How far should timber be cut back for dry rot?

All timber must be removed at least 1 metre (1000mm) beyond the last visible sign of dry rot (Serpula lacrymans) infection. This includes apparently sound timber, as the fungal hyphae can extend well beyond visible mycelium and fruiting bodies. This is a PCA (Property Care Association) standard requirement and the most critical step in dry rot eradication.

Why does masonry need treating for dry rot?

Dry rot is unique among wood-decay fungi because its mycelium (root-like strands) can grow through and across masonry, brick and plaster to reach new timber food sources. The fungus can travel several metres through walls. Masonry irrigation involves drilling holes at 150mm centres and injecting fungicidal solution under pressure to kill fungal growth within the wall. Without masonry treatment, dry rot can re-emerge on the other side of a wall.

How much fungicide spray do I need for dry rot?

For timber treatment, the standard application rate is 5 litres per 3 m² of timber surface (approximately 1.67 L/m²). This heavy application rate ensures deep penetration into the timber. The fungicide (typically a boron-based solution) is sprayed liberally onto all remaining timber surfaces within the treatment zone. For masonry irrigation, each injection point receives approximately 0.5 litres of solution.

What is the difference between dry rot and wet rot?

Dry rot (Serpula lacrymans) is far more serious than wet rot. Dry rot can spread through masonry, produces distinctive red-brown fruiting bodies and white/grey mycelium, and requires only 20-30% moisture content to thrive. Wet rot (Coniophora puteana and others) only grows on wet timber, does not spread through masonry and is treated by removing the moisture source and replacing damaged timber. Dry rot requires specialist treatment and is a notifiable condition in property surveys.

What standards cover dry rot treatment in the UK?

The PCA (Property Care Association) publishes the Code of Practice for the Identification and Remedial Treatment of Dry Rot, which is the industry standard. BS 7913:2013 (Guide to the conservation of historic buildings) also covers dry rot in the context of heritage properties. All dry rot treatment should be carried out by PCA-registered contractors who can issue insurance-backed guarantees, typically for 20-30 years.

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

Calculate dry rot treatment quantities including 1m cut-back distance, fungicide spray for timber, masonry irrigation and replacement timber estimates. Based on PCA guidelines.

Affected Area (visible rot)

Total area of visible dry rot on timber

Wall Length Affected

Length of masonry wall behind affected timber

Masonry wall behind affected area

If yes, masonry irrigation with fungicide is included

Timber Replacement Needed

Area of timber to be cut out and replaced (within cut-back zone)

Fungicide Price Per Litre (optional)

Enter fungicide price per litre for cost estimate

How We Calculate This

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

All timber within the affected zone must be removed to at least 1m beyond the last visible sign of infection. The calculator adds a 1m margin on all sides of the affected area to determine the total treatment zone.

Cut-back area ≈ (√Affected area + 2)²

Fungicide spray for timber

Remaining timber surfaces within the treatment zone are sprayed with fungicide at a rate of 5 litres per 3 m² of timber surface.

Timber fungicide = (Treatment area ÷ 3) × 5 litres

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 PCA Code of Practice for Dry Rot and BS 7913:2013 (Guide to the conservation of historic buildings). Fungicidal products should be HSE-approved. All work should be carried out by PCA-registered specialists for insurance-backed guarantee eligibility.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Verify with your supplier.