Choosing a species
Thermally modified timber species guide
Thermal modification works on many species, but the base timber still shapes the finished look, strength and price. This guide compares the species most often specified for cladding and saunas. Availability changes, so ask us what is in stock or can be sourced.
The table gives a quick comparison. The sections after it explain what actually differs in use: weight, hardness, knots, colour, and how each group of species is best fixed and finished.
Species at a glance
| Species | Type | Appearance | Best for |
|---|---|---|---|
| Pine (Scots) | Softwood | Warm brown, lively grain, some knots | Cladding, decking, sauna walls |
| Spruce | Softwood | Fine, even grain, light brown | Cladding, interior lining |
| Ash | Hardwood | Strong grain, deep chocolate brown | Cladding, decking, sauna benches, interiors |
| Aspen | Hardwood | Pale and even, fine texture | Sauna benches and lining |
| Alder | Hardwood | Soft red-brown, even | Sauna lining and interiors |
| Oak | Hardwood | Distinct figure and rays, rich brown | Cladding, flooring, feature interiors |
| Radiata pine | Softwood | Uniform pale brown, few defects | Cladding, decking |
| Beech | Hardwood | Fine, even, light to mid brown | Interiors, flooring, joinery |
Softwoods: pine, spruce and radiata
Softwoods are the workhorses of thermally modified cladding. Pine and spruce are the classic ThermoWood® species, with a strong track record and a broad range of profiles. Radiata pine is a uniform plantation softwood that gives very clean boards. All three are light, easy to work and good value.
Knots are part of the character of pine and spruce. After modification knots can be more brittle and occasionally loosen, so boards are normally sold in grades based on knot size and frequency. Ask which grade is being quoted when you compare prices, because a clear grade and a rustic grade look very different once they are on the wall.
- Pine: more visible grain and larger knots, with a warm brown colour. Resin is greatly reduced by modification, although small traces may remain around knots.
- Spruce: smaller, tighter knots and a paler, calmer face than pine.
- Radiata pine: grown on plantations, with fewer defects and a very even colour. Some suppliers offer it cut for vertical grain, which helps stability in wide boards.
Hardwoods: ash, aspen, alder, oak and beech
Thermally modified hardwoods give richer, darker colours and finer grain. Ash is the best known: strong, handsome and very stable after modification. Aspen and alder are prized for saunas because of their pale, smooth, low-resin surfaces. Oak and beech bring distinctive character to feature cladding and interiors.
Hardwoods are denser and harder than the softwoods, so they resist dents and wear better. That is useful for decking, benches and flooring. They are also heavier and more prone to splitting at fixings, so pre-drill every fixing rather than only those near the ends.
Several of these species, including ash, aspen, alder and beech, have little natural durability when untreated, so modification is what makes them suitable for exterior or hot, humid use. Aspen and alder are usually used indoors or in saunas rather than as exterior cladding. Oak is different: it already has good natural durability, so it is chosen mainly for stability and colour.
Practical differences on site
| Consideration | Softwoods | Hardwoods |
|---|---|---|
| Weight | Lighter, easier to handle on site | Heavier |
| Hardness | Softer, marks more easily | Harder, wears better underfoot |
| Fixing | Pre-drill near board ends | Pre-drill throughout |
| Knots and grain | Knots common, graded by size | Finer grain, fewer large knots in aspen and alder |
| Price position | Usually lower | Usually higher |
Colour and weathering
Every thermally modified species darkens in proportion to the temperature and time of the process, so Class D is darker than Class S for the same species. Colour also varies between batches and sometimes within a board, so do not judge a species from a photograph alone. Order samples and view them in daylight.
Outdoors, untreated surfaces fade and grey at broadly similar rates whatever the species. A pigmented oil or stain makes far more difference to colour retention than the choice of wood.
How to choose
- Outdoor cladding on a budget: thermally modified pine or spruce, Class D.
- Premium or contemporary cladding: thermally modified ash for a darker, more refined finish.
- Decking: pine or ash, Class D, laid with gaps and good ventilation beneath.
- Sauna benches: aspen, alder or ash.
- Sauna walls and ceilings: aspen, alder, pine or spruce depending on budget.
- Interior feature walls: oak, ash or beech.
- Need a fire rating as well: talk to us about treated options.
Not sure? Request free samples, and compare colour and grain in your own light. When you contact us, tell us the use, the species or budget you have in mind, the profile and thickness, and the quantity. Prices in the shop include VAT and UK delivery, and you can call 0161 504 0011 for advice.
Shop and specify
Frequently asked questions
Talk to our timber team
Tell us about your project and we will recommend the right species, profile, finish and fire performance. Free samples are available, and we can quote for bespoke milling.