Exterior Wood Oil Calculator — Decking & Outdoor Timber

It depends on the product, and the figure manufacturers print is for the recommended two coats — not per coat. On softwood decking Osmo Decking Oil covers roughly 10–12 m²/L over 2 coats (so a 2.5 L tin covers about 25–30 m²), Ronseal Ultimate Protection about 5 m²/L over 2 coats (a 2.5 L tin covers ~10–12 m²), and Barrettine All-In-One around 5 m²/L. Smooth dense hardwood drinks roughly half the oil of softwood (Osmo quotes ~24 m²/L on smooth hardwood). Across both coats that works out near 0.09 L/m² for Osmo on softwood — the heavier first coat takes the larger share and later coats less as the pores fill. Rough-sawn or grooved boards absorb more than smooth planed ones, so always treat these figures as indicative and check the tin.

Softwood decking needs oiling annually. Porous hardwood (oak, iroko) and cedar around every 2 years. Dense tropical hardwoods (ipe, cumaru) every 1–2 years — DeckWise Ipe Oil recommends recoating every one to two years with at least an annual clean. South-facing decks in full sun may need more frequent treatment. The water bead test works well — pour water on the deck: if it soaks in rather than beading up, it's time to re-oil.

For UK conditions, Osmo Decking Oil and Owatrol Aquadecks are top professional choices. They penetrate deeply, repel water and allow the wood to breathe. Danish oil is good for furniture but not durable enough for decking. Boiled linseed oil is sometimes used as a cheap option but is not recommended for decks — it offers no UV protection and can actually feed mildew growth, so it needs frequent reapplication and gives poor weather durability. Avoid cooking oils and motor oil — they don't protect against UV and can go rancid.

Yes — lightly sand with 80-120 grit to open the wood grain for better oil absorption. This removes the grey weathered layer and any raised fibres. For new timber, sanding removes mill glaze (a shiny surface from sawing) that prevents oil penetration. Always clean off dust before oiling. For previously oiled decking, a light sand is usually sufficient.

No — the timber must be dry for oil to penetrate properly. Allow at least 48 hours of dry weather after rain before oiling. Use a moisture meter if possible; the timber should be below 18% moisture content. Oiling damp wood traps moisture underneath, which causes the oil to peel and can promote fungal growth. Choose a dry spell with temperatures above 10°C.