Length times height is the wrong number
Ask anyone how big their fence is and they will multiply the run by the height. That figure is the outline of the fence, and it is wrong in two directions at once.
It is too big because a spaced picket fence is not solid. With 5.5 inch boards and half inch gaps, only 92 percent of the outline is actually wood, so the face area is lower than the rectangle suggests.
It is too small because boards have edges. Every picket has two of them, each the thickness of the board by the height of the fence, and on a spaced fence both are exposed and both need coating. A hundred foot run of six foot fence at that spacing has two hundred boards. Two hundred boards, two edges each, three quarters of an inch by six feet, is 150 square feet — about fourteen percent added on top of the faces, and it is fourteen percent nobody has ever included in an estimate made at the fence with a tape.
Then there are the posts, whose three exposed faces on a typical fence add up to more than people expect across fourteen of them, and on a fence being finished on both sides, the back of every rail.
Rough-sawn wood and where the gallon goes
The coverage figure on a tin is measured on smooth, sound, new timber. Rough-sawn stock is a different material as far as a finish is concerned. The saw leaves a surface of torn and lifted fibre, and the real area of that surface is far greater than its measured dimensions — the way a towel has more surface than a sheet of paper the same size. Every lifted fibre also acts as a wick, drawing finish in by capillary action rather than merely holding it on the face.
The practical result is that a gallon rated at 250 square feet delivers something closer to 120 on new rough-sawn stock, and less again if the wood has weathered and the grain has opened. This is the single largest error in fence finishing estimates, and it always fails in the same direction: you buy for the label figure, run out two thirds of the way along, go back for more, and the second batch is a slightly different colour.
| Surface | Roughly how thirsty | 250 sq ft label becomes |
|---|---|---|
| Planed smooth, new | As labelled | about 250 sq ft |
| Planed smooth, weathered grey | Around 1.5 times | about 170 sq ft |
| Rough-sawn, new | Around 2 times | about 120 sq ft |
| Rough-sawn, weathered | Around 2.5 times | about 95 sq ft |
The second coat behaves differently again, and better. The first coat has filled the most absorbent parts of the surface, so the second sits more on top and covers appreciably further — this calculator uses about a third more coverage for coats after the first. That is why a two coat job does not need twice the material of a one coat job.
Spraying, and the material that never lands
A sprayer is the fastest way to get finish onto a fence and the least efficient way to get it into one. Between overspray drifting past the fence, atomised material carried off by any breeze, and the gaps between pickets that a spray fan passes straight through, a meaningful fraction of what leaves the gun does not end up on wood. On a solid wall the loss is modest; on a spaced fence with gaps in it, a good deal of the fan is aimed at nothing.
Back-brushing after spraying recovers most of the quality problem without recovering the material. It pushes finish into the grain, works it into the picket edges, and evens out the heavy and light passes that a moving sprayer always leaves. A sprayed fence that is not back-brushed frequently looks fine when wet and blotchy a week later.
The one safety point that is not optional
Rags, brushes, pads and roller covers wet with oil-based stain or sealer can generate enough heat on their own as they cure to ignite without any flame present. A bunched rag in a bin is the classic case. Lay them out flat and separately until they are bone dry, or put them into a sealed metal container with water. This is worth doing every single time.
Before the finish, and after it
New pressure-treated timber is often too wet to accept a finish, and a fence stained the week it was built can shed the finish as the boards dry and move. The usual test is to sprinkle water on the wood: if it beads and sits, the wood is not ready; if it soaks in within a minute or so, it will take a finish. That may be a few weeks or several months depending on the stock and the weather.
Weathered grey wood usually wants cleaning first, and possibly brightening, before any finish goes on, and those quantities are covered in the deck cleaning and sealing calculator, which handles the cleaner and brightener side of the same job on decks and railings. For finishing quantities on general woodwork rather than fences, the finish coverage calculator works from area directly, and the wood finishing schedule calculator deals with coat sequence and drying times.
If the fence does not exist yet, the board count comes from the privacy fence board calculator for spaced boards or the shadowbox and board-on-board calculator for overlapping ones, and an overlapping fence has substantially more surface to finish than its outline suggests. Posts and concrete are in the fence post calculator, and the gate, which is the part that gets handled and wears its finish off first, is in the gate sag and brace calculator.
Questions people ask
How much stain do I need for a 100 foot fence?
For a six foot spaced picket fence finished on both sides, in rough-sawn timber, with two coats, the answer lands in the low to mid twenties of gallons, and spraying rather than brushing pushes it toward the upper end — and almost every part of that sentence is doing work. Halve it for one side only. Cut it by roughly half again if the timber is planed smooth rather than rough-sawn. Add to it if the wood is weathered. The reason a single number for a hundred foot fence does not exist is that the surface condition changes the answer by a factor of two and a half, which is more than any other variable on the page.
Why does rough-sawn wood use so much more?
Because its real surface area is much larger than its measured area, and because it wicks. A sawn face is a mass of torn fibres standing up from the surface, each with sides of its own, so the area a liquid has to wet is several times the area a tape measures. Those same fibres draw finish inward by capillary action rather than letting it sit on top. The combination means a first coat on rough stock can absorb double what the label predicts. It is not waste, the finish is in the wood doing its job, but it has to be bought.
Do I need to stain both sides of the fence?
The wood does not care which side faces your garden. Both faces are exposed to sun and rain, and finishing one side only means one side is protected and the other weathers normally, which shows up as differential movement in the boards before it shows up as appearance. The practical obstacle is usually access rather than opinion, since the far side of a boundary fence may not be yours to stand on. Where you can only reach one side, finish that one and accept the fence will not last as long as one done properly.
Can I stain a brand new pressure-treated fence?
Often not straight away. Treated timber is frequently still carrying a great deal of moisture when it is sold, and a finish applied to wet wood either will not penetrate or gets pushed off as the wood dries and shrinks. The standard check is to flick water onto the surface: if it beads and stands, wait; if it soaks in within a minute, the wood is ready. Depending on the stock and your weather that can be a few weeks or the better part of a season. A water-repellent product designed for new treated wood is a different case and its own instructions govern.
What do I do with the rags afterwards?
Lay them out flat and separately outdoors until they are completely dry, or put them in a sealed metal container with water. Rags soaked in oil-based stain or sealer generate heat as they cure, and a bunched pile holds that heat in until it reaches ignition. There is no flame involved and no warning. This applies to brushes, pads and roller covers as much as to rags, and it applies to the ones you were going to throw away as much as to the ones you are keeping.