South Downs Green Wood Centre - Timber Frame

Green Oak Timber Frames
We design our green oak timber frames to architects and engineers requirements. This means that our buildings will comply with all current building regulations. However Green Oak is not a "regular" building material (although along with stone it may be one of the oldest) and it requires special attention to ensure that the current building regulations are met including insulation and draft exclusion. Surprisingly oak and some other timbers out perform a lot of more modern building materials with regard to fire resistance since it chars on the outside, protecting the inner core of the timber from damage, steel especially warps expands and twists when exposed to high temperatures and other materials are just very combustable in comparison. For out-buildings we use traditional designs dating back hundreds of years together with traditional jointing techniques to simulate a traditional granary or barn, using the same materials and methods a new authentic building can be produced. For some buildings where it is necessary to minimize costs more modern building materials are necessary to minimize labour and time with Oak being used where it's natural strength and durability are required, such as in open fronted garages. These buildings can be made with as much or as little Oak as required. A full oak garage with framed upper floor is entirely possible. This page attempts to answer some of the more common questions that we are asked. To enquire about what we can do for you please contact us by email or by phone on our contact page. Frequently Asked Questions What is green oak? What is Shrinkage & Movement? Why? References & Further Reading
What is Green Oak? The term "Green Wood" is not a specifically defined one. Since all wood has water in it to some extent even if kiln dried, it is incorrect to say that Green Wood contains water and dried wood does not. The moisture content of Oak will vary depending on a number of factors including the following non-exhaustive list: how recently it was felled; what point of the year it was felled; what environment it has been stored in. The moisture content in Oak is made up of water in the cell walls known as "bound water" and liquid or "free water". The bound water is held chemically within the cell walls of the wood. The moisture content figure can be calculated in different ways, the moisture content as a percentage of the oven dried weight as a basis can be calculated from the following formula: MC = 100 * (Wc - Wd) / Wd [1] where MC = percentage moisture content Wc = weight current of specimin Wd = weight when "completely" kiln dried This tells us the amount of total moisture within wood when compared to the dried specimin. 100% moisture content means that half the mass of the timber is water and half is the timber itself. Mostly English and European Oak, when freshly felled, has a moisture content of between 60% and 80%. This will vary tree to tree and other species may have up to 200% moisture content. As timber dries from it's freshly cut state the free water leaves the wood. When there is no free water left inside the wood it is said to have reached it's fibre saturation point. This is approximately at 30% moisture content for most timber species. Up until this point the water leaving the timber will not have had much of an effect on the dimensions of the timber (though other factors such as the releaving of forces during milling will). From 30% to the final moisture content value is when the timber will shrink and warp. The shrinkage will be approximately linear over this range however the shrinkage is not consistent across all dimensions of the grain. See shrinkage & movement below. The final moisture content value will directly depend on the environment it is in. If moved from a dry environment to a wet one the timber will absorb water, the moisture content increase and the timber will swell. Shrinkage & Movement Movement in the timber can be attributed to a number of factors. The main two will be release of forces in the felled timber as it is processed and the drying of the timber to below the fibre saturation point. The former is related to how the tree has grown and from where in the tree the timber is being milled from. As the section of timber is removed from the rest of the timber it may crown or "banana" due to the release of the force holding it within the tree. It is advisable to leave the timber to settle for as long as possible before use so that any major splitting defects that might occur have started. Sometimes it is possible to hear the timber creaking and popping as the timber moves during and after the milling process. Excessively bent timbers should be discarded or stored for use in particular situations. It is tempting to re-saw a smaller timber straight again after the bending has occured from the timber, however particular attention should be made to the cross grain that would result, in the same way that milling a straight beam out of a curved tree would result in the grain traveling from one side of the timber section to the other rather than along it's length. As mentioned above the drop in moisture content will cause oak timbers to shrink and warp. For this reason certain timbers in a frame may be specified as air-dried or kiln-dried. These timbers will carry a cost that would be prohibitive if applied to all the timbers in the frame. An understanding of the way that timber shrinks is essential to the design of the jointing of green timber frames. Various techniques have been developed over the many centuries that timber has been traditionally used. They will not prevent the shrinkage , movement or appearance of checks in the timber but instead ensure that the frame itself does not suffer any weaknesses due to this movement. As an estimate the following figures can be used as a guide: Green to oven dry Longitudinal shrinkage 0.15%. Green to 10% MC radial shrinkage is around 4.5%. i Green to 10% moisture content tangential shinkage is around 7%. [2] i It is due to this uneven shrinkage that proper selection of material must be made and traditional methods of jointing have been developed over the years. Also it should be accomodated within the design stage for all joinery and envelope details of the frame, since fissures and checks will appear within the wood and where green oak is exposed on the inside and outside of the frame a passage for drafts may result. Why? If Green Oak is so prone to movement why not air or kiln dry it? Air dried oak is exceptionally hard to work. It requires effort on a small scale and and a lot of effort on a large scale. Only certain timbers are selected to be air or kiln dried within a frame for special purposes or for repairing existing frames where movement would be deemed unacceptable. Due to the storage required the kiln and air drying of oak would become prohibitively expensive. There are also other benefits. As the frame begins to season ( dry ) it will move and warp slightly, this causes the joints to bind together and helps fix and strengthen the frame further. Oak as well as most other timbers become stronger as they dry. Since a frame needs to be erected whilst it is green before the movement has taken place it is designed usually in the "green" state to take the design loads of the building, this results in a seasoned building being stronger than required when dried, ensuring the longevity of the building. Oak is also a durable timber and as long as it is allowed to dry and is far enough from the ground on a sub wall containing a damp proof course, will last a life time (whose life is up to you). References [1] WOOD HANDBOOK Wood as an Engineering Material, Algrove Publishing, Ch 12 page 264 ISBN 1-894572-54-8 [2] Green Oak In Construction, TRADA Technology, Ch 4 page 24 ISBN 1-900510-45-6