ABOVE: Curved roofs are very popular in contemporary house design
Roofs are one area of housebuilding which is changing. In Victorian and Edwardian times, there was fundamentally just one way to build a roof and its still widely used. But since then we have seen the flat roof, the curved roof, the panellised roof and especially the trussed roof. All have their advocates and all have their pros and cons.
Traditional or Cut Roof?
The traditional roof is a steeply pitched roof, rarely less than 35°. This reflects the weatherproofing requirements of older roofing materials like thatch and peg tiles. The roof timbers are cut on site and erected one at a time: roof building is one of the more complex aspects of site carpentry and is a highly skilled job. This makes it both relatively slow and relatively expensive. One big advantage accruing from traditional roof building is that the finished structure is readily adaptable: not only is the volume large enough to convert into living space, but the structure can generally be easily altered.
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There is nothing technically challenging about building flat roofs, which, incidentally, are never flat but are always constructed to a fall in order to stop rainwater pooling. But it wasn’t until the 1950s that cheap and reliable materials for covering flat roofs became widely available and this started a boom in flat roofed extensions across the country. Whilst the flat roofed extension has gone out of fashion, flat roofed modernist housing is enjoying a revival, although the roof covering is often done in an expensive metal.
Trussed roofing was an innovation that appeared in the UK in the 1960s and rapidly took over the new housebuilding scene. Trussed rafters promised a pitched roof that was both cheaper and faster to install than the traditional cut variety. Instead of individual timbers being prepared on site, the trusses were made to order in a factory and delivered to site for placing on the wallplates. A simple roof shape might consist of 15 or 20 trusses, which could be placed in a few hours.
The disadvantage of roof trusses is that they cant readily be altered so it becomes harder (though not impossible) to later convert the roofspace. This problem has to a large extent been countered by the creation of the open or attic truss which, although rather more expensive, provides room for a loft conversion.
ABOVE: Roofs can be cut from timbers on site or delivered as trusses, ready made up, as in this example
Panellised or SIPs Roofs
Panellised roofing is very common in the industrial and warehousing markets and is widely used in some other countries, notably the Netherlands. Instead of erecting complex timber roof configurations, a panellised roof just needs supporting beams, typically one at the ridge and a pair halfway down the roof, known as purlins. Once these are in place, the roof panels can be craned in in a matter of minutes.
As you can imagine, this is a very quick way to erect a roof and it has the added advantage of coming pre-insulated, which saves a lot of additional work. Plus you get open roofspace, which is increasingly sought after these days. However, the speed advantages of panellised roofing begins to significantly disappear when the roof shape becomes particularly complex. If you have a roof with hips, valleys and dormer windows, you will probably do better to use one of the other roofing methods.
Curved roof shapes are currently the height of fashion, at least on upmarket builds. They are generally covered in either a metal like zinc or copper or with a green roof, made up of turf planted with sedums. In structural terms, they tend to be variants on flat roofing using curved timber glumam beams under a plywood covering. They certainly add visual interest but are realistically only an option where the overall roof shape is simple.
There is currently enormous interest in creating green or turf roofs, which are perhaps the most obvious sign of a green-minded home. In fact, these are nothing new – being commonly used in parts of Scandinavia and Scotland for hundreds of years – but today’s green roof is a very different beast. You have to build such a roof up in layers, paying careful attention to waterproofing details, and the planting regime has to be carefully thought through. The benefits are that green roofs absorb CO?, absorb a proportion of rainwater and provide a degree of insulation.
Insulation and Warm Roofs
The requirements for ever-higher levels of insulation have had a dramatic effect on how we build rooms in the roof. If you leave the loft space empty, you can simply roll out insulation above the ceiling. But if you want to use the loft as living space, you have to insulate directly under the sloping roof. However, the amount of insulation required is deeper than the rafters or trusses. Extra insulation can be added to the underside, but this reduces headroom, so a common solution is to build a ‘warm roof’, where the insulation is placed above the rafters. Another approach is to do away with rafters and use insulated panels laid across a series of beams. This works best on simple roof shapes.
In Scotland, they routinely build roofs with a solid decked covering, known as sarking. Its more expensive than open-felted roofing as built elsewhere but it provides a more wind-resistant structure.
Making the Decision
First you have to decide how you want to use your roof. If you have no intention of using it as living space either now or in the future, you are probably best off using a simple fink roof truss. However, if you want to ‘live in the loft’, then you’ll have to choose one of the other methods. There may not be that much difference in cost between them, and it may pay to go with what your designer and/or builder is most comfortable with. If you want a cathedral or vaulted roof, then you’ll have to work with the designer. It’s unlikely to work with any kind of truss, but can be done using traditional rafters laid on beams, or sometimes with panelised roofing elements.
Note: Article updated June 2011