What Fasteners Does Thermo-Aspen Sauna Paneling Need?

Why does thermally modified wood need stainless fasteners?

Thermal modification makes wood more acidic. The handbook gives ThermoWood® products a pH of about 4, against 4.5 to 5 for a comparable standard product. The scale is logarithmic: a drop of 0.3 pH units doubles the amount of acid ITWA handbook, p. 12.

That acidity can affect the corrosion of metal fasteners, so the handbook concludes that fasteners used with thermally modified timber should be stainless or acid-resistant steel ITWA handbook, p. 12. Section 9.1 names the visible result: other fasteners react with the wood and stain the area around the fastener ITWA handbook, p. 44.

The stainless rule applies indoors and outdoors, and the pH drops during the controlled kiln process itself ITWA handbook, pp. 12, 44. The process pagewalks through that process. None of these figures is specific to aspen: aspen is on ITWA’s list of species used for ThermoWood® products, in both classes, and the fastener rules cover ThermoWood® products of every species ITWA handbook, p. 9.

What Table 10 requires

Every row in Table 10 starts at the same grade: class A2 stainless, which the handbook equates with AISI 304. US suppliers sell it as Type 304. All seven uses in the table share it: wall and ceiling cladding in dry rooms, bathrooms, and saunas; dry-room floors; sauna benches; decking; and exterior cladding ITWA handbook, p. 44. English terms follow the 2025 edition ITWA handbook (English), p. 44.

For a sauna, two points follow. The sauna rows ask for the same grade as a dry room, not a higher one. And no row goes below stainless: the table has no entry for galvanized, coated, or plain steel.

Type 316 is the step up. The handbook calls class A4 (AISI 316) the most common acid-resistant class and says acid-resistant fasteners can also be used ITWA handbook, p. 44. Table 10 does not address pool rooms or salt air. If your sauna sits beside either, ask the fastener maker, with 316 as the handbook’s named alternative.

For Thermory panels, follow Thermory’s fixing method

Section 9 of the handbook opens with a priority rule. ThermoWood® products must be installed according to the manufacturer’s instructions, and the handbook’s own text is general guidance ITWA handbook, p. 44. The same order holds for any board: the fixing method comes from the maker of the board you bought.

For its sauna wall paneling, Thermory USA’s guide recommends a staple gun and stainless steel clips Thermory USA guide, p. 2. It says to fix with staples from the groove side, for vertical and horizontal runs alike, and states that finishing nails are not suitable for fixing the paneling. Set the staple depth so the clips neither stand proud of the surface nor go deep enough to damage the wood Thermory USA guide, p. 3.

The handbook’s Table 11 gives examples of fixing methods for ThermoWood® cladding. For indoor walls and ceilings it shows hidden clips, and Dyckert nails as a visible fixing through the side, with heads set just below the surface ITWA handbook, p. 45. For a Thermory board, follow Thermory’s guide: clips at the groove, not finishing nails.

The same guide sets the battens the clips land in: at least 13/16 in thick and 1-13/16 in wide, with a recommended spacing of about 15-3/4 in Thermory USA guide, p. 2. The handbook drawings and Thermory USA’s guide both cap spacing at 23-1/2 in ITWA handbook, p. 38. The post on sauna framing, battens, and the air gap covers that layer.

Thermory’s STS4 1×4 thermo-aspen panel measures 0.59 × 3.54 in Thermory catalog, p. 8. You can see the STS4 1×4 thermo-aspen panel at Sauna Marketplace, where you can also order a sample, or compare it with other profiles on the profiles page.

Length, head depth, and distance from the ends

Length

The handbook sets two limits. Nails and screws must be long enough to extend through the mounting batten or board. Their length must also be chosen so they do not pierce any air or moisture barrier or similar layer ITWA handbook, p. 44.

