Timber frame construction using English oak has always been popular at the premium end of the domestic market, and for good reason. A well-designed and constructed home that uses beautiful English oak as a key part of the aesthetic is truly impressive.
In recent years there has been a degree of concern as to the viability of oak timber frame when pursuing the highest levels of energy efficiency. However, thanks to the application of the latest insulation technologies, the highest levels of thermal efficiency are now being achieved. This application of modern technology will help to ensure England oak remains a popular choice.
For designers looking to use English oak it is worth understanding not only the wood itself, but also the ways in which it is cut and processed, as this can make all the difference to the effectiveness and longevity of the design.
Many people regard English oak (Quercus robur) to be stronger than imported oak because the wood is coarser and features a more interlocking grain. The closeness of the grain around knots not only gives the timber more strength, it also produces a more interesting aesthetic. With French and German oak the grain typically runs in a very straight line, which is excellent for joinery, but the wood is ‘mellow’ and without the interlocking grain. Another benefit of the coarser wood found in English oak trees is that it provides more resistance to worm attack, which is endemic in oak logs from the continent.
Oak beams are usually cut from trees between 90 and 120 years old, ideally felled during the winter when the sap is down. For construction use beams tend to be fresh sawn or ‘green’, which means that they are in fact wet. While this is of concern to many architects because of oak’s natural tendency to shrink and split, it is the ideal way to use the timber. The alternative, dry oak, is very hard and therefore more difficult to work.
Oak dries very slowly (a large beam can take 8 – 10 years to fully dry) and is consequently expensive. There are four options for the dryness of the timber, and this will have an impact on the price – kiln dried oak beams, air dried oak beams, semi seasoned/weathered structural grade oak beams, and fresh cut structural oak beams cut from old felled logs which have been ‘settling’ for over three years. Fresh sawn beams are available up to 9.5m long, and up to 500 x 500mm section size.
In the UK, structural oak (beams) are graded to the British standard BS 5756, Specification for Visual Grading of Hardwood, and EN14081. For large sections (20,000mm square +) the grades are THB (lower grade) and THA (higher grade). For smaller sections the grades are TH2 (lower grade) and TH1 (higher grade). The higher grades will be stronger than the lower grade, allowing for the use of smaller sections while maintaining their performance. In order of strength these are THA, THB, TH1, TH2. There is also an overarching grading system defined in standards BS EN 338:1995, Structural Timber Strength Classes, and BS 5268, Structural Use of Timber. This system allows for comparison between species and includes grades such as C24 for soft woods and D30 for hard wood.
The direction of the grain is also a consideration, if a tree is nice and straight, so the grain is likely to be as well. If the tree is bent or twisted the grain can present a structural problem when cutting a straight beam. However, this also presents another benefit of English oak, which due to our fragmented and open woodlands, can often be misshapen.
This becomes a distinct advantage when manufacturing curved oak beams as a curved log will have definite benefits. When cutting curves from a straight log the grain will inevitably ‘run off’ the arc of the final cut. Drying cracks will run along the grain and therefore across the face of the curve, with the likelihood of the curve snapping. With English oak it is common practice to store curved logs for just this purpose, upon which a curve template can be laid for cutting. The grain of the wood will follow the curve of the template and the produced piece will have much greater inherent strength when in use.
It is important also to appreciate from where exactly in the tree the timber is cut. Beams cut from the core (heart) of the tree will be stronger as the grain is more compact. When a sawmill addresses a newly-felled tree, full-length logs are cut from a butt end at the bottom of the trunk, typically 2.5 to 4.0m, depending on where the first bough usually appears. This butt end is generally clean and free of knots and will be planked and dried to produce joinery quality wood. The sawmill will take out the middle of the tree up until the point where knots become problematic, and that is generally where beam quality wood is cut from. The very top of the tree, with lots of knots, becomes the lower quality wood, often cut for products such as landscaping sleepers.
Larger timber sections will typically be cut with a ‘boxed heart’, meaning the heart of the tree runs through the length of the beam. This cutting technique produces a very strong and normally straight beam. Other cutting techniques include half-cut, where the heart runs down one face of the beam, or quarter cut, where four beams are cut out of the log leaving the heart behind. Alternatively smaller pieces are often cut away from the heart or in sawmilling parlance ‘off the heart’.
Better understanding of natural materials, particularly wood, is key to its effective use. English oak delivers many benefits over imported oak, and it will continue to be a popular choice.