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Building with oak frame

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Anna-Marie DeSouza explains why oak framing remains a popular build method in the UK

What do Shakespeare’s Globe, Liberty of London and the Stirling prize-winning Savill Building have in common? They were all constructed using oak.

Oak has been a popular building material in Britain for more than 4,000 years, and this typically British tree is also the most profitable forestry product. Oak frames are perfect for anyone wanting to build a house with instant character and, as an increasing number of self-builders are proving, they're flexible enough to create both modern- and traditional-style properties.

Green oak

One of the major differences between building with oak and most other wood is that it’s used when green – within two years of being felled and while still carrying a high moisture content, when the timber is still easy to work with before it hardens. As the frame dries out over a number of years – at a rate of approximately 25mm per year, a bit longer for thicker wood – it will develop the distinctive cracks and texture that create the material’s charm. More importantly, the drying process is one of oak’s best assets: as the frame seasons, it tightens, and in the process becomes stronger and harder.

Oak is fairly easy to maintain and does not suffer from any of the drawbacks often mistakenly associated with it, such as a high risk of fire or insect infestations. Its natural preservative, tannin, protects it against both rot and insect attack. And the denser oak becomes as it dries, the harder it gets, meaning insects can’t penetrate its solid core, nor tolerate the higher level of tannin. It is also this high density of oak that provides the material with a good level of fire resistance. In a fire, oak will maintain its structural integrity for longer than many other materials, including steel, which would distort and lose strength.

The structure

The traditional craft of oak framing has stayed almost exactly the same for centuries. The only big change to the construction method is that, these days, the frame is craned into place rather than being erected by hand. Oak beams create a structural frame in four main sections – wall, roof, floor and cross. These are connected and linked using tenon and mortise joints to create the skeleton of the house, upon which the external envelope is hung.

The mortise is a hole cut into the end of a piece of timber that holds the tenon, a projection at the end of another timber member. The mortise can either pass entirely through the timber or be cut shallower. Once the tenon is inserted into the mortise, holes are drilled into the wood and a peg is driven through both, which locks the tenon into the mortise. The pegs can either be driven right through the joint so they are visible on either side of the mortise, or they can be ‘blind’ pegs, which can only be driven into the joint from one side and don’t come right through the timber. These pegs are made from drier oak than the timber to be joined, ensuring that the joint shrinks around the peg and tightens over time.

Once the frame is complete, work can start on infilling the walls and placing the roof. Traditionally, wall spaces between the timbers were filled with wattle and daub – a mixture of earth, cow dung and straw – with a thatched roof placed on top. Nowadays, walls are constructed from a variety of materials, including bricks, render and cladding, and you can leave the external structural frame exposed or enclosed, depending on your preference. Inside, much of the appeal of this building method is the exposed frame, but is possible to conceal this if you prefer.

Building large-frame houses like this is obviously a hazardous task if it is undertaken by anybody other than a trained professional. While it is possible to use separate architects, carpenters and construction teams to help you to build your house, the safest option for self-builders is probably to use one of the huge number of reputable firms that offer packages from design to completion. One option is to take your architect-drawn plans to an oak-frame company and let them take it from there.

Eco credentials

Oak is a great building material for the eco conscious. Wood’s main environmental plus is its natural absorption of CO2. If burnt as fuel, timber will release the CO2 it absorbed in its lifetime. But if the timber is put to long-term use in a house, a significant amount of carbon is removed from the atmosphere. The oak trees used in timber framing are on average at least 60 years old. If a home is looked after properly it could last hundreds of years, making the timber used in its construction a completely carbon neutral building material.

Compared to other construction materials, oak can also have low levels of embodied energy (the amount of energy used to source, manufacture and transport it). If locally sourced green oak is used to build, there would be minimal energy used in transportation, coupled with the minimal amount of energy used to fell and saw it. Bear in mind, though, that not all oak is sourced in the UK - much of it makes the journey over from France, for example.

Also, unlike many other timbers, green oak is very natural and does not need to be treated with preservatives or fire retardants, which can be toxic. And what’s more, a great deal of the oak produced in the UK is certified by the Forest Stewardship Council, meaning that it has come from a well-managed sustainable source.

Costs

Like any build method, the cost of an oak-framed house depends on the complexity of the design. Oak frames will cost more than softwood timber frames, but you’re paying for a distinctive, high-quality material. Expect an oak frame to cost 30-50 per cent more than softwood, and if you work on the frame being approximately 35 per cent of the house value, that means a price increase of 10-17 per cent on a standard timber-frame house.

Case study: The McManus house

When the McManus’ children had grown up and left home, the couple decided to build a home of their own. They wanted something that they could enjoy and a place where their children, and in time grandchildren, could visit. Once they’d found a plot near some woodland in Clevedon, Somerset, Edmund – an architect by profession – designed a split-level house to suit the steeply sloping site. The full-height split-level allows a galleried landing for the first floor, with access to the bedrooms, which looks down on the oak frame in the living areas.

Much of the home is faced in Cotswold stone, which blends in well with the stained horizontal tongue-and-groove boarding used to clad the other elevations. This in turn complements the Redland Landmark double Roman concrete tiles used on the roof.

Once planning permission was granted, a green-oak frame, as per their design, was ordered from Westwind Oak.

The main works on the £325,000 project started on site in early April 2007, and the couple moved in towards the end of November of that same year, although completion of the stone facing and external works carried on until the end of February 2008.

Photo: Andrew Wall

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