Where the vapor barrier sits right behind the battens, the second limit decides. Keep the fastener shorter than the wood it passes through plus the batten, so the tip stops before the barrier. Thermory’s guide calls for battens at least 13/16 in thick Thermory USA guide, p. 2. A thicker batten gives the fastener more depth to hold in before it reaches the barrier.

Head depth

Nail and screw heads should sit level with the wood surface. The exception is the Dyckert nail, whose head sits just below the surface ITWA handbook, pp. 44, 45. With clips at the groove, check each one before the next board covers it.

Distance from edges and ends

Section 9.2 asks you to keep nails and screws far enough from board ends that they do not split the wood, and it allows pre-drilled holes ITWA handbook, p. 44. Figures 51 and 52 give recommended distances for ThermoWood® products fastened without pre-drilling. They apply to nails and screws driven through a board, such as bench screws or face-fixed trim, not to clips at the groove ITWA handbook, p. 48.

The handbook’s drawings for ThermoWood® cladding show two ways to hold that end distance at a joint. One sets an end-matched joint just beside a batten. The other, a plain butt joint, uses two battens side by side so each board end has its own support ITWA handbook, pp. 44, 48. Thermory’s catalog lists end-matched versions of its STP and STS4 profiles Thermory catalog, p. 20.

If you pre-drill, the handbook’s sizes are 50 to 80% of the nail diameter for ring-shank nails, and 50 to 70% of the screw diameter for screws, never wider than the threaded core. Those rows cover outdoor cladding, decking, and indoor flooring; none is a sauna row ITWA handbook, pp. 45, 46, 47. Plan your cuts so board ends land where you can keep the distance. The post on cutting, sanding, and storing thermo-aspen covers the saw side.

Bench screws and what carries the weight

For ThermoWood® products, Table 10 puts sauna benches on the same Type 304 floor as the walls ITWA handbook, p. 44. Thermory’s guide adds two rules. Bench weight must be carried by supports added behind the paneling, never by the paneling itself. At least once a year, check the bench and support fastenings and tighten the screws if needed, since heat and humidity make the wood swell and shrink, which can loosen them Thermory USA guide, p. 4.

In the handbook’s screw tests, thermal modification lowered screw holding strength somewhat. The tests cover pine, spruce, and several other species, but not aspen ITWA handbook, p. 22. The two ThermoWood® classes are Thermo-S, modified at 374°F, and Thermo-D, modified at 414°F, for pine and spruce ITWA handbook, p. 18.

In these averages, pine lost about 10 to 12% and spruce about 17 to 32%. The handbook adds that ordinary variation in density affects screw holding more than modification does, and that in lower-density material, screws held better in smaller pre-drilled holes ITWA handbook, pp. 22, 23. For bench screws, that points to pilot holes at the small end of the handbook’s range, plus the yearly check.

Fastener checklist for a thermo-aspen sauna

• Stainless steel for every clip, staple, nail, and screw that touches the wood. Thermory’s guide calls for stainless steel clips on its paneling Thermory USA guide, p. 2. For ThermoWood® products, the handbook sets Type 304 as the minimum and names Type 316 as the acid-resistant step up ITWA handbook, p. 44.
• For Thermory paneling: stainless steel clips from a staple gun, fixed at the groove side, and no finishing nails Thermory USA guide, pp. 2, 3.
• Staple depth set so the clips are not proud of the surface and not deep enough to damage it Thermory USA guide, p. 3.
• Fastener lengths that never pierce the air or vapor barrier ITWA handbook, p. 44.
• For nails and screws through a board without pre-drilling, at least 13/16 in from edges and 2-13/16 in from ends, per the handbook’s figures for ThermoWood® products ITWA handbook, p. 48.
• Bench weight on supports behind the paneling, with fastenings checked once a year and screws tightened as needed Thermory USA guide, p. 4.

To lay out the wall before you buy fasteners, open the profiles in the sauna designer. The spec guide and the buyer’s guide carry the rest of the install details, and the questions page answers the short versions.