How Much Timber Treatment Do I Need?
Quick Answer
4–5 litres for a 20m² area (spray application)
Based on: 4–5m²/L coverage rate for woodworm/rot treatment by spray. Brush application uses more
Coverage Rates
- Spray application: 4–5m² per litre (most common for large areas)
- Brush application: 3–4m² per litre
- Injection (paste): As directed per flight hole
Common Treatments
- Woodworm treatment (permethrin-based): For common furniture beetle. Apply to all exposed timber surfaces
- Wet rot treatment: Remove and replace affected timber, treat surrounding area with fungicide
- Dry rot treatment: Professional treatment essential. Affected timber removed 1m beyond visible damage. Masonry treated with fungicidal solution
- Preservative (boron-based): Preventative treatment for new or replacement timbers
How to Calculate Area
For loft timbers, measure the area of all exposed rafters, purlins, and joists. A typical loft has roughly 1.5–2× the floor area in exposed timber surface:
- Loft floor area: 40m²
- Estimated timber surface: 60–80m²
- Treatment needed: 12–20 litres
Professional vs DIY
- DIY treatment: £5–15 per litre, suitable for minor woodworm
- Professional spray: £15–30/m² (supply and labour)
- Dry rot specialist: £1,000–£5,000+ depending on extent
Last updated: April 2026