How much woodworm treatment spray do I need per m²?

Insecticidal spray for woodworm treatment covers approximately 5 m² per litre per coat. For moderate to heavy infestations, 2 coats are typically required. The total timber surface area is greater than the floor area because joists have multiple faces — for floor joists, multiply your floor area by approximately 1.8 to get the total treatable surface area.

How do I know if woodworm is still active?

Active woodworm produces fresh frass (fine, powdery sawdust) below exit holes, and the holes will have clean, sharp edges with pale-coloured wood visible inside. Old, inactive holes appear dark and dusty. The flight season for common furniture beetle is May to September, so check for fresh holes and frass during these months. You can place paper beneath suspected timber to check for fresh frass over several weeks.

When is structural repair needed for woodworm damage?

Structural repair is needed when woodworm damage has significantly reduced the cross-section of load-bearing timbers such as floor joists or roof rafters. If you can push a screwdriver or penknife easily into the timber, or if the timber sounds hollow when tapped, it may have lost structural integrity. Heavy infestations with dense exit holes covering more than 50% of the timber surface typically indicate the need for structural assessment by a timber specialist.

What types of woodworm are found in UK properties?

The most common is the Common Furniture Beetle (Anobium punctatum), which produces 1-2mm exit holes in softwood. Death Watch Beetle (Xestobium rufovillosum) attacks hardwood, particularly oak, in older buildings with 3mm holes. House Longhorn Beetle (Hylotrupes bajulus) is mainly found in Surrey and the south-east, attacking softwood roof timbers. Wood-boring Weevil (Euophryum confine) attacks already-decayed timber and indicates a damp problem.

Do I need to treat all timbers or just affected areas?

PCA (Property Care Association) guidelines recommend treating all accessible timbers in the affected area, not just those showing visible signs of infestation. Larvae may be present inside timber that shows no external signs yet. For roof spaces and sub-floor voids, all accessible timber surfaces should be treated. This preventative approach ensures complete protection and is the standard practice for insurance-backed guarantees.

Woodworm Treatment Calculator — How Much Treatment Fluid Do I Need?

Calculate insecticidal spray, paste or injection quantities for woodworm-infested timber. Covers joists, floorboards and rafters with PCA-compliant treatment rates.

Floor Area Affected

Total floor area over affected timbers

Timber Type

Type of timber affected by woodworm

Severity

Based on density of exit holes and timber condition

Treatment Type

Spray is standard; paste for localised areas; injection for deep treatment

Price Per 5L (optional)

Enter price per 5L for a cost estimate

How We Calculate This

This calculator estimates the quantity of woodworm treatment product needed based on the floor area affected, the type of timber, severity of infestation and your chosen treatment method.

The formula

Spray: Litres = (Floor Area × Surface Multiplier ÷ 5) × Number of Coats

Paste: Kg = Floor Area × 1 kg/m²

Surface area multipliers

Floor joists: ×1.8 (top, bottom and both sides)
Floorboards: ×1.2 (top and underside)
Roof rafters: ×1.6 (exposed faces)

Coverage rates

Insecticidal spray: ~5 m² per litre per coat (typically 2 coats for moderate infestation)
Insecticidal paste: ~1 kg per m² of directly affected area
Micro-injection: injection points at 150mm centres along timber, ~20ml per point

Severity guidance

Light infestations (scattered holes) typically need 1 coat. Moderate and heavy infestations require 2 coats. Heavy infestations where timber is softened may require structural repair — replacing affected joists, rafters or floorboards after treatment.

Standards

Treatment should follow PCA (Property Care Association) Code of Practice for Remedial Timber Treatment. Products used should be HSE-approved for use against wood-boring insects. For insurance-backed guarantees, work should be carried out by a PCA-registered contractor.

Frequently Asked Questions

Last updated: February 2026

All calculations are estimates. Verify with your supplier.

Woodworm Treatment Calculator — How Much Treatment Fluid Do I Need?

How We Calculate This

The formula

Surface area multipliers

Coverage rates

Severity guidance

Standards

Frequently Asked Questions

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