The question comes up on almost every structural project we take on. Oak or Douglas Fir? The honest answer is that it depends on three things: budget, how much movement you can live with, and what you want the finished frame to look like in twenty years. Here is how the decision actually breaks down.
Economy
Douglas Fir costs significantly less than structural oak — typically 30 to 50 percent less for comparable section sizes and grade. For a garden room or house extension where the frame will be largely concealed within the build-up, that difference is straightforward to justify. For an exposed internal frame where the timber is the architectural feature, the calculus becomes more nuanced.
Oak commands a premium because it earns one. A well-maintained oak frame has a demonstrable lifespan measured in centuries rather than decades. Medieval barns still standing in Powys are oak. The oak in a new building today will outlast several generations of owners. If longevity over an architectural timescale matters to your project, oak’s premium is real value. If the building is a garden studio or a single-storey extension that will be refurbished within fifty years regardless, Douglas Fir is the more rational economic choice.
At our sawmill we grow and mill both species ourselves. That means the price difference reflects genuine material differences rather than supply chain markups. We will tell you plainly which species makes sense for your project and budget — and we will not push oak for its margin.
Movement After Installation
This is where the practical difference between the two species is most visible, and where clients most often have questions after handover.
Both oak and Douglas Fir are almost always installed green — that is, freshly milled with a moisture content above fibre saturation point. As the frame dries to equilibrium with its environment, the timber moves. It shrinks, and because tangential shrinkage is greater than radial shrinkage, it does so unevenly. The result is checking: surface cracks that follow the grain, most visible at the ends of beams and at corners. This is normal, structural, and expected. It is not a defect.
Oak moves more than Douglas Fir. Tangential shrinkage in oak runs to around seven or eight percent; in Douglas Fir it is closer to five. Oak checks are deeper, more defined, and more numerous. In a traditional or rustic context, this is often exactly what clients want — it is the look of a building that has been built with real material that is doing what real material does. In a contemporary setting with smooth plaster and minimal detailing, the checking of green oak can look out of place.
Douglas Fir dries more quietly. The checks are smaller, less dramatic, and the overall movement of the frame is more contained. For clients who find the idea of visible cracking in their new frame unsettling — even after being told it is structural and normal — Douglas Fir is the more reassuring choice. For clients who want the frame to develop character over time, oak’s movement is not a drawback. It is the point.
Most significant movement happens in the first one to two years. By year three to five, a well-ventilated frame has largely equilibrated to its environment and the checking stabilises.
Aesthetics
Oak is visually complex. Quarter-sawn boards show medullary rays — the silver fleck pattern unique to oak among British structural species. The heartwood is a warm gold-brown when freshly cut. It deepens to amber in the first few years, then silvers and darkens over decades until it reaches the rich, almost chocolate tone you see in old barn timbers. It is a wood that improves visually with age in a way that few materials in construction do.
Douglas Fir is cleaner. Straight, consistent grain with a warm honey colour and a subtle rosy undertone when fresh. It does not have the complexity of oak but it does not compete with it either — in contemporary interiors with pale plaster and steel windows, Douglas Fir sits in the background in the right way. It takes oil finishes cleanly and holds them well. Over time it silvers gently at exposed surfaces without the dramatic patina shift of oak.
The short summary: if the frame is the feature and you want it to tell a story over decades, oak. If the frame is the structure and you want the interior to read as contemporary and resolved, Douglas Fir. We build extensively with both and carry no preference either way — the right species is the one that fits the project.
The Practical Summary
Choose Douglas Fir when budget is a constraint, when the project is a garden room or extension that will be insulated and lined, when the aesthetic is contemporary, or when clients are likely to find heavy checking uncomfortable.
Choose oak when the frame will be exposed and visible as the architectural character of the space, when the project has a traditional or heritage context, when longevity over an architectural timescale is a genuine priority, or when the client wants something that will develop character for the next hundred years.
Both species are grown in our North Wales woodland. Both are milled at Bryncoch Sawmill before they enter the workshop. If you are weighing the decision for a specific project, get in touch — we can talk through the structural requirements and give you an honest steer.