Rotten Timber Repair: How to Fix Rotten Wood Without Replacing It

Rotten timber doesn't have to mean new timber. If the rot is in one area and the rest of the piece is still sound, you can usually cut out the damage, harden what's left and rebuild the missing section with epoxy. You keep the original window frame, sill, door frame or fascia, and you skip the cost and disruption of replacing it.

We've been restoring timber sash windows in Sydney since 2005, and this is how we repair rot. The method below works on window frames, sills, door frames, architraves, weatherboards, fascias and other exterior joinery, using the Evermend timber repair system.

How to Tell if Timber Is Rotten

The quickest check is the screwdriver test. Press the tip of a screwdriver firmly into the timber. Sound wood resists. Rotten wood feels soft or spongy, and the screwdriver sinks in.

Other signs to look for:

• Paint that is peeling, blistering or cracking, especially at joints and the bottom of frames
• Dark, discoloured or crumbling patches
• Timber that breaks away in small cubes or splinters when you scrape it
• Gaps opening up at joints, or putty falling away from window glass
• A sash or door that has started to stick or sag

Check the places where water collects or sits: the bottom of window frames and sills, the base of door frames and thresholds, the lower edges of weatherboards, and the ends of fascia boards.

Should You Repair or Replace Rotten Timber?

Where a section is too far gone, a common approach is to cut it out, splice in a new piece of timber, and use epoxy to bond the joint and shape it to the original profile. If you're not sure which way to go, send us a photo and we'll give you our view. You can also find a local tradesperson through our trusted installer list.

What Causes Timber Rot?

Wood rot is a fungus, and it needs moisture to grow. It starts when water gets into timber and can't dry out. Cracked paint, failed putty, open joints and missing flashing all let water in, and the damage is usually worst wherever water sits. In Australian conditions, sun and heat crack paint faster, which lets the water in sooner.

That's why the last step of any repair, sealing it with primer and paint, matters as much as the repair itself. It's also worth fixing the cause, such as a blocked gutter or a gap that lets rain in, so the rot doesn't come back.

What You'll Need

Materials

• Evermend Epoxy Wood Stabiliser, to harden the soft timber left after you cut out the rot
• An Evermend epoxy filler: a 1 Hour or 4 Hour cartridge, or the 4 Hour Self Mix Pot if you don't have an extrusion gun
• The twin cartridge extrusion gun, if you're using cartridges
• Exterior primer and paint

Tools

• Moisture meter
• Chisel, multi-tool or screwdriver to cut out the rot
• Small brush for the stabiliser
• Putty knife or filling blades, and a mixing board
• Sandpaper or a sanding block
• Gloves and eye protection

Not sure how much filler you need? Our free Evermend Epoxy Calculator works it out from the size of your repair.

How to Repair Rotten Timber, Step by Step

Step 1: Cut out all the rotten timber

Remove every bit of soft, decayed wood until you reach solid timber. Be thorough. Rot left under a repair will keep spreading. It's normal for the hole to end up bigger than you expected.

Step 2: Make sure the timber is dry

Check the exposed timber with a moisture meter. It should read below 18% before you apply anything. If it's higher, cover the area from rain and let it dry out first. Epoxy on damp timber is one of the most common reasons repairs fail.

Step 3: Harden the timber with stabiliser

Mix the Evermend Stabiliser and brush it onto the exposed wood. It soaks into the soft fibres, binds them together and primes the surface so the filler bonds to a sound base. Leave it to soak in for at least 20 minutes, then wipe off the excess.

Step 4: Fill and shape with epoxy

Mix the filler until the colour is even all the way through. Press a small amount in first so no air is trapped, then build up the repair a little proud of the surface and shape it to the original profile. The non-sag formula holds its shape on vertical surfaces and overhead, so frames and fascias are no harder than sills.

On corners and edges, clamp clear acrylic offcuts either side of the repair as a former. The epoxy cures to their shape, and you can see through them to check the fill as you go.

Step 5: Sand it flush

Once the filler has cured, sand it flush with the surrounding timber. You can also drill and screw into it, so hinges, catches and window hardware can go straight back on.

Step 6: Prime and paint

Apply an exterior primer, then at least two coats of paint over the repair and the timber around it. This seals out water and is what keeps the repair, and the rest of the timber, sound for years to come.

Repairing a sill? Our rotten window sill guide goes through each step for sills in more detail.

1 Hour or 4 Hour: Which Filler Should You Use?

Doing your first repair? The Pot & Stabiliser Bundle has everything you need with no gun to buy. If you have a few repairs to do, or work in the trade, see the best ways to buy Evermend for bundles and bulk packs.

Common Mistakes to Avoid

• Leaving rot behind. If any soft timber remains under the repair, the rot keeps spreading.
• Working on damp timber. Always check it reads below 18% first.
• Skipping the stabiliser. Filler bonds best to a hard, primed surface, not soft fibres.
• Mixing too much at once. The 1 Hour gives you about 5 minutes of working time, so mix small amounts.
• Working in the heat of the day. In summer, work in the shade or early morning, or use the 4 Hour.
• Leaving the repair unpainted. Prime and paint it to keep water out of the timber around it.