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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.
Length of visibly decayed timber
Width of the timber section
Depth of the timber section
Area for preservative (leave 0 for auto)
Average cost per litre of treatments
Each end, beyond visible decay (300–500mm typical)
m² per litre (Ronseal Wet Rot Hardener ≈ 10)
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How We Calculate This
This calculator estimates the materials for wet rot repair from the extent of decay. It gives quantities for two alternative repair paths — cut-out-and-replace, or consolidate-in-place (hardener + epoxy) — so you can compare; choose one per section.
The formula
Cut-Back Length = Affected Length + cut-back distance each end (300mm default; 300–500mm typical)
Hardener = Affected-zone Surface Area ÷ Coverage — coverage defaults to 10 m²/L (Ronseal Wet Rot Wood Hardener); only the retained soft-decay length is consolidated, not the replaced wood.
Epoxy Filler = Decayed Volume × 1200 kg/m³ — decayed volume assumes ~30% of the affected cross-section is voided and rebuilt; 1200 kg/m³ (≈1.2 kg/L) is a typical filled epoxy wood-repair compound density.
Preservative = Treatment Area × 0.2 L/m² — 0.2 L/m² (≈5 m²/L) is a liberal 2-coat rate for rough/decayed timber (Ronseal Total Wood Preserver covers up to 5 m²/L over 2 coats). When no area is entered, the tool auto-estimates the cut-back run's surface area then doubles it to allow for surrounding sound timber; enter a measured area to override.
Frequently Asked Questions
Wet rot is fungal decay of timber that needs persistent free water in the wood. Decay needs moisture content above the fibre saturation point (~28–30%), with active growth optimal around 40–80% MC; timber kept below ~20% is not at risk and ~22% is the practical decay-onset threshold (not 50%). Unlike dry rot, wet rot does not spread through dry masonry — it stays localised to the wet timber. Common species include Coniophora puteana (cellar fungus). It makes timber soft, dark and crumbly.
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.
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 roughly 10m² per litre on solid timber (Ronseal Wet Rot Wood Hardener) — soft, absorbent decayed wood drinks more, so allow extra. Hardener and epoxy are for the consolidate-in-place repair path, not the timber you cut out and replace.
These are alternative repairs for a given section, not both at once. Epoxy filler (with hardener) suits localised, non-structural decay — window sills, frame corners, decorative mouldings — where sound wood remains around the void. If more than ~30% of the cross-section is decayed, or the member is structural (joist, rafter, beam), cut the decayed section out and replace it with new preservative-treated timber instead. This calculator gives quantities for both options so you can compare; pick one per section.
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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Last updated: March 2026
Verified against UK standards · estimates only, confirm with your supplier.