How much timber preservative do I need per m²?

For brush application, wood preservative covers approximately 6 m² per litre per coat. Spray application is more efficient at around 8 m² per litre, but some product is lost to overspray. Most manufacturers recommend 2-3 coats for full protection. Always check the specific product data sheet as coverage can vary between brands.

How many coats of timber treatment do I need?

For general preservative treatment, 2 coats are standard for interior timbers and 3 coats for exterior or high-risk areas. Fire retardant treatments typically require 2-3 coats to achieve the required fire rating. Wood hardener is usually applied in a single coat, flooding the timber until saturated, but badly degraded timber may need 2 coats.

What is the difference between preservative, fire retardant and wood hardener?

Wood preservative protects against fungal decay and insect attack using biocides — it is the standard treatment for structural timbers. Fire retardant treatment contains intumescent chemicals that swell when heated to form a char barrier, slowing fire spread. Wood hardener penetrates and consolidates soft, partially decayed timber by binding fibres together with a resin.

Is brush or spray application better for timber treatment?

Spray application gives faster coverage over large areas such as roof timbers and floor joists, with slightly better coverage per litre (around 8 m²/L vs 6 m²/L for brush). However, brush application ensures better penetration into the grain, end grain and joints. For best results, spray the main surfaces and then brush-coat joints, end grain and hard-to-reach areas.

What standard covers timber preservation in the UK?

BS 8417:2011+A1:2014 covers the preservation of wood, including use classes for different exposure levels. Use Class 1 covers interior timber above DPC, Use Class 2 covers timber under cover but with risk of wetting, Use Class 3 covers exterior timber, and Use Class 4 covers timber in ground contact. The treatment type and retention level depends on the use class.

Timber Treatment Calculator — How Much Preservative Do I Need?

How We Calculate This

This calculator determines the volume of timber treatment product you need by dividing the total surface area by the coverage rate for your chosen treatment and application method, then multiplying by the number of coats.

The formula

Litres needed = (Surface Area ÷ Coverage Rate) × Number of Coats

Coverage rates used

Preservative (brush): 6 m² per litre per coat
Preservative (spray): 8 m² per litre per coat
Fire retardant: 4 m² per litre per coat (brush or spray)
Wood hardener: 3 m² per litre per coat (brush or spray)

These are typical coverage rates for smooth-planed softwood. Rough-sawn timber, end grain and porous hardwoods will absorb more product and reduce coverage by 20–30%. Always check the manufacturer’s product data sheet for specific rates.

Tin sizing

Results are rounded up to the nearest 5-litre tin, which is the most common trade size. Products are also available in 1L, 2.5L and 25L containers. For large projects, 25L drums offer better value.

Standards

Timber preservation should comply with BS 8417:2011+A1:2014 (Preservation of wood). Use classes and treatment specifications are defined in this standard. Fire retardant treatments should be tested to BS 476 or BS EN 13501-1 for reaction to fire classification.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Verify with your supplier.