The use of timber in Britain goes back a long way. Since at least the early Saxon era, the timber frame has been the traditional material for roof construction. Regardless of whether we are building in a classic or modern design, or whether we are producing pitched or flat roofs, it is still the roof material of choice for house builders and roofers like us.
The popularity of wood as a roofing material is due to its historical availability, general lightness and ease of usage, excellent strength-to-weight ratio, potential for cost savings, typical aesthetic appeal, and potential for environmental friendliness when sourced responsibly. (It is biodegradable as well, which means that it is easily disposed of; no crazy waste costs there!)
In the past, wood was the most accessible and appropriate material to use to construct the roof. Before England and Britain was deforested, there was abundant supplies of wood here (Nowadays, wood is a little more expensive, because there is less of it around locally!)
The primary determinant of the roof’s shape was its roof covering. Roofs in Britain’s past would almost always be pitched, or sloping, and would typically originate from a central ridge. The roof’s type of covering and level of exposure decided the degree of pitch. For example, a house very exposed to the elements requires a specific ratio so that the load is light on the house, (including the added weight of rain or snow) but also does not catch the wind. (We’ll be writing more about roof pitch and gradient at a later date, so keep your eyes peeled!)
For covering a timber roof, the smaller the pieces of covering, the steeper the pitch, due to the greater likelihood of rain intrusion through the many gaps. With many little and irregular-shaped Delabole slates, for example, the pitch of the roof will need to be significantly steep, so that the rain can’t enter the house!
Lead, copper, and zinc sheet can be laid at extremely shallow, nearly flat slopes, whereas thatch typically requires a pitch of roughly 50°, plain clay tiles 45°, and thick slates and stone flags a minimum of 35°.
The size of the timbers and the roof pitch were both determined by the roof covering. Stone slates (so much heavier than clay or thatch) require very sturdy posts and rafters.
Here’s a fun fact: In the colder parts of Europe, such as in the Alps or some parts of Germany, many more timbers are used to keep the roof up, as they must not only bear the weight of whatever is covering them, but also bear the weight of snow!
You can use a timber frame for your flat roof, however the structure and covering will likely be very different from a timber pitched roof. A timber frame with a flat roof is typically the most affordable and straightforward alternative for home builders. But there are a few drawback to flat roofs. Though this style of roofing is popular in some mid-century, modern architecture, many residential buildings are built with pitched roofs. There are only certain styles of house that suit this kind of roof, though on the right building, it can look great! However, a flat roof lacks loft area for water tanks and storage, and unlike a pitched roof, it does not “shed” water as easily. Finding leaks is also more difficult.
A timber roof is made up of two main parts: the supporting framework and the covering.
In a straightforward pitched timber roof, the rafters are supported by the horizontal purlins (beams), which run parallel to the roof ridge. The rafters are typically notched with a “birdsmouth” to fit over the purlins in order to prevent a roof from being blown off by the wind. When supporting masonry walls, rafters are often slotted over a wall plate made of wood that is fastened to the top of the brickwork or blockwork. Typically, steel straps that have been galvanised are used to secure them. Rafts are often notched at the eaves over a purlin that is bolted to timber posts in timber frame construction.
The distance between the supports and the roof structure, or the span, can be made much wider by forming the rafters into trusses, which are triangular constructions. This is accomplished by creating a triangle out of the lower ends of the rafters at the eaves and a wooden beam. The wooden beam holds the rafters together and keeps them from splaying.
Modern construction offers the prefabricated timber truss as an alternative to building this arrangement on-site from individual timber lengths (which is the traditional way and is simple to execute but takes more time and may produce enormous timbers).
Timber trussed rafters can be a cost-effective option for simple spans without internal supporting walls. They are the end product of years of research into triangular roof systems.
If you’re interested in getting a new timber frame roof, get in contact with us on tel:01288381256.