When it comes to your roof, your choice of structure can have a drastic affect on costs. Here we explain the cost implication of the more traditional structural choices.
With the loads that the roof has to take and the stresses caused by the wind and weather, building any roof is not a simple matter and its design and construction must follow strict rules and conform to known strengths.
The costs outlined below apply to a 10x8m rectangular roof.
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Roof Trusses: What Options are Available?
- Fink roof truss: The cheapest option. It’s lightweight, uses relatively small timber sections, and can be erected on most roofs in a day. However, it’s no good if you want to use the roof space for more than light storage or on more complex roof shapes.
- Traditional cut roof: In cost terms, there is not much to choose between an attic truss and a traditional cut roof. In fact, it can often be hard to tell how an open-attic roof has been built, but it tends to come into its own when the roof shapes are complex.
- Attic truss: As its name suggests, the attic truss gives you an empty attic space within the simplicity of a trussed roof design. However, attic trusses are made from much heavier timbers than fink trusses, and therefore cost considerably more to buy.
- Panelised roofing: Panelised roofing uses large preinsulated sheets, laid across roof beams, so works best on simpleshaped roofs. It is more expensive to buy a roof this way, but fitting costs are reduced because the panels are pre-insulated.
(MORE: Roof Structure Options)
What Affects the Roof Cost?
One of the main factors that affects the price of the roof structure is the same factor that impacts on the price in so many other elements of the building: complexity. Consequently, the simpler the shape, the cheaper the price.
Undoubtedly, moving away from a fink trussed roof to either an attic truss or a purlin and spar roof will add to the costs. But, even so, it provides the opportunity for ‘cheap’ space either immediately or at a later date and is therefore cost-effective in real terms. Thought does, however, have to be given as to how one is to gain access to this additional storey and the knock-on effects, such as the need for fire doors if a third storey is provided.
Fink Trussed Roof
Undoubtedly the cheapest form of roofing for most situations, a ‘fink’ truss is identified by having a ‘W’-shaped formation of cross beams and supports within the basic triangle formed by the rafters and the ceiling joists. There are other slightly different forms but they employ the same principles.
The trusses are light and often seem quite flimsy when taken or handled individually. But by being very light allows them to be lifted on to the scaffolding and the roof plate by one man and an assistant, and then stood up and held into place with a temporary batten, nailed across each truss, while the binders and bracings are fixed, locking all of them into a homogenous structure.
An average cost for each truss, assuming a span of around 8m with a 30° pitch, including supply of the necessary binding, bracing and truss clips, would be £56.
|17 trusses with bracings, bindings and truss clips||£966.67|
|Labour (carpenter plus mate) for one and half days||£462.00|
Attic Trussed Roof
The huge advantage of the traditional purlin and spar roof was the fact that it provided clear space within the attic that could easily be converted for use as living space.
The attic truss, however, provides that option by doing away with the ‘W’-shaped supports within the main triangle and replacing them with upright supports close to the eaves, together with a cross tie support at what would be ceiling level. Effectively this