Thermo-aspen and thermowood sauna floor grids

Updated November 2, 2026 · 4 min read

A sauna floor grid made from thermo-aspen or other thermally modified woods gives you a surface that's comfortable underfoot, resists moisture, and is easy to lift and clean. These removable wood platforms are a practical solution for home saunas with concrete sauna floors, tile, or even waterproofed plywood. Here’s what you need to know before choosing and installing a sauna floor grid.

What Is Thermally Modified Wood?

Thermally modified wood—sometimes called thermowood—is natural wood that's been heated to high temperatures (usually 400°F or more) in a controlled, oxygen-free environment. This process changes the wood’s cell structure, making it more stable, less likely to absorb water, and less attractive to mold or decay. You’ll often see thermally modified aspen, alder, or spruce sold for sauna use. Unlike pressure-treated wood, thermowood contains no chemicals, so it’s safe for bare feet and high-heat environments.

Thermo-aspen is a popular choice for sauna floor grids because the modification process brings out a rich brown color and a pleasant, subtle scent. The heat treatment also makes the wood more dimensionally stable, so it resists cupping and shrinking better than untreated aspen. Thermo-spruce and thermo-alder are also available, but aspen is usually the most splinter-free.

Aspen, Thermo-Aspen, and Other Low-Resin Woods

In sauna construction, untreated aspen, alder, hemlock, and spruce are chosen because they’re low in resin. This matters: high-resin woods like pine can get sticky or ooze sap when heated, which isn’t ideal for a floor you’ll walk on.

Thermo-aspen takes untreated aspen and improves its performance by reducing its ability to absorb water. This makes it less likely to swell or discolor on a wet sauna floor. Thermo-spruce and thermo-alder perform similarly. Untreated aspen is still a reasonable option if you’re on a tight budget, but it will need more care to prevent mold or warping. For any grid that will be in regular contact with moisture, thermally modified wood is usually worth the extra upfront cost.

Here’s how they stack up for sauna floor grid use:

Loose-Lay Grids Versus Fixed Floors

A sauna floor grid is almost always a loose-lay design: you just set it on your finished floor and lift it out for cleaning. This is different from a fixed wood floor, which is attached to joists or sleepers and isn’t meant to be moved. Loose grids allow water to drain through (if your floor is sloped and has a drain), making them a good match for tile sauna floors or interlocking sauna floor tiles.

Loose grids are usually built from slats spaced about 3/8" to 1/2" apart, with the whole panel standing 1" to 1-1/2" high on supporting runners. Most commercial sauna grids are made for DIY trimming, so you can cut them to fit your space. Fixed wood floors are rare in home saunas, since they’re harder to clean and can trap moisture underneath.

Sizes and Cutting a Grid to Fit

Most sauna floor grids come in standard panel sizes: 2' x 2', 2' x 4', or sometimes longer. You’ll need to measure your sauna’s usable floor area, leaving a gap around the heater and, if possible, a few inches from each wall for air flow and to avoid trapping debris. If your sauna has a floor drain, make sure to cut a hole or notch in the grid for access.

A typical project looks like this:

1. Measure the floor area you want to cover, avoiding the space under the heater and benches.
2. Buy panels that slightly oversize the area, so you can trim for a snug fit.
3. Mark and cut the panels with a circular saw or handsaw. Sand any rough edges.
4. Test-fit the grid, checking that it lays flat and doesn’t rock.
5. If you need to cover a drain, cut an access hole or leave a removable section.

Most grids can be cut to fit around room jogs, drains, or other obstacles. If you’re not comfortable with the cuts, a handy local carpenter or flooring installer can handle it quickly. For a full install, see how to install a sauna floor.

Care and Lifting Grids to Dry

Thermo-aspen and other thermowood grids are much less prone to mold and rot than untreated wood, but they still need to dry between uses. After each sauna session, prop up or remove the grid to allow air to circulate underneath. This helps both the grid and the floor dry out, especially if you have a sauna floor drain and slope for water runoff.

Routine care is simple:

• Sweep or vacuum debris off the grid after use.
• Wipe with a damp cloth or mild soap if needed.
• Inspect for any loose slats or splinters.
• Once or twice a year, take the grid outside and wash it with a hose, then let it dry fully before reinstalling.

Never use varnish, polyurethane, paint, or standard wood sealers on a sauna grid. Only consider finishes that are specifically tested and certified for use in sauna environments at high temperatures, though most heater manufacturers and building codes recommend leaving sauna wood unfinished for safety and to avoid off-gassing. Most thermally modified woods do not require any finish; bare wood is generally safer and dries more quickly in the heat.

If you’re pricing out a custom or prefabricated grid, local material costs and labor rates can vary widely. Expect a broad range from the low end (DIY-cut untreated aspen) to the premium end (custom thermowood panels), with most projects falling somewhere in between. For a detailed quote, always check with local pros—see free quotes from local pros for options in your area.