Finishing Mass Timber: Glulam, CLT, and Exposed Structure
Mass timber is structure you are meant to see. The glulam beam is the beam and the ceiling both, which means a finish has to do two jobs that usually belong to different products: protect and beautify a visible architectural surface, and get out of the way of the building's moisture management.
Our finishes were originally developed for the unique requirements of large timbers and logs, which is the same problem at a different scale. They work equally well on glulams, CLT and NLT panels, DLT, and exposed heavy timber, and they enhance the patina and grain of whatever species you have specified rather than covering it.
Permeability is the central question in mass timber
Every mass timber project has a moisture story, and it is usually the riskiest part of the schedule.
Panels arrive at roughly the moisture content they will live at, typically around 12 percent, and then they sit exposed during erection while the building is not yet dry-in. Rain lands on them. Water pools in connections and at panel edges. Concrete topping slabs add more. The panels take some of that moisture on, and the whole question is whether they can give it back up.
This is where the choice of finish stops being cosmetic. A film forming finish, meaning most varnishes, polyurethanes, and high solids coatings, has low water vapor permeability. It does not prevent water from getting into the wood, because water finds its way through checks, at connections, at end grain, and through the top of a panel before the roof goes on. What it prevents is moisture getting back out. Wood decay fungi need sustained moisture content above roughly 20 percent to establish, and elevated moisture inside a panel is invisible until it is a structural problem.
A penetrating oil cures inside the cell structure rather than bridging over it. The wood stays permeable, so it can take on moisture and release it again. It also sheds bulk water, since liquid water beads on the cured, hydrophobic surface while vapor continues to pass. That combination, water shedding and vapor open, is exactly what an exposed structural wood surface needs.
The same property removes an entire failure mode. There is no film to crack as panels move, no film for water to get underneath, and therefore no peeling and no stripping. What happens instead is that the oil slowly thins with exposure and gets another coat.
What makes these finishes different
They are premium penetrating oil finishes, hand crafted in small batches from natural ingredients from sustainable sources.
No heavy metal driers. Conventional oil finishes use metallic catalysts to force fast curing. We use none in any product. Instead the oils are polymerized before packaging, so the curing chemistry happens under controlled conditions in our shop rather than in your building. That matters practically: the oil can sit on the surface for an hour or overnight without gumming or hardening, so it keeps absorbing the whole time rather than skinning over. It continues wicking into fibers, grain, and checks long after the surface feels dry. It also means a long, forgiving working window on the large continuous surfaces mass timber presents, where lap marks would otherwise be a real risk.
No petroleum products.
Full ingredient disclosure. We publish every ingredient, which matters if you are documenting materials for a project.
High solids content. More finish stays in the wood per coat, which means a more durable protected surface, greater coverage per gallon, and less maintenance over the life of the building.
The result dries to a natural matte sheen. The wood reads as wood.
Which product for which condition
Interior exposed structure: Original Finish. Two coats. Apply only as much as the wood will absorb, then wipe off the excess. 100% bio-based and food safe once dry.
Interior, where VOC content is specified: LEED Finish. Same penetrating oil approach, formulated for lower VOC content. If your project is pursuing credits tied to low emitting materials, or your specification sets a VOC ceiling, this is the product to look at. Contact us for current figures before you write it into a spec.
Exterior exposed structure and exterior glulam: Exterior Finish. Two to three coats, or as much as the wood will absorb. Wipe off the excess. Contains a mildewcide and fungicide plus UV blockers, both of which matter on anything the weather reaches.
Application
The method is the same at every scale, and it is short.
Apply only as much finish as the wood will absorb. Then wipe off everything that has not soaked in.
Wiping is not a finishing touch, it is the step that determines the result. Excess left standing does three things you do not want: it leaves drips, it dries too quickly into glossy patches where finish sat on the surface instead of entering the wood, and it breaks up the even glow that makes a large panel look right. On mass timber the surfaces are big and continuous, so an uneven sheen is far more visible than it would be on trim.
Avoid direct sun and hot surfaces during application, since heat flashes the solvent off before the oils can absorb. On logs and heavy timber, work the finish into checks, which are the fastest route moisture has out of the wood.
Shop or site? Both are done. Shop application gives controlled conditions and full access to every face before panels are lifted, but finished surfaces then have to survive rigging, transport, and erection. Field application after dry-in avoids that risk and lets you finish the surfaces as installed, but access to soffits and high ceilings costs scaffolding or lifts. Decide early, because it changes both the protection plan and the schedule.
Maintenance
Add more oil when the wood looks dry. That is the whole procedure.
No stripping, no sanding, no film edges to feather, no scaffolding a building for a refinishing campaign. Clean the surface, apply a maintenance coat, wipe the excess. Interior work in a conditioned building goes a long time between coats. Exterior surfaces need attention when water stops beading or the color begins to fade, which is as often as annually on hard exposures and typically every two to three years elsewhere.
For a building owner, this is the difference between a maintenance line item and a capital project.
For specifiers
LEED Finish carries third-party VOC emissions certification. It has been tested by Berkeley Analytical, an independent ISO/IEC 17025 accredited laboratory, to the California Department of Public Health Standard Method v1.2-2017, the emissions test more commonly known as CA Section 01350.
It met the acceptance criteria for both defined exposure scenarios, School Classroom and Private Office, on individual VOCs of concern and on formaldehyde. The tested sample was applied to Douglas fir tongue and groove at a loading of 186 grams per square metre.
To be precise about scope, since specifiers reasonably ask: that certificate covers LEED Finish. Our other products are formulated from the same family of natural ingredients but have not been through the same emissions testing.
That standard is recognized by USGBC LEED v4 and v4.1 under BD&C, ID&C, and Residential BD&C Multifamily, by the WELL Building Standard v2 under Feature X06, and by the Green Globes ANSI/GBI 01-2019 assessment protocol.
One distinction worth being precise about, because the two get conflated constantly. VOC content, measured in grams per litre, describes what is in the can. VOC emissions, measured in a chamber under CDPH 01350, describe what actually reaches the air in an occupied room. For interior low-emitting materials credits it is the emissions test that counts, and that is the one LEED Finish has.
If you are writing our products into a specification or assembling a submittal, contact us directly rather than working from the website. We can supply the emissions certificate and test report, ingredient disclosure, VOC content figures by product, and coverage rates for estimating. We would much rather answer those questions before the spec is issued than after.
What these finishes will not do
They will not build a film. If your design intent requires a hard, glossy, sealed surface, a penetrating oil is the wrong product and we will tell you so.
They will not prevent checking. Glulam and heavy timber check as they equalize. Filling checks with finish slows drying through the fastest route the wood has and reduces further checking, but nothing eliminates it.
They are not a preservative or an insecticide. Where mold, insects, or sap stain are concerns, a borate treatment goes on before the oil.
They cure slowly, by design. No driers means patience. Build the cure window into the schedule rather than discovering it.
They will not work through a barrier. Mill glaze, dust, form release, and existing coatings all have to come off first.
Safety: oily rags will start a fire
Natural drying oils generate heat as they polymerize. On a project scale this is a real jobsite hazard, not a footnote. A pile of oil soaked rags holds its own heat, the temperature climbs, and the pile can ignite with no spark and no flame anywhere near it.
Lay used rags flat and separated on a non combustible surface outdoors until stiff and fully dry, submerge them in water in a sealed metal container, or use listed oily waste cans. Make it part of the daily close out, and make sure the crew knows why.
Simple. Beautiful. Natural.
For ease of maintenance, durability, performance, and indoor air quality in the buildings you are putting up, a permeable penetrating oil is the finish that works with mass timber rather than against it.
- Specifying, estimating, or planning a finishing sequence? Call us at 541-844-8748 or write info@heritagenaturalfinishes.com. We are glad to talk through a project before it is bid.
- Further reading. Boyce Thompson's Innovations in Mass Timber: Sequestering Carbon with Style in Commercial Buildings surveys what is actually being built in mass timber and why, which is useful whether you are specifying your first CLT project or explaining the carbon case to a client.
- Photo: our Exterior Finish on exterior glulam after a previous film forming coat was removed.