The Railings and Steps Are Two Fifths of a Deck to Stain

Rough timber drinks 43 % more of the same product

The same deck, the same stain and the same two coats need 4.35 gallons on planed timber, 5.44 on weathered and 6.22 on rough sawn — a quarter more and then nearly a half. Nothing about the area changed; the surface did, and a rougher one has more actual surface per nominal square foot and more places for liquid to sit. The choice of product moves the figure by a comparable amount: a transparent finish covers further than a semi-transparent one, and a solid stain covers least of all. Read the coverage figure on the tin for the condition of the wood in front of you, because the number printed for smooth new timber is the best case and the tin is unlikely to be opened on smooth new timber.

The second coat is not the same as the first

The calculation multiplies by the number of coats, so two coats come out at exactly twice one. On a smooth surface that is close to true; on bare, thirsty timber it is not, because the first coat is partly absorbed into the wood and the second sits largely on a surface that has already been sealed. The practical consequence runs the other way from the arithmetic: budget generously for the first coat on new or weathered wood and expect the second to go further than the tin claims. Where a job spans two sessions, buy the first coat's worth, apply it, and measure what is left before buying the rest — the wood answers the question better than any table.

Frequently asked questions

Should I stain the underside of the boards?

If the boards are not yet down, yes — and the edges too, which are the most absorbent faces of a sawn board. A board sealed on top and open underneath takes up moisture from below and releases it from above at different rates, and that difference is what makes boards cup and pull at their fixings. Once a deck is built the underside is usually unreachable and the question is moot, which is why the moment to decide is while the timber is still stacked. The area doubles if you do, and the calculation above needs the floor entered twice.

How do I convert gallons to litres on the tin?

The tool does it, and the factor it uses is the US gallon at 3.785 litres — 4.352 gallons becoming 16.47. The trap is that an imperial gallon is 4.546 litres, a fifth larger, so a coverage figure quoted in gallons is only comparable with another once it is clear which gallon each meant. Where a product is sold in litres, work in litres throughout; where a coverage figure is quoted per gallon with no country attached, the safest move is to convert it to square metres per litre before comparing it with anything.