External cladding guide
Thermowood and thermally modified cladding
Thermally modified timber is one of the most popular choices for contemporary external cladding: stable, uniform in colour and free of chemical preservatives. This guide covers what to specify and how to install it so it performs.
Why choose thermally modified cladding?
- Dimensional stability means tighter joints and less movement through the seasons.
- Class D has enhanced decay resistance without preservatives.
- A consistent warm brown that can be left to silver or kept with a finish.
- Machined to a wide range of profiles, from shadow gap to board-on-board.
- Lighter than many dense hardwoods and easy to cut and fix on site.
Choosing a profile
The profile affects appearance, water shedding and how forgiving the installation is. Common options for thermally modified cladding include:
| Profile | Look | Notes |
|---|---|---|
| Shadow gap (tongue and groove) | Crisp, contemporary, deep reveal | Hidden fixing possible; good for horizontal or vertical runs |
| Square edge / rainscreen | Open-joint, architectural | Needs a dark breathable membrane behind the open joint |
| Rhombus (open or closed) | Angled, strong shadow lines | Common on vertical facades and fins |
| Board-on-board | Traditional, layered | Good water shedding, cost-effective |
| Featheredge / weatherboard | Classic British look | Overlap shed water; suits rural and heritage settings |
We can also quote for bespoke milling if you need a profile that is not in the standard list. Browse our timber profiles page for dimensions and cover widths.
Design and detailing
Good detailing matters more than the timber itself. Thermally modified cladding should be installed as a ventilated rainscreen.
- Fix to treated battens of at least 25 mm over a breathable membrane, creating a ventilated and drained cavity.
- Provide ventilation openings at the top and bottom of the cavity, protected with insect mesh.
- Keep the lowest board well clear of the ground and any splash zone. Follow the supplier guidance, commonly 150 to 200 mm minimum.
- Allow for a small amount of movement with correct board gaps and expansion allowances.
- Seal or protect all end grain, especially at window reveals and cut ends.
- Avoid ponding or trapped water at ledges and abutments.
Fixings
Use stainless steel fixings, A2 as a minimum and A4 in coastal or highly polluted locations. Ordinary or galvanised steel can corrode and stain the timber black.
- Face fixing: stainless ring-shank nails or screws, pre-drilled near board ends to avoid splitting.
- Secret fixing: stainless clips suit tongue and groove profiles and give a clean face.
- Allow for the fixing to hold the board without restricting natural movement.
Finishes and weathering
You can leave thermally modified cladding untreated and let it weather to an even silver-grey, or you can protect the colour.
- Untreated: weathers to silver-grey over months to a couple of years depending on exposure. Sheltered areas weather slower, so expect some variation.
- Pigmented oil or stain: slows UV greying and maintains the brown tone. Reapply as the manufacturer recommends.
- Paint or opaque coatings: possible on suitably prepared thermally modified timber. Check adhesion guidance with the coating supplier.
Specifying and ordering
- Species and class (for external use, Class D).
- Profile, thickness, cover width and length range.
- Orientation and board layout, including how end joints are staggered or hidden.
- Finish: untreated, oiled or stained, or factory finished where available.
- Fixings and accessories: stainless fixings, trims, corner details, insect mesh and membrane.
- Volume and delivery schedule. Tell us your area and we will confirm quantities, wastage and lead times.
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.