Decking and Cladding in Modified Timber
Timber outdoors does not rot from rain: it rots from water that cannot leave. The whole technique of decking lies in a few millimetres of air under the board and a couple of degrees of fall.
Overview
TAV. 00Modified timber exists to solve an old problem: making species durable outdoors that would not last, without toxic preservatives and without importing tropical hardwoods. Acetylation chemically modifies the cell wall and takes away the timber's appetite for water, bringing it to stability and durability comparable to the best exotic species; thermal modification achieves a similar effect physically, by cooking the wood at high temperature, gaining durability and losing mechanical strength; strand woven bamboo comes from yet another direction, with a density and hardness no European timber reaches. The material changes, the installation rules do not: air underneath, water that runs away, stainless fixings and no direct contact with the ground.
The site score
TAV. HThe detail of the craftwhere the water through the gaps ends up
The substrate and the fall
Checking or creating the fall of the supporting surface, setting the adjustable pedestals and checking where the water collects.
Before you lay anything, throw a bucket of water down and watch where it goes. If it sits there, you've already lost the timber.site notebook — editorial synthesis
click a beat · arrow keys ← → walk the site · dashed pauses are the chemistry at work
The deformation that follows a rule
Which way a board cups, and why
It is not random and it does not depend on the quality of the timber: a board curves towards the face that dries more. If the top face is in the sun and the underside is closed against a damp surface, the top shrinks and the board becomes concave; if both faces breathe equally, the board stays flat. From this follow the two non-negotiable rules of decking — ventilate underneath and treat every face — which are not recommendations but the direct consequence of how timber loses water.
This holds for any timber, modified or not. Modification reduces the amplitude of the movement, not the mechanism: a badly ventilated acetylated board cups less than a spruce one, but it still cups.
Three routes to durability
Acetylated, thermally modified, strand woven bamboo
Three different ways of achieving the same result — timber that does not rot outdoors — with different trade-offs. Acetylation works on the chemistry of the cell wall and takes away no strength; thermal modification works on the physical structure and takes away a little, in exchange for a lower cost; strand woven bamboo modifies nothing, but starts from a density and hardness European timber does not have. The choice comes down to three parameters: how long it must last, how much it must carry, how much it may cost.
Durability class and mechanical performance must be read from the manufacturer's documentation for the specific product: two thermally modified timbers with different cycles are not the same material, and the degree of treatment changes both durability and strength.
How it ages (and what betrays it)
TAV. PQuestions from the site
TAV. QIs the grey a defect or not?
It is not, and it matters to say so before laying rather than afterwards. Silvering affects fractions of a millimetre of surface, where ultraviolet light degrades the lignin and rain washes it away leaving silvery cellulose. The strength, durability and stability of the board do not change at all. Anyone wanting to keep the original colour must budget for re-oiling every one or two seasons: a legitimate choice, but recurring maintenance, not a permanent treatment. Anyone who accepts the grey has a floor that needs nothing for decades.
How long does a modified timber deck really last?
Manufacturers declare warranties that for acetylated timber reach decades above ground, and the declared durability is comparable to the best tropical species. But real life is not decided by the material: it is decided by the installation details. Acetylated timber sitting on a surface that ponds lasts less than well-ventilated larch. In practice, if ventilation, fall and fixings are right, the deck comfortably outlasts its supporting structure, and the first thing to replace is the joists, not the boards.
What changes at the poolside?
Two things change. The first is chemical: chlorides attack ordinary stainless steel, so the fixings move up a grade and the quality of the hidden ironmongery matters as much as that of the visible screws. The second is safety: a wet surface walked on barefoot needs verified slip resistance, achieved through board grooving or specific finishes, and it is a requirement to be documented, not estimated. In practice it is also worth allowing slightly wider gaps, because large volumes of water have to leave quickly.
Can the same material be used for facade cladding?
Yes, and cladding is in fact where modified timber performs best, because water never stands on a vertical surface. The constructional logic is the decking one rotated ninety degrees: a sub-frame forming a ventilated cavity, openings at bottom and top so the air rises, a dark breather membrane behind the open joints, stainless fixings and clearance from the ground. The main difference is exposure: south and west facades silver quickly and unevenly where a partial overhang shelters part of them, which is why you often see one facade grey and another still golden on the same building.