The short answer
A green oak timber shrinks by roughly 4 to 7 per cent across its width as it dries, so a 200 mm post loses 8 to 14 mm over about four years. Its length hardly changes. Cracks along the grain are a normal part of this and do not weaken a properly designed frame.
On this page
The first loud crack from a new oak frame usually comes on a cold evening a few months after the heating goes on. It sounds like something breaking. It is the timber drying, and every oak building ever made has done the same.
The numbers
Wood shrinks as it loses water, but not equally in every direction.
| Direction | Typical shrinkage, green to dry | On a 200 mm by 200 mm post, 3 m long |
|---|---|---|
| Along the grain (length) | About 0.1% | About 3 mm |
| Across the grain (width and depth) | About 4 to 7% | 8 to 14 mm |
The difference is why oak frames work at all. The lengths that set the shape of the building barely alter. Only the thickness of each piece reduces.
How long it takes depends on size. The usual guide is 25 mm of drying a year from each face, which gives about two years for a 100 mm brace and four for a 200 mm post. In a heated room the first two winters do most of the visible work.
Why it cracks
The outside of a beam dries first and tries to shrink round a core that is still wet and full size. Something has to give, and the surface splits along the grain. Framers call these splits shakes.
Shakes follow the grain, so they run lengthways and often spiral gently. They can be a few millimetres wide and more than a metre long. They open in dry weather and close slightly in damp.
They do not cut across the fibres that carry the load. The strength figures engineers use for oak already assume that shakes will be present, so the beam was sized with them in mind.
What would be a concern is different in kind: a crack running across a timber, a joint that has opened so that a peg is visibly bent or sheared, or a beam that has sagged. Those are rare and should be shown to the framer.
Where the movement shows
At joints
A tenon shoulder that met its post perfectly on raising day may show a gap of a millimetre or two later, as the post it sits against becomes thinner. Pegs driven through offset holes hold the joint tight in the direction that matters.
Against plaster
Run plaster hard up to green oak and a crack will open along the edge within a year. The better detail is a small planned gap or a bead, so the line stays neat as the timber draws back.
Round glass
Glazing must never be trapped in a groove between two green timbers. It is fixed to the outer face on a compressible gasket, with a cover board over it, so the oak can move behind while the seal stays put. Ask to see this detail on a drawing before you order a frame.
Where the frame meets the house
Brick and stone do not shrink. The junction between an oak frame extension and the existing wall needs a flexible, lapped weathering detail instead of a rigid mortar fillet.
At doors
An opening door hung straight on a green post will bind or drop. Doors should sit in their own dry sub-frame.
What a good frame design does about it
Shrinkage is not a fault to be fixed. It is a known quantity to be designed round. In practice that means:
- all joints draw-pegged, so drying tightens them;
- beams and plates sized for their dried dimensions;
- face-fixed glazing with dry oak cover boards;
- movement gaps wherever oak meets plaster, masonry or joinery;
- floors and worktops kept independent of the posts;
- a return visit after one or two heating seasons to adjust and re-seal.
If a quotation says nothing about any of this, ask.
What you can do as the owner
Go gently with the heat. Fast drying makes wider shakes. In the first winter, bring underfloor heating up by a degree or two a week instead of running it at full temperature from the first day. A wood-burning stove close to a post will dry that post much faster than the rest.
Do not fill the cracks. Filler will be squeezed out or left hanging as the shake goes on moving. If a split bothers you after three or four years, when the timber has settled, it can be filled then with a flexible, oak-coloured sealant. Most people stop noticing them long before that.
Leave finishes until later. Oil or wax applied in the first year will craze as the surface moves. Wait for two winters.
Keep steel away. Wire wool and steel brushes leave fragments that turn black on oak that is still damp.
Expect noise. Creaks and the occasional bang are normal for the first couple of years and then fade.
Does it ever stop?
Mostly. After four or five years the frame reaches balance with the air in the room. From then on it swells and shrinks a little with the seasons, as all timber does, but by fractions of a millimetre.
If you want to know more about the material, start with what green oak is. For typical prices see the cost page, and to ask a framer about a design for your house, use the form below.