Wet rot is fungal decay of timber caused by prolonged exposure to moisture (typically above 50% moisture content). Unlike dry rot, wet rot does not spread through masonry — it stays localised to the wet timber. Common species include Coniophora puteana (cellar fungus). It makes timber soft, dark and crumbly.

How far should I cut back beyond the decay?

Cut back at least 300mm (12 inches) beyond the last visible sign of decay to ensure all affected timber is removed. The fungal hyphae extend beyond the visible damage. Sound timber at the cut point should be firm, light-coloured and show no discolouration or softness.

What is timber hardener used for?

Timber hardener is a liquid consolidant that penetrates into soft, partially decayed wood and hardens it. It is brushed or poured onto timber that is too soft to hold fixings but not decayed enough to remove. It binds the fibres together and provides a base for epoxy filler. Coverage is approximately 5m² per litre.

When should I use epoxy filler vs replace the timber?

Epoxy filler repair is suitable for localised decay — window sills, frame corners, decorative mouldings. If more than 30% of the timber cross-section is decayed, or if the member is structural (joist, rafter, beam), the decayed section should be cut out and replaced with new preservative-treated timber.

How do I prevent wet rot returning?

The key is eliminating the moisture source. Fix leaking gutters, pipes, or flashings. Improve ventilation to sub-floor voids. Apply 2 coats of wood preservative to all exposed timber within 1.5m of the affected area. Ensure DPC and cavity trays are functioning. Maintain paint/stain on external joinery.

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Wet Rot Treatment Calculator — Timber Hardener, Filler & Preservative

Calculate treatment quantities for wet rot: cut-back 300mm beyond decay, timber hardener litres, epoxy filler kg and preservative coverage.

Affected Length

Length of visibly decayed timber

Timber Width

Width of the timber section

Timber Depth

Depth of the timber section

Surrounding Area to Treat

Area for preservative (leave 0 for auto)

Treatment Price Per Litre (optional)

Average cost per litre of treatments

How We Calculate This

This calculator estimates the treatment materials needed for wet rot repair based on the extent of decay.

The formula

Cut-Back Length = Affected Length + 300mm each end

Hardener = Surface Area / Coverage (5 m²/L)

Epoxy Filler = Decayed Volume × 1500 kg/m³

Preservative = Treatment Area × 0.2 L/m²

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.