Timber Resin Repair Guide

Unlike standard wood fillers, which often shrink and fall out, specialist epoxy resins bond permanently with the wood fibres. This creates a flexible, waterproof seal that moves with the timber, preventing future rot and significantly extending the life of your windows, doors, and decorative mouldings.

What Does the Work Involve?

• Inspection and Probing: The tradesperson identifies the full extent of the rot, which often goes deeper than it appears on the surface.
• Removal of Decay: All soft, decayed wood is routed out until "bright" (healthy) timber is reached.
• Stabilisation: A liquid timber stabiliser or primer is applied to the healthy wood to ensure the resin bonds deeply into the grain.
• Resin Application: Two-part epoxy resin is mixed and moulded into the cavity, often using shuttering for larger sections.
• Curing and Shaping: Once hardened, the resin is sanded flush to the original profile of the wood, making the repair invisible.
• Priming and Painting: The repaired area is sealed with high-quality exterior paint to match the rest of the joinery.

Typical Costs

Prices are influenced by the depth of the rot, the height of the repair (scaffolding requirements), and the complexity of the timber mouldings. While the initial cost is higher than standard filler, it is usually 60-70% cheaper than a full joinery replacement.

How Long Does It Take?

• Minor repairs: 3–5 hours (can often be completed in a single visit).
• Extensive window restoration: 1–2 days, allowing for resin curing times and multi-stage sanding.
• Full house of sash windows: 5–10 days depending on the number of units and condition.
• Weather impact: Heavy rain can delay work, though many resins can be applied in damp conditions if the area is sheltered.

DIY or Professional?

While basic wood filler is a common DIY task, structural timber resin repair requires specialist equipment and training. The resins used are chemical-heavy and have specific mixing ratios and "open times" that can be difficult for a novice to manage.

Professional application is essential for listed buildings or structural elements like joist ends and main door frames to ensure the repair lasts decades rather than months.

Choosing the Right Tradesperson

• Check Certifications: Look for decorators or joiners trained by brands like Repair Care (Window Care Systems) or Timbabuild.
• Portfolio: Ask for "before and after" photos of previous rot repairs, specifically looking at the finish of the sanded resin.
• Red Flags: Avoid anyone who suggests using "car body filler" (P38) or standard "two-part wood filler" for deep exterior rot, as these will fail within two seasons.
• Questions to Ask:
  • Do you remove all the rot or just cover it?
  • What brand of resin system do you use?
  • How long do you wait for the resin to cure before painting?
  • Do you offer a guarantee on the repair?

UK Regulations

• Listed Building Consent: If your home is Grade I or II listed, resin repairs are often preferred by Conservation Officers over replacement, but you must still check if consent is needed.
• Part L Building Regs: Generally do not apply to repairs, but if you replace more than 25% of a window, you may need to consider thermal upgrades.
• Health and Safety: Professionals should provide COSHH data sheets for the resins used and ensure proper PPE is worn during the routing of old lead-based paints.

Common Problems

• Trapped Moisture: If the timber isn't dry enough (usually below 18% moisture content), the resin may trap dampness inside, leading to further rot behind the repair.
• Poor Bonding: Failing to use the specific system primer means the resin will eventually pull away from the wood.
• Incorrect Mixing: If the two-part epoxy isn't mixed perfectly, it will remain tacky and never fully harden.
• Inadequate Sanding: Visible "humps" where the resin meets the wood can ruin the aesthetic of a period window.

Frequently Asked Questions

How long does a timber resin repair last?

When applied correctly by a specialist, resin repairs are designed to last as long as the surrounding healthy wood—often 10 to 20 years or more, provided the paintwork is maintained.

Can you paint over the resin immediately?

No. Most resins require a curing period of anywhere from 4 to 16 hours depending on the product and ambient temperature before they can be sanded and painted.

Is resin repair better than splicing in new wood?

Resin is often superior for complex shapes or where rot is localized. Splicing (Dutchman repairs) is excellent for straight sections but creates joints that can eventually let in water; resin creates a seamless, monolithic bond.

Can it be used on structural beams?

Yes, there are specific high-strength structural resins and "timber ties" designed for repairing the ends of floor joists or roof rafters that have suffered from damp.

What is the difference between resin and standard wood filler?

Standard fillers are usually polyester-based; they are brittle and don't bond well to wood fibres. Timber resins are epoxy-based, meaning they are flexible, stronger than the wood itself, and do not shrink.