If you commission a structural timber frame, the timber in it will almost certainly be green. Understanding what that means — and what it does not mean — is one of the most useful things you can know before the frame goes up.
The Definition
Timber is described as green when its moisture content is above fibre saturation point, roughly 28 to 30 percent. Below that threshold, the cell walls of the wood begin to dry and the timber starts to shrink and move. Above it, the wood is dimensionally stable but still carrying the moisture it held as a living tree.
All freshly milled timber is green. The moment the log is sawn into beams it begins drying, but for structural sections — posts and beams of 150mm and above — that process takes years under natural conditions. The traditional rule of thumb is one year of air drying for every 25mm of section thickness. A 200mm x 200mm post takes eight years to air-dry to a stable moisture content. Kiln drying large structural sections is possible but energy-intensive, expensive, and not widely available for green oak or large Douglas Fir at the volumes structural frame builders actually use.
The practical consequence is that unless you are specifically sourcing reclaimed timber — beams salvaged from a demolished building that have already dried over decades — the frame going up in your building will be green. This is not a compromise or a shortcut. It is the normal and correct way to build with structural timber.
Why Green Timber is Preferred for Framing
Green timber is easier to work than seasoned timber. It is softer, cuts more cleanly, and responds to hand tools in a way that dry timber does not. Mortise and tenon joints cut in green oak fit together with a precision that is difficult to achieve in dry stock. As the frame dries, the tenon shrinks into the mortise and the joint actually tightens. Traditional timber frame joinery has been designed around this behaviour for centuries — the connection improves as the building ages.
Green timber also gives the frame builder more latitude. Sections can be adjusted, joints refined, and the structure tuned during fabrication in ways that seasoned timber — which is harder, more brittle, and more prone to splitting under the mallet — does not always permit. At Bryncoch Sawmill, timber goes from the mill to the workshop in weeks rather than years. That speed is part of the integrated supply chain, and it produces better joinery.
What Happens as the Frame Dries
Checking is the most visible sign of a green frame drying. Checks are surface cracks that follow the grain — most prominent at beam ends, at corners, and at the faces of wide boards. They form because the outer surface of the timber dries faster than the core. The differential shrinkage creates tension, and the wood releases it by splitting along the grain lines of least resistance.
Checks are cosmetic. They do not affect structural performance. A checked beam carries its load exactly as a smooth beam does — the checks run parallel to the grain and do not interrupt the fibres that give the timber its strength. This is well established in structural timber engineering and has been for as long as timber frames have been assessed.
Beyond checking, the frame will undergo minor movement — slight changes in dimension as sections dry and stabilise. Some twist or bow is possible in longer members, particularly if air circulation is uneven during the drying period. In a well-built frame these movements are absorbed by the joinery and the racking resistance of the structure. They are not a sign that something has gone wrong.
How Species Affects the Behaviour
Oak checks heavily. It has relatively high tangential shrinkage and the ray structure that gives it its silver-fleck figure also makes it prone to deep, defined checks at the ray surfaces. This is characteristic of oak — the checking pattern in an old oak frame is part of its identity. It stabilises, but it takes longer to reach equilibrium than most other species, and the final checks are larger.
Douglas Fir checks less. Lower differential shrinkage between the tangential and radial axes means the tension is released more gradually and in smaller increments. The checks are narrower, less dramatic, and the overall movement of the dried frame is more contained. For projects where the aesthetic needs to read as clean and contemporary, Douglas Fir’s quieter drying behaviour is an advantage.
Ash and sweet chestnut fall between the two in checking behaviour. Reclaimed timber of any species has already dried and checked and is essentially stable from the day it is installed — which is part of what makes it valuable, and part of why it costs more.
Timeline and Aftercare
Most of the movement in a green frame happens in the first twelve to eighteen months. The frame is drying most actively during this period, and checks that are going to open will largely do so in year one. By year two or three, movement has slowed significantly. By year five, a frame in a well-ventilated building has typically equilibrated to its environment and the timber has reached a stable moisture content.
There is nothing to do during this process except leave it alone. Do not fill checks with flexible sealant in the first year — the timber is still moving and the sealant will be pushed out. Do not cover green timber with vapour-impermeable finishes that trap moisture inside. Make sure the building is ventilated during the first drying season. After year two, checks can be filled if appearance requires it, and any oil or wax finish can be applied without the risk of trapping residual moisture.
Every frame we deliver at our workshop is built green. We explain the drying process to clients before the frame goes up, not after. If you are commissioning structural timber work and want to understand exactly what to expect from the material as it dries, we are happy to walk through it in detail at any stage of the project.