconds, the boards need re-oiling or re-staining.

Cleaning methods compared

Composite decking caution: many composite boards have a capped surface that can be damaged by harsh acids, bleach-heavy cleaners, or stiff wire brushes. Use only products specified as safe for composite decking, and avoid pressure washing above approximately 100 bar (1,450 psi).

How much does professional deck cleaning cost?

Indicative UK costs, last reviewed 2026-06-01. Quotes vary by region, deck condition, access, and whether treatment is included.

These figures are indicative. Heavily soiled decks, awkward access, or additional structural repairs will affect the final price. Always obtain at least two to three quotes.

What to ask before accepting a quote

• Is pressure washing, biocidal treatment, and sanding all included, or are they quoted separately?
• What oil, stain, or sealant will be used, and is it compatible with my decking material?
• How many coats of treatment will be applied?
• Will you move garden furniture, or should I clear the deck before your visit?
• Is VAT included in the price?
• What happens if rotten boards or structural defects are discovered during cleaning?

Treating and sealing timber decking after cleaning

Cleaning alone does not protect timber from future weathering. Once the deck is clean and fully dry (at least 48 hours after washing in dry weather), apply a suitable treatment:

• Deck oil penetrates the wood and is the most common choice for hardwoods and many softwoods. Re-apply every 12–18 months.
• Deck stain (opaque or semi-transparent) provides colour, UV protection, and water resistance. It is harder to re-coat than oil, as the old layer must be stripped before re-application.
• Decking paint gives an opaque finish with the longest service life between applications, but requires full stripping before re-coating and can peel if moisture is trapped beneath.

Apply in temperatures between 10–25°C and avoid direct midday sun, which causes oil to dry at the surface before it has fully penetrated into the wood.

Annual maintenance checklist

• Clear debris (leaves, twigs, soil) from between boards using a stiff brush or flat-bladed tool
  
• Check and tighten any loose screws; replace rusty or raised fixings before they cause trips
  
• Inspect joists and posts through any accessible gaps; look for dark staining, softness, or fungal growth
  
• Run the water-bead test to check whether the sealant is still active
  
• Clean the surface using the appropriate method for your decking type
  
• Apply oil, stain, or treatment once the deck is fully dry
  
• Check the deck for level and for any movement that might indicate structural failure below
  

Red flags: when cleaning is not enough

Stop and seek professional advice if you notice any of the following:

• Boards that feel soft, springy, or spongy underfoot — this may indicate joist or bearer rot beneath the surface
• Visible fungal growth (white, orange, or black mycelium) on the underside of boards or on joists
• Boards lifting at the edges or warping significantly despite previous treatment
• Fixings pushing up through the board surface, which can indicate structural swelling or movement
• The deck has not been treated in five or more years and shows deep surface cracking and pervasive grey weathering throughout

When to get professional help

Routine annual cleaning is within reach for most homeowners with the right equipment and care around pressure settings. Professional help is worthwhile when:

• The deck is large (over 30–40 m²) and full cleaning and treatment would take more than a day
• Several years of neglect mean the deck needs sanding and a full treatment programme to restore it
• You suspect rot in the joists, posts, or ledger board — a carpenter or builder should assess the structure before any cosmetic work begins
• The decking is composite and the manufacturer specifies professional maintenance to preserve the warranty

If your deck sits beneath a flat roof or parapet gutter, ensure the roof drainage is functioning correctly before the next outdoor season. Overflowing gutters are a common cause of accelerated algae growth on decking below. A vetted roofer to inspect guttering and flat-roof details can clear blockages, check drip-edge condition, and confirm that water is being directed clear of the deck surface.

How Housey can help

Housey can match you with vetted local tradespeople when your deck needs more than a DIY clean and re-oil. Where organic growth on your deck is linked to poor roof drainage above — such as a blocked gutter or a failing flat-roof drip edge — our roofer to inspect guttering and flat-roof details can combine a roof inspection with practical advice on protecting the deck surface below.

Frequently asked questions

Can I use a standard pressure washer on composite decking?

Yes, with care. Keep pressure below 100 bar (approximately 1,400 psi) and use a fan-tip nozzle rather than a pencil jet. Hold the nozzle at least 30 cm from the surface and work along the board direction. High pressure on composite can strip the capped surface layer, which cannot be repaired and typically voids the manufacturer's warranty.

How long does a professional deck clean take?

For a typical 20–30 m² garden deck, professional cleaning and treatment usually takes half a day to a full day. If sanding is included, allow a full day or longer. DIY cleaning often spans a weekend once you factor in the drying time between washing and re-oiling.

Does deck oil need to dry before it rains?

Most deck oils need at least 24–48 hours of dry weather after application. Check the specific product datasheet. Applying oil before rain can wash the product off before it has penetrated, and may stain adjacent paving or gravel.

Is it worth cleaning an old deck, or should I replace it?

If boards are structurally sound but weathered and grey, cleaning and re-oiling can make a significant difference. If the joists, bearers, or more than around 25% of boards are rotten, replacement is usually more cost-effective in the long run. A carpenter or builder can give an honest structural assessment before you commit to cleaning costs.

Sources and further reading

• Timber Decking Association: guidance on installation and maintenance — Timber Decking Association
• TRADA: technical guidance on timber in construction — TRADA (Timber Research and Development Association)
• HSE: working safely with wood dust — Health and Safety Executive