Seasonal Maintenance for Wooden Jetties and Decking

Start With a Thorough Autumn Inspection

The best window for waterside timber work in the UK is late summer through mid-autumn. The wood has had a full season to dry out, daytime temperatures are still high enough for treatments to cure properly, and you are working ahead of the first real gales. A jetty or deck that has shrugged off a damp summer can still be opened up by a single November storm, so treat this as a job worth doing properly rather than a quick tidy-up.

Walk the whole structure slowly, ideally at low water, so you can see the piles, bearers and the underside of the deck rather than just the surface you stand on. Work from the shore outwards and be systematic. Look for:

• Soft or spongy patches in deck boards, especially where boards meet bearers and around fixings.
• Dark, damp staining, blackening or a musty smell — all classic early signs of fungal decay.
• End grain, cut ends and drilled holes, which soak up water faster than any other part of a board.
• Green algae, moss and slippery biofilm, which hold moisture against the timber and become a genuine trip hazard.
• Pile tops, the waterline and the splash zone, where constant wetting and drying does the most damage.
• Movement, springiness underfoot, or a step that was solid last year and now is not.

Deal With Rot Before It Spreads

Decay needs two things: moisture and still air. Most rot on jetties starts where leaf litter, silt and debris collect — between boards, behind handrail stanchions, where a bearer meets a pile, or wherever timber sits permanently damp. Clearing those traps is as important as any treatment you apply afterwards.

Test suspect timber with a sharp bradawl or thin screwdriver. If the point sinks in more than a few millimetres with little resistance, you have found soft wood. Cut back to sound timber, then either splice in a new section or replace the board or bearer entirely. For small, isolated pockets, drilling out the affected area and filling with a suitable epoxy repair system, combined with a boron-based preservative rod inserted into the timber, can buy several more years of service. Anything structural — a pile, a main bearer, a load-bearing joist — should be replaced rather than patched. Never simply paint or oil over rot; you will hide the problem and trap moisture underneath.

Check and Tighten Every Fixing

Fixings fail quietly. Winter wind uplift and wave action work screws and nails loose over a season, and a deck that looks sound can lift a board in a storm.

• Use marine-grade stainless steel (A4/316) for anything in salt spray. Standard galvanised fixings corrode surprisingly quickly in coastal air.
• Avoid mixing metals in the same joint, as dissimilar metals accelerate galvanic corrosion.
• Re-drive loose screws, or replace them with a slightly larger gauge. Where a hole has worn oval, drill it out, glue in a hardwood dowel, then re-fix into fresh timber.
• Check through-bolts, coach bolts, washers and backing plates, and re-torque where they have slackened.
• Inspect gangway hinges, handrail stanchions, cleats, brackets and joist hangers — these take the most load and are often the first to go.

Seal countersunk heads and any freshly drilled holes with end-grain preservative as you go.

Clean the Timber Properly Before You Treat It

Treatment will not penetrate dirty, algae-covered timber, and it will not last on a damp surface. Scrub along the grain with a stiff brush, then use a low-pressure washer if you have one — keeping the nozzle moving and working with the grain. Too much pressure frays the fibres, opens the grain and actually makes the deck absorb more water next winter. A suitable deck cleaner or oxygen-based product will lift algae and grey surface grime; follow the label, rinse thoroughly and then let the timber dry for at least 48 hours of dry weather before you apply anything else.

Choose the Right Treatment for the Job

For most waterside decking, a penetrating oil-based treatment is the better choice. Film-forming varnishes look smart for a season and then crack, letting water in behind the coating. Look for products formulated for exterior marine use, with water-repellent and UV protection built in. Two thin coats, applied in dry conditions above about 10°C with no rain forecast for 24 to 48 hours, will outperform one thick coat every time.

Pay particular attention to cut ends, notches and drilled holes, which need end-grain preservative even on naturally durable species such as oak, iroko or ekki. Older, grainier boards will drink more product than new ones, so budget accordingly and expect to re-oil annually for the first few years.

Prepare for Storms and Check Again in Spring

Before the worst weather arrives, secure anything that could become a missile: loose boards, fenders, planters, covers and furniture. Check mooring cleats and rings, and add sacrificial rubbing strips where tenders and fenders chafe against the timber. Keep a simple written log with a few photographs each year so you can spot gradual changes rather than reacting to surprises.

After a heavy blow, walk the structure again and note any lifted boards, displaced fixings or new movement. Then, once spring arrives and the timber has been through another cycle of wetting and drying, repeat the fixing check and re-oil where the surface has dulled. That rhythm — inspect, repair, clean, treat — is what keeps waterside timber safe, handsome and serviceable for decades rather than years. Do check whether your local authority or landlord requires consent for anything beyond routine maintenance, particularly on tidal foreshores.