The short answer
Use green oak for the structural frame and air-dried oak for the parts that must stay tight: glazing cover boards, door and window frames, and sometimes braces. Green oak is cheaper, easier to cut and available in large sections. Air-dried oak moves less after fitting but costs more and is rarely dry all the way through in big sizes.
On this page
Some suppliers offer frames in air-dried oak and describe green oak as the budget option. Others say the opposite: that green is the proper material and air-dried a needless expense. Both are selling something. The honest answer depends on which part of the building you are talking about.
Three states of the same timber
Green oak is freshly sawn. It can hold 60 to 80 per cent of its dry weight in water and has had no drying beyond the weeks between the sawmill and the workshop.
Air-dried oak has been stacked outdoors under cover, with spacers between the pieces so that air can pass through, and left. The traditional rule is a year for every 25 mm of thickness. The surface of a well-seasoned beam settles at around 20 to 25 per cent moisture, in balance with outdoor air.
Kiln-dried oak has been taken down to 8 to 12 per cent in a heated chamber. That works for boards. It is not practical for structural sections, because a thick piece dries so unevenly in a kiln that it tears itself apart.
The catch with large air-dried sections
Apply the year-per-25 mm rule to a 200 mm square post. Drying from each face towards the centre, the heart is 100 mm from the nearest surface. That is four years at the very least, and in practice longer, because drying slows as it goes deeper.
Very little oak is stored that long in framing sizes. Much of what is sold as air-dried in sections above 150 mm is dry on the outside and still wet at the core. It will go on shrinking after it is built in, only more slowly than green oak and by a smaller amount. If you are paying extra for air-dried beams, ask how long they have been stacked and what a moisture meter reads at depth, not just at the surface.
How they compare
| Green oak | Air-dried oak | |
|---|---|---|
| Moisture when delivered | 60% or more | About 20 to 25% at the surface |
| Movement after fitting | Up to about 7% across the grain | Perhaps a third to a half of that |
| Ease of cutting | Cuts cleanly with hand tools | Hard, slow, heavy on tool edges |
| Large sections | Readily available to order | Limited stock, long lead times |
| Surface | Clean and pale when raised | Often already split and weathered |
| Cost | Lower | Markedly higher |
The movement row is the one buyers care about. The cost and cutting rows are the ones that explain why most frames are green.
Where air-dried oak earns its price
Around glass
The cover boards that hold glazing against the frame need to stay flat and tight. If they cup or shrink, the seal suffers. Most careful framers make these from air-dried or kiln-dried oak even when the frame behind is green.
Doors and windows
An opening casement or a door has to keep its shape within a millimetre or two. These are joinery, not framing, and should be made from dry stock, often engineered oak, and hung in a sub-frame that lets the green posts move behind them.
Braces and small members
Curved braces are often cut from air-dried pieces because suitable bent timber has usually been collected and stored over time. In sections of 75 to 100 mm, air-dried stock really is dry through, so there is a true benefit.
Additions to an old frame
When repairing or adding to a historic frame that finished moving centuries ago, dry oak avoids introducing a new piece that shrinks away from old joints.
Where green oak is the better choice
For the posts, plates, tie beams and principal rafters of a new oak frame extension, green oak is the sensible material and has been for as long as frames have been built.
It can be ordered to exact sizes and lengths from the sawmill instead of picked from whatever a yard has in stock. Joints cut in it are crisp. The frame goes up clean, without the grey surface and open splits that long-stored timber carries. And the shrinkage, which sounds like a drawback, is used on purpose: pegs driven through offset holes pull each joint tight, and drying tightens it further.
The movement is predictable. A competent designer details the glazing, the plaster edges and the junction with the house so that 8 to 14 mm of shrinkage across a 200 mm post causes no harm.
Questions to ask a supplier
- Which parts of the frame are green and which are dried? A mixed answer is a good sign.
- For anything sold as air-dried, how long has it been stacked, and at what section size?
- How is the glazing fixed, and what are the cover boards made from?
- Is a return visit included after the first year or two to adjust and re-seal?
A framer who answers those easily is worth more than a brochure that promises timber that never moves. No oak does that. Good design allows for it.
To see what a frame might cost, try the cost page, or start a quote with the form below.