How to repair wet rot in timber

Video by Ronseal. This guide draws on the "How To Repair Damaged Wood | DIY Tips from Ronseal" video from the official Ronseal UK channel. Ronseal is a Sheffield-based brand whose wet rot wood hardener and wood filler are the go-to products for this repair in most UK homes. Their video demonstrates the technique clearly and is worth watching alongside this guide.

1. Confirm it is wet rot and not dry rot

Wet rot and dry rot are different problems. Wet rot stays local to where the moisture is — the wood is soft and dark, often with a slightly spongy feel when you press it. Dry rot spreads aggressively through masonry and timber and requires specialist treatment.

If you see white or grey strands spreading beyond the rotted timber onto brickwork or plaster, stop and call a damp specialist. That is dry rot and it needs a different approach entirely. Everything below assumes you are dealing with straightforward wet rot in an isolated area of timber.

2. Find and fix the source of moisture

Wet rot cannot survive without a persistent source of damp. Common culprits are failed paintwork that lets water in, blocked or broken gutters directing water onto the frame, a cracked mastic joint between the frame and wall, or a missing drip groove on the underside of a window cill.

Fix whatever is causing the moisture first. Repairing the rot without addressing the source is pointless — the rot will return before the paint on the repair has even dried properly.

3. Let the timber dry out

You cannot treat or fill wet timber effectively. Strip off the paint, open up the area, and let it dry. In summer this might take a few days. In a damp British autumn it could take a couple of weeks. If you are in a hurry, a heat gun on a low setting can speed things up, but do not rush this step.

While it dries, probe around with a screwdriver. Push firmly into the wood. Anywhere the blade sinks easily is soft and needs to come out.

4. Cut and gouge out all soft material

Remove every bit of soft, spongy, or discoloured wood. A sharp chisel and a stiff wire brush do this well. You want to get back to sound timber — if the screwdriver does not sink in, the wood is solid enough to build on.

Do not be tempted to leave soft patches because they are inconvenient to get to. Filler over rotten wood does not bond and will just fall out, or worse, trap moisture and make the rot worse underneath.

5. Apply wet rot wood hardener

Wet rot hardener — Ronseal, Cuprinol, and similar UK brands all make one — soaks into the remaining soft fibres and consolidates them. Brush it generously onto all the prepared surfaces, getting it into any cracks and end grain.

Apply at least two coats, letting the first soak in before adding the second. The hardener is thin and absorbs quickly. You are not painting it on — you are saturating the timber. Let it cure fully before filling. That typically means 24 hours, though the label will confirm.

6. Fill with a two-part wood filler

Once the hardener has cured, fill the repair with an exterior-grade two-part wood filler. Mix the filler and hardener according to the manufacturer’s ratio — too little hardener and it will not cure properly, too much and it sets before you have shaped it.

Work the filler firmly into the cavity, building it up in layers if the repair is deep. Overfill slightly so you have material to sand back. Two-part fillers are hard when set, so it is worth getting close to the right shape before it cures rather than chasing it with sandpaper afterwards.

7. Sand, prime, and paint

Once cured, sand the filler flush with the surrounding timber. Start with 80-grit to shape it and finish with 120-grit for a smooth surface that paint will key to.

Apply a coat of exterior primer, then at least two topcoats of good quality exterior paint. Pay particular attention to the edges and end grain — that is where paint fails first. A well-painted repair should be invisible from a normal viewing distance.

When to call a handyman

Call Richard if the rot is in a structural element such as a lintel or main post, if the area is larger than you can comfortably deal with, or if you suspect dry rot rather than wet rot. Wet rot repairs look straightforward but doing them badly — leaving soft spots, skipping the hardener, painting over damp timber — just defers the problem a year or two.