Structural and base materials · Timber & Wood
Chemically Modified Timber
Sets out the chemical routes to modified timber alongside the heat route the library already covers, and explains why a change that runs through the section behaves differently from a treatment concentrated near the surface.
Educational reference entry. Suitability depends on the complete assembly, substrate, exposure, installation method, manufacturer documentation, local requirements and qualified professional review.
Overview
What chemically modified timber is
Chemically modified timber is ordinary timber that has been put through a process that changes the chemistry of the wood substance itself. Two routes account for most of what is available. In acetylation, the timber is reacted with a chemical that converts a large share of the hydroxyl groups in the cell wall — the sites that attract and hold water — into acetyl groups, so the wood has far fewer places for water to attach. In furfurylation, the timber is impregnated with a liquid derived from agricultural by-products, which is then polymerised in place inside the cell wall so that the wall is permanently bulked out.
Both routes share a defining characteristic with the heat route: what is left afterwards is a modified wood, not wood carrying a chemical reservoir. That is the line the library draws between modification and preservative treatment. It matters practically because a modification runs through the section, so a board that is cut, ripped, notched or drilled has the same material at the new face as at the original one. That is precisely where preservative-treated timber, where the treatment is concentrated in the outer zone, calls for an end-grain treatment at every cut.
The chemical routes and the heat route are not interchangeable. Thermal modification is typically associated with an increase in brittleness, which is why the published guidance around it is so concerned with pre-drilling and fixing. The chemical routes do not carry that particular trade-off in the same way, which is why they turn up in exposed positions where the timber is doing more than cladding a wall. What each process actually does to a given species is described by whoever carried it out, and their documentation is the source.
Appearance and handling differ by route. Acetylated timber usually stays close to the natural colour of the species and takes coatings readily; furfurylated timber darkens through the section, sometimes to a deep brown. Both are more dimensionally settled than the unmodified species, which is why boards move less across their width with the seasons. Both can affect metals in contact with them, so the process holder's guidance on fixings, fasteners and any metal detailing is part of the material rather than an afterthought. This entry stays generic: the processes are what it describes, and named products are not its subject.
Terminology
Common names and aliases
These names describe the same thing. The library keeps one entry per material so that an acronym, a trade name or a regional spelling never becomes a second, thinner page.
- Acetylated timber
- Furfurylated wood
- Acetylated and furfurylated timber
- Chemically modified wood
Applications
Common applications
Where this material is typically encountered. A typical application is not a statement that it suits your project.
- External cladding and rainscreen boarding where seasonal movement of unmodified boards would be visible.
- Decking, terrace boards and external walkway boards exposed to weather on all faces.
- Window and door components within manufactured joinery, where the maker has designed around the material.
- External joinery, screens, louvres and landscape structures in a domestic setting.
- Waterside and exposed timberwork where a designer has specified a modified material rather than a treated one.
Consideration
Where it is commonly considered
- Where dimensional movement of external timber is a visible or functional problem in the design.
- Where a project prefers a change to the wood substance over the introduction of preservative chemicals.
- Where components will be cut and machined a great deal and re-treating every cut would be unrealistic.
- Where the appearance of a pale timber or of a through-coloured dark one is part of the design intention.
- Where the supplier's fixing and detailing guidance can realistically be followed by whoever does the work.
Professional review
Where additional professional review is especially important
These are the situations where the answer depends on the specific building, and where a qualified professional should be involved before anything is decided.
- Any structural or loadbearing role, where the modified material's behaviour is assessed by an engineer with the process holder's data.
- Ground contact, permanent immersion or buried situations, which are specific questions for the process holder.
- Positions where fixings, fasteners or metal components are in prolonged contact and no guidance has been sought.
- Substituting a chemically modified product for a thermally modified one, or the reverse, on the assumption they are equivalent.
- Applications where an eventual weathered appearance would be unacceptable and no maintenance is planned.
Properties
Key properties to discuss
Qualitative topics worth raising. Measured values, classes and grades come from the manufacturer's technical documentation for the specific product, not from a reference page.
- Moisture behaviour
- The whole purpose of modification is to change how the cell wall interacts with water, so boards take up and give off moisture differently from the unmodified species and move less with humidity. It does not make timber impervious, and detailing still has to let water drain and the wood dry.
- Repairability
- Because the change runs through the section, a cut end is the same material as the face. That removes the end-grain treatment step that preservative-treated timber depends on, and is one of the clearest practical differences between the two.
- Appearance
- Acetylated material usually stays near the natural colour of the species; furfurylated material darkens through the section. Left outside unfinished, both weather at the surface towards grey while the interior keeps its tone.
- Installation dependency
- Fixings, fasteners and any metal in prolonged contact follow the process holder's guidance rather than habit, and that guidance is part of what is being bought.
- Weathering and exposure
- Exposure to sun and rain changes the surface regardless of the modification, so how the timber is expected to look as it weathers is a conversation to have before it is ordered.
- Maintenance
- Whether a coating is applied, and how often it is renewed, is a decision about appearance and upkeep, and coating behaviour on modified timber differs by route.
- Documentation
- The process holder's own documentation for the species and the process is the only authority on what the material does, and it is what a designer and an installer both work from.
Exposure
Exposure and environmental conditions
- How exposed is this position, and is the timber wetted on all faces or only on one?
- Will the material be in ground contact, submerged, or splashed regularly?
- Is this a coastal or aggressive environment where metals in contact with the timber are more of a subject?
- How much sun does this elevation take, and how quickly will the surface change?
- Does the detailing let water drain away and let the boards dry out between wettings?
Assembly
Substrate and system dependencies
What this material sits on, is fixed to or depends on. This is usually where performance is won or lost.
- Fixings, brackets and any metal in contact are chosen with the process holder's guidance, not by default.
- Supporting battens, carriers and substructures may be a different material and have their own requirements.
- Ventilation and drainage behind cladding or decking are part of what lets the timber dry.
- Where modified and unmodified timbers meet, the junction has two materials moving differently.
- Coatings, if used, are chosen for the modified material rather than carried over from an unmodified job.
Appearance
Appearance and finish considerations
- Colour depends on the route: one family stays pale, the other darkens through the section.
- Because the change is through the thickness, machined faces and cut ends match the rest of the board.
- Left unfinished outdoors, the surface silvers while the body keeps its tone, which is a deliberate design choice.
- Coating behaviour differs between the routes and from unmodified timber, so trial areas are worth agreeing.
- Board-to-board variation still exists, because the starting material is timber and the species still shows.
Durability
Durability questions
Questions to raise rather than a service life to expect. No material has a universal lifespan — it depends on the assembly, the exposure and the upkeep.
- Which process has this timber been through, and what does the process holder document about it?
- What species is underneath the process, and does that matter for this application?
- Is the position within what the process holder describes for this material?
- What fixings have been specified, and are they the ones the process holder identifies?
- Does the detailing let the timber dry, or are there traps where water will sit?
Upkeep
Maintenance and repair considerations
- Is a coating intended, and how often would it need renewing to keep the appearance wanted?
- How would a damaged board be replaced, and would a new board match the weathered ones?
- Are offcuts or spare boards being kept for future repairs?
- How is the timber cleaned, and does that differ from cleaning unmodified timber?
- What is the plan when the surface has weathered as far as it is going to?
Installation
Installation dependencies
What the installation depends on — not how to do it. Method belongs to the manufacturer's instructions and the trades carrying out the work.
- Fixing patterns, pre-drilling and fastener choice follow the process holder's guidance for the product supplied.
- Cut ends need no end-grain treatment for the modification itself, which is a real change to site routine.
- Storage on site keeps boards flat, supported and out of standing water before they go up.
- Metal components in prolonged contact are selected deliberately rather than taken from stock.
- Where a coating is used, its application follows the coating maker's instruction for modified timber.
Documentation
Documentation to request
Ask for these in writing, for the specific product being proposed, and keep them with the project record.
- The process holder's documentation for the process and the species supplied.
- Their fixing, fastener and metal-contact guidance for the material.
- Coating guidance where a finish is intended, including any preparation required.
- Any information about the positions and conditions the process is described for.
- A record of what was actually supplied, kept with the building information for future repairs.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- Which modification route is this, and why that route rather than the heat route?
- What species is the underlying timber, and does that affect anything here?
- Is this material doing anything structural, and if so who has assessed that?
- What fixings and fasteners have been specified, and on whose guidance?
- How is the detail letting water drain and the timber dry?
- What will this look like as it weathers, with and without a coating?
- How would a damaged board be replaced, and would it match?
- Is anything in the specification treating this as interchangeable with treated or heat-modified timber?
Blind spots
Commonly overlooked points
- The modification runs through the section, so cut ends need no end-grain treatment — a genuine difference from preservative-treated timber and an easy one to get backwards.
- Chemical and thermal modification are different routes with different trade-offs and are not substitutes for one another.
- Fixings and metal in contact are part of the specification, not a site choice made from the van.
- Modified timber is not waterproof; it is less inclined to take up water, which is not the same claim.
- The unmodified species is still in there, so figure, knots and variation come through as they would in any timber.
- Weathering still changes the surface, so the appearance in year one is not the appearance the building keeps.
What this page does not do
- What any modified timber does in a given position is described by the process holder's own documentation, and nothing here substitutes for it.
- Nothing here indicates that modified timber is structurally adequate, decay-proof or appropriate for ground contact or immersion.
- Structural use, fixing design and exposure decisions are matters for qualified professionals.
- Requirements vary by location and building type and must be verified against local requirements before work is planned.
Related entries
Alternatives to consider
Entries that may be considered instead of this one for a similar job. Not a statement that they are interchangeable, or equal in any respect.
Related entries
Common comparisons
Entries met at the same decision but which behave differently, so the comparison is about the difference rather than a swap.
Inspiration
Related Ideas Library pages
Design directions where this material commonly appears.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this material comes up.
Go deeper
Related Build Design Hub guides
Planning guidance and neutral comparisons behind the decisions on this page.
Timber & Wood-Based Materials
Reference entries for solid timber and wood-based panels and engineered products — from structural softwood and plywood to cross-laminated timber, OSB, MDF and veneers.
Browse all timber & wood entries →