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Roof trusses

Most modern house roofs use trussed rafters in the construction of the roof. They consist of quite slender pieces of wood, which are fixed together at the junctions with metal plates. The really clever thing about them is that they derive their strength from their geometry, which is always based on a system of triangles. The alternative way to construct a timber roof is for a carpenter to cut timbers on-site, to the correct length, and then nail them together. This process takes longer than unloading the ready-made trussed rafters from a lorry and positioning them on the house. Invariably, the size of the timbers in a cut, or framed up roof, as it is usually called are much bigger than the timbers used in a trussed rafter roof, this is because you cannot rely on the strength of the junctions to transmit the loads in the same way. In a trussed rafter, the metal plates used to join the timbers together have protruding ‘teeth’, which are forced into the wood by a press, and because this is done in a factory, the quality can be more precise than if you were to rely on a carpenter hammering nails in on the building site. Also, the wood used in trussed rafters is selected to ensure that the design strength is achieved. Apart from the economies in wood, and the time taken, the other significant factor is that the trussed rafter can often span considerably further than a traditional framed roof, often from one outside wall to the other outside wall, meaning the internal walls may not be load-bearing, which in turn means less foundations need to be constructed. All of this can result in large savings in house building costs.

There are of course some disadvantages. The resulting roof space will be a bit more cluttered due to the timber members forming the triangular geometry, and the trusses have to be handled and stored with care, as they can be easily be damaged. They can be more easily affected by wood rot than a cut roof because the timbers are smaller, however, it is now usual practice to have trusses treated with wood preservatives, which help to resolve this issue. In the past, the metal plates have also been known to suffer from corrosion, but these too are now treated to stop them from rusting – it may surprise you to know how the damp the internal space of a roof space can be, which is why we now ventilate roof spaces.

Another disadvantage of a trussed rafter roof is that they are usually (but not always) more difficult to convert to a useful space, like a bedroom. Here at Building Tectonics, we do the design for many loft conversions, and we have derived techniques for both types of roof. but generally a little more steel work is required for a roof consisting of trussed rafters. Of course, with a little forethought from the house builder, this can be overcome by using what we call “room in the roof trussed rafters”. These are more expensive than the alternatives, but still a lot cheaper than constructing the roof on-site out of timbers. It is a shame that more developers do not use the room in the roof trusses as it would allow many houses to later be given an additional bedroom more easily if they ever wanted to convert the space, but of course, house builders want to maximise their profit. Self builders should really consider spending the bit extra to give themselves that flexibility later. We usually do recommend this to clients, we have also had some past conversion/alteration projects where because of the drastic nature of the work, we have suggested that while the client is going to all this effort, they may as well remove the existing roof and replace it with room in the roof trusses. Even though taking a roof completely off is not for the faint hearted and can only be done after much preparatory work such as creating a temporary tent over the house, it is sometimes a brilliant success, it is also more often than not, a lot cheaper than many alternatives too.

The trussed rafter roof was a major innovation in the construction industry when they started being used more generally in the 1960’s. Once the early issues of rot and some manufacturing problems were overcome, the only real problem we are left with in their use is that they are not being handled on site with the care that they should be. They create a very strong roof when it is complete, but the slender nature of the wood elements makes trusses very susceptible to damage until they are in place, therefore good on-site management is required. Apart from the framed roof, and trussed rafter roofs, there have been some other types of roof structures formed in wood, one such system is often called a Trussed roof, this is where timbers are bolted together to form a very strong element, which is then used to support purlins and rafters, which can be smaller than those used in a corresponding framed roof. These were used a lot in in the post housing boom of the 1950’s, but even though they were more economic in timber (meaning that there was less timber involved) than the traditional framed roof, they still lost out to the even more efficient Trussed rafter.  However the terminology can cause some confusion when discussing older buildings, but since many (younger) builders have never seen such a thing, getting the terminology correct is now less important.

This is what one happy client wrote after we advised him of the benefits of using trussed rafters:

Thanks Tony for recommending I use prefab roof trusses on my loft conversion and extension. Not only did it give me the ability to have a much wider open plan kitchen family room but it was significantly cheaper. My builder quoted me 12k but I ended up paying just under 7k.

Written by Tony Keller – Building Tectonics Ltd.

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