How Much Cladding Does Your Sauna Actually Need? A Board-Feet Walkthrough

Ordering too little cladding means a second supplier run mid-build, hoping the new boards match the first batch closely enough. Ordering too much means paying for lumber that sits in a garage indefinitely. Both are avoidable, because the quantity you actually need isn't a guess — it's a straightforward calculation from your room's dimensions, and walking through it once with real numbers makes the whole process far less mysterious than a supplier's order form makes it look. Every figure below comes from actually running a real room's dimensions through the same calculation this site's cladding calculator uses, step by step, so you can follow exactly where each number comes from rather than taking a final total on faith.

Start with surface area, not volume

Cladding is a covering, so the calculation starts with area, not the room's cubic footage the way heater sizing does. Wall area is simply the room's perimeter multiplied by its height — for a 6-by-6-foot room with a 7-foot ceiling, that's a 24-foot perimeter times 7 feet, or 168 square feet of wall. Whether you're also cladding the ceiling is a separate decision that adds the floor's own area on top — 36 square feet for that same 6-by-6 room — bringing the gross coverage to 204 square feet before any adjustments.

Subtract the door, add the waste

From that gross figure, subtract the door opening, since it's not being clad — a typical 12 square foot opening brings that 204 down to a 192 square foot net area. Then add a waste allowance, typically around 10%, to cover trimming, corner cuts, and the coverage lost to the tongue-and-groove joint itself where boards interlock. That 10% on 192 square feet works out to 211.2 square feet, adjusted — the number that actually goes into the final board-feet calculation, not the raw gross figure you started with.

From area to board feet: where thickness comes in

Board feet is a volume measurement, not an area one — one square foot of coverage at one inch of thickness, which is the standard unit lumber suppliers price by because it lets boards of any width or length be compared on equal footing. That means the final step is simple multiplication: adjusted area times nominal board thickness. At the industry-standard 3/4 inch tongue-and-groove thickness, that 211.2 adjusted square feet for our 6-by-6 example works out to 158.4 board feet, ceiling included — the number you'd actually bring to a supplier.

What skipping the ceiling saves

Cladding the ceiling isn't mandatory, and it's worth knowing the real tradeoff before deciding either way. Drop the ceiling from that same 6-by-6 room's order, and the adjusted area falls to 171.6 square feet, bringing the board-feet total down to 128.7 — a meaningful savings, roughly 19% less material, but at the cost of leaving the ceiling as whatever substrate sits behind it rather than matching the walls. Most builders clad the ceiling anyway for a finished, enclosed look and because an unclad ceiling in a hot, humid room needs its own separate finish that's still appropriate for the conditions; it's a real cost-versus-finish decision worth making deliberately rather than defaulting either way.

How much thickness moves the order

Because board feet is area times thickness, thickness scales the order directly and proportionally — there's no shortcut around it. That same 6-by-6 room at the thinner 5/8 inch (0.625in) profile some suppliers offer comes to 132 board feet instead of 158.4, while stepping up to a full 1-inch board pushes the order to 211.2 board feet. None of the room changed; only the board's thickness did, and the order size followed in exact proportion. This is worth checking against your chosen supplier's actual stocked thickness before ordering, since suppliers don't all carry the same profiles, and a mismatch between what you calculated and what's actually available is an easy, avoidable surprise.

What waste allowance actually buys you

The waste percentage isn't padding for its own sake — it's a genuine allowance for real losses: boards cut short around corners and the door frame, occasional milling defects you'll want to cut around rather than install, and the small amount of coverage lost at every tongue-and-groove seam. Trim that allowance down to 5% on the same 6-by-6 room and the order drops to 151.2 board feet; push it up to 15% for a more complex room with lots of corners or trim cuts, and it climbs to 165.6. A simple, mostly rectangular room with an experienced installer can often get away with the lower end of that range; a room with unusual angles, a lot of trim detail, or a first-time DIY install is safer erring toward the higher end, since running short mid-installation, and waiting on a second order that may not perfectly match the first delivery's grain and tone, is a far bigger hassle than having a few extra boards left over.

Scaling up: a larger room's numbers

The same walkthrough applies at any size. An 8-by-8-foot room with a 7-foot ceiling, the same 12 square foot door, standard 3/4 inch thickness, and a 10% waste allowance comes to 227.7 board feet — nearly 44% more material than the 6-by-6 example above, tracking the larger room's greater wall and ceiling area rather than any change in the underlying formula. Whatever size room you're planning, the process is identical every time: gross area, minus the door, plus waste, times thickness.

Common ordering mistakes worth avoiding

A handful of mistakes show up repeatedly in first-time cladding orders, and every one of them is avoidable once you know to check for it. Measuring only the walls and forgetting the ceiling is probably the most common, especially when a builder is pricing options and comparing a ceiling-included quote to a walls-only one without realizing they're not comparing the same scope. Forgetting to subtract the door opening is another — a 12 square foot door is a real chunk of a small room's wall area, and skipping that subtraction means over-ordering by roughly that same amount. Rounding a waste allowance down to zero because a room looks simple on paper is a third: even a straightforward rectangular room loses some coverage to trim cuts and joints, and running out mid-install, waiting on a second delivery that may not match the first batch's color and grain exactly, is a far worse outcome than having a modest surplus. Finally, ordering by the wrong thickness — using a generic 3/4 inch assumption when your supplier's actual stocked profile is 5/8 inch, or vice versa — throws the whole board-feet figure off proportionally, so it's worth confirming the exact thickness you're being quoted before finalizing an order built on a different assumption.

What's not counted in board feet

Board feet measures the visible cladding only — it says nothing about what goes in behind it. A correctly built sauna wall needs its own vapor barrier, typically a foil-faced layer installed with the foil side facing into the hot room, to keep the high humidity a session generates from migrating into the wall cavity and insulation, where it can condense and cause problems inside a wall you can no longer see once the cladding is up. That's a separate line item on any real materials list, priced and ordered on its own, and it's worth budgeting for it alongside the cladding rather than treating the board-feet total as the whole materials cost. Most suppliers also sell cladding in bundled units rather than exact board-foot quantities, so a calculated figure like 158.4 board feet is really a target to round up to the nearest bundle size your specific supplier offers, not a number you can order to the decimal. Getting the vapor barrier's orientation and seam-taping right is worth following exactly to your specific product's installation instructions, since an incorrectly installed barrier can trap moisture rather than block it, and that's a mistake that only becomes visible once real damage has already set in behind a finished wall.

It's also worth repeating, because it matters regardless of which species or thickness you order: whatever wood you choose, leave it bare. Staining, painting, or varnishing the interior risks a finish that off-gasses or becomes uncomfortably hot at sauna temperatures, and it defeats the reason you chose a sauna-appropriate species in the first place, no matter how tempting a glossy showroom finish might look next to a raw board.

Species doesn't change the math, only the price

One thing board feet does not account for is which wood species you've chosen — cedar, aspen, hemlock, or thermally modified wood all get quantified in board feet the same way, since it's a measure of volume, not material. What differs by species is price per board foot and, in some cases, standard stocked thickness. Our guide on choosing sauna cladding wood walks through those species tradeoffs in depth once you've got a board-feet quantity in hand to price out.

Use our Cladding Board-Feet Calculator to run your own room's exact dimensions, door size, thickness, and waste allowance. See cladding figures for eight common room sizes side by side, alongside heater and bench numbers, on the Sauna Room Size Reference.