How often should I treat outdoor timber?

Decking should be oiled every 12-18 months. Fences should be treated every 2-3 years with preservative or fence paint. Exterior joinery (windows, doors) needs varnish or stain refreshing every 2-3 years depending on exposure. South-facing surfaces may need more frequent treatment.

How many coats of treatment do I need?

Most timber treatments need 2 coats for adequate protection. Bare, untreated wood should have a preservative base coat followed by 2 coats of stain, oil, or paint. Previously treated timber that is still in good condition may only need 1 refresher coat.

What coverage do timber treatments give?

Coverage varies by product: Wood preservative covers about 6 m²/L, decking oil 8 m²/L, fence paint 8 m²/L, exterior stain 12 m²/L, and exterior varnish 14 m²/L. Rough-sawn timber absorbs more than planed timber, reducing coverage by up to 30%.

Can I apply treatment to pressure-treated timber?

Yes, but wait at least 6 months for new pressure-treated timber to dry out before applying oil, stain, or paint. The treatment chemicals need time to fix in the wood. You can apply water-based treatments sooner (after 2-3 months) than oil-based ones.

WoodCalcs

Timber Treatment Calculator — Preservative & Oil Quantities

Calculate preservative and treatment quantities for outdoor timber protection.

Treatment Type

Select based on the condition being treated

Surface Area

Total area to be treated

Number of Coats

Container Size

Price per Container (optional)

Enter your supplier price for a cost estimate

How We Calculate This

This calculator estimates the quantity of timber treatment needed based on the surface area, number of coats, and treatment type.

The formula

Litres needed = (Surface area x Number of coats) / Coverage per litre

Containers = ceil(Litres needed / Container size)

Coverage rates used: Preservative 6 m²/L, Decking oil 8 m²/L, Fence treatment 8 m²/L, Exterior stain 12 m²/L, Exterior varnish 14 m²/L. These are average rates for smooth planed timber. Reduce by 20-30% for rough-sawn surfaces.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Always verify measurements and material quantities before cutting or purchasing.