Oak-framed buildings are enjoying a renewed popularity due to the material’s strength characteristics. Ed Hamilton of Oakmasters explores why green oak has a place in modern construction and addresses structural questions. Centuries-old barns, houses and churches all prove that oak can withstand the test of time. According to European standards, which classify timber into five classes from very durable (1) to not durable (5) based on a number of biological tests, oak gets a score of 2, surpassed only by a handful of tropical hardwoods.
In bending tests, steel proved to be three times stronger than oak, however, it is also ten times heavier, which works in favour of oak in terms of its weight/strength ratio.
Other tests looking to establish comprehensive and shear strength place oak in the medium to high strength category. It is notable that oak’s strength increases as it dries.
It is important to use strength-graded timber in order to ensure its structural suitability. TRADA, the leading authority on wood, allows timber to be graded visually by certified organisations. Many oak-framing companies are certified
to stress grade oak during the production process. There are four stress categories – TH2, THB, TH1 and THA – in increasing order of strength. Main oak frame beams and posts will typically be in the THB category while THA is usually applicable to oak flooring.
Moisture and drying
As oak dries, it shrinks and cracks until it reaches its equilibrium level of moisture content. The majority of shrinking occurs in two directions. Typical shrinkage values from green to 12 per cent moisture content are as follows:
- • longitudinal shrinking (along the grain) ≈ 0.15 per cent
- • radial shrinking (across the grain) ≈ 4.5 per cent
- • tangential shrinking (parallel to growth rings) ≈ 7 per cent
Fissures tend to appear only from the centre to the outer edge of the log, which means that any beam will retain a minimum of half of its structural integrity at any one time. Skilled craftsmen take this into consideration, while advances in design technology for oak framing mean that allowances are made for loading stress and shrinking very early on in the process.
Air-dried vs green
This age-old conundrum still seems to create confusion but it all comes down to the environment for which the oak is intended.
Typically, oak placed in an indoor heated environment will eventually reach a moisture content equilibrium of approximately 7-9 per cent. Oak placed in an indoor unheated environment will stabilise at around 12-14 per cent and where exposed to sun and rain, it will vary between 12 and 20 per cent.
In short, an oak frame will carry on shrinking and expanding whether it is green or air-dried. However, the combination of hardness, drying time, drying movement and pre-existence of fissures makes air-dried oak a less viable commercial proposition.
Provided that shrinking and cracking are taken into consideration when designing the frame, the main difference is that air-dried oak takes longer to produce due to storage requirements; is harder to work with due to existing fissures and hardness and is ultimately more expensive, while offering the same structural guarantees as green oak.
Creating an energy efficient building envelope
In creating a seamless envelope, conforming to tightening Building Regulations is the first requirement. While an oak frame can be combined with conventional wall build up methods including masonry, render or cladding, more sophisticated methods using structural insulation panels (SIPs) are gaining popularity.
The combination of the SIP system and oak frame provides the structural strength and aesthetics of the oak superstructure with the speed, rigidity and thermal advantages of SIPs. The use of SIPs can also avoid the need to use softwood studwork and rafters or trussed rafters, which reduces time spent on site.
Compared to alternative construction methods, the open structure of an oak frame pairs up perfectly with roof and wall panel insulation systems. Once the oak frame has been constructed, the next step is to apply SIPs which fit on the outside of the frame, creating an extremely energy-efficient, uninterrupted envelope.
On top of this layer is a waterproof membrane and external cladding, whereas on the inside the oak frame is exposed while incorporating a service void and interior wall covering. This high level of insulation is excellent at allowing a building to retain heat. The encapsulation can easily be increased in specifications towards meeting Passivhaus standard, making the oak frame a perfect complimentary material to a high performance building envelope.
Leak-proof glazing system
Traditionally, oak frame glazing solutions invariably incorporated PVCu, aluminium glazed systems or varnished hardwood frames. Contrary to popular belief, it is possible to successfully glaze a green oak frame.
The solution comes in the form of a TRADA-approved encapsulated glazing system (EGS). Our own EGS uses drip profiled, fully air-dried oak capping, which is carefully conditioned for outside use. Spacer bars, routed drainage channels and weather strips are combined with a high plasticity sealant, flashing and EPDM (Ethylene Propylene Diene Monomer) rubber seals. There is even a cavity for humidity venting and extra insulation if required.
This highly sophisticated system eliminates the possibility of leakages either around the glass or through the oak frame itself. Furthermore, the insulation properties of this system are far superior to other systems, greatly reducing cold bridging, thermal resistance and keeping the building warm and dry.
The system is designed to work with any glass, including acoustic, triple glazing and various coating options, although a typical solution will involve toughened 28 mm double glazing filled with argon gas, creating high levels of insulation. This is then bedded onto EPDM black rubber glazing tape. The whole system provides the necessary flexibility needed to accommodate the movement of the green oak frame.
Ed Hamilton is managing director of Oakmasters