What is green oak?

Ask a framer what the posts in an oak frame extension are made of and the answer will nearly always be green oak. The word puts people off. It sounds unfinished, as if the timber has been used before it was ready. In fact green oak is the traditional material for structural frames, and almost every timber-framed house still standing in Britain was built with it.

What “green” means

Green describes moisture, not colour. A living oak is full of water. When the tree is felled and sawn into beams, the wood can still hold 60 to 80 per cent of its dry weight as water, and sometimes more. Timber in that state is green.

Compare that with the oak in a dining table, which has been dried to around 10 per cent so that it stays stable in a heated room. Getting a 200 mm square beam down to that level in a kiln is not practical. The outside would dry and split long before the heart caught up.

So framers do what they have done for 800 years. They cut the joints while the oak is wet, raise the frame, and let it dry where it stands.

Why framers want it wet

Fresh oak cuts cleanly. A sharp chisel takes a shaving off green oak in a way that it simply will not from a beam that has sat in a barn for a decade. Seasoned oak is closer to stone. It blunts tools, and long, accurate mortices become slow and expensive.

There is a structural reason too. As a green frame dries, each timber shrinks across its width. The pegs through the joints are made from drier oak, and the peg holes are bored slightly out of line so that driving the peg pulls the joint tight. Framers call this draw-boring. Shrinkage then tightens the frame further instead of loosening it.

Cost matters as well. Green oak is the cheapest way to buy large structural sections, because nobody has paid to store it for years.

How it dries

The rule of thumb in TRADA’s guide to building with green oak is that oak dries at about 25 mm a year, measured inward from each exposed face. A 200 mm post has 100 mm to its centre, so it takes around four years to reach a steady state. A 100 mm brace gets there in two.

Indoors, in a heated room, the surface dries faster than that. This is where the visible changes come from.

Shrinkage

Oak shrinks very little along the grain, around a tenth of one per cent. A 4 m beam stays, for practical purposes, 4 m long. Across the grain is different: expect 4 to 7 per cent as the timber goes from green to dry. On a 200 mm post that is roughly 8 to 14 mm.

A good frame design expects this. Glazing is fixed to the outside face of the oak on flexible gaskets, never set tightly into a groove. Plaster stops short of the timber with a small shadow gap. Where oak meets a rigid material such as an existing brick wall, the detail allows a few millimetres of movement.

Splits

As the outer layer dries and shrinks around a still-swollen core, it has to give somewhere. The result is a split along the grain, called a shake or a check. Some run for a metre or more and open to the width of a pencil.

They are normal. The engineer who sizes the frame works to strength classes for oak that assume these splits will appear, so they do not reduce the safety of the structure. They tend to open most in the first two heating seasons and then settle.

Twist

Timber with spiral grain can turn slightly as it dries. Framers sort for this when they choose which piece goes where, putting the straightest stock in long posts and beams. A small amount of twist in a brace or rafter is part of the look.

Colour and staining

Freshly sawn oak is pale and slightly pink. Inside, it mellows to a honey brown over a few years. Outside, with no finish, rain and sunlight turn it silver-grey within two or three.

Oak is rich in tannin, and in the first months rain will wash some of it out. Tannin leaves brown streaks on paving, render and stone beneath the frame. Most of it weathers away or scrubs off, but it is worth protecting a new limestone terrace until the worst has passed.

Tannin also reacts with iron. Ordinary steel screws or a stray filing from an angle grinder will leave blue-black marks on wet oak. That is why fixings into green oak should be stainless steel, and why framers keep steelwork away from the timber during the build.

Does it need treating?

Structurally, no. Oak heartwood is classed as durable under the British and European standard for timber durability, which means it resists rot without preservative. Sapwood, the paler band under the bark, is not durable and should be cut away or kept to a minimum in any timber that will get wet.

What oak cannot stand is sitting in water. Posts are raised off the ground on a brick, stone or concrete plinth, usually with a damp-proof layer under the foot.

Finishes are a matter of taste. Many people leave the frame bare. An oil or wax indoors will deepen the colour and make marks easier to wipe off, but it is best left until the first couple of years of movement are over.

Green oak in an extension

For a room joined to a house, the wetness of the timber has three practical effects.

1. Expect noise. A drying frame creaks and occasionally cracks loudly, mostly in the first winter with the heating on.
2. Budget for a return visit. A careful builder will come back after a year or two to adjust doors, re-seal where oak has pulled away from plaster and check the glazing gaskets.
3. Bring the heat up slowly. Running underfloor heating at full power from the first day dries the surface too quickly and makes the splitting worse. A gentle first winter is kinder to the frame.

None of this is a defect. It is the timber doing what oak has always done, and a frame designed with it in mind will be tighter in ten years than on the day it was raised.

If you would like prices for a frame of your own, the cost page gives typical ranges, and the form below starts a quote.