Dry Rot Treatment Calculator — Fungicide, Cut-Back & Replacement Timber

This calculator estimates the materials needed for dry rot eradication based on the visible affected area, wall length, whether masonry treatment is needed and the volume of timber to be replaced.

Cut-back calculation

Infected timber should be cut back to at least 500mm beyond the last visible sign of decay (the figure quoted by major remedial suppliers and PCA-aligned practice). For severe or uncertain cases a more conservative margin of up to 1m is sometimes specified. The calculator uses a worst-case 1m margin on every side, so its quantities are deliberately cautious. It models the affected zone as a compact square, adding the margin around it:

Cut-back area ≈ (√Affected area + 2)²

This square model suits compact patches. A long, linear run of rot along a joist or wall plate (e.g. an 8m × 0.5m strip) should instead be measured along its length plus a margin at each end — the square approximation will under-estimate the treatment zone for such runs. The precise cut-back is a surveyor’s judgement; eliminating the moisture source is the most critical step.

Fungicide spray for timber

The timber that remains within the treatment zone is surface-sprayed with a boron-based fungicide. Manufacturer datasheets (e.g. ACS Boracol 10, Safeguard ProBor) quote roughly 4–6 m² per litre by spray for preventive surface treatment. The calculator defaults to 4 m²/L (adjustable in Advanced Options):

Timber fungicide = Treatment area ÷ coverage (m²/L)

Heavier “apply to refusal” rates of 2–3 L/m² are reserved for deep brush or injection saturation, not routine surface spraying. Always follow the product datasheet.

Masonry irrigation

If the dry rot has affected masonry walls, holes are drilled at 150mm centres in 3 rows and injected with fungicidal solution at approximately 0.5L per hole.

Drill holes = (Wall length ÷ 0.15) × 3 rows

Masonry fungicide = Drill holes × 0.5 litres

Replacement timber

All removed timber must be replaced with pre-treated timber or timber treated in situ with preservative. New timber in contact with or close to masonry should be isolated using DPC or joist hangers to prevent future moisture transfer.

Standards

Treatment should follow the PCA Remedial Timber Treatment Code of Practice, BRE Digest 299 (Dry rot: its recognition and control) and, for heritage work, BS 7913:2013 (Guide to the conservation of historic buildings). Fungicidal products should be HSE/biocide-regulation approved. All work should be carried out by PCA-registered specialists for insurance-backed guarantee eligibility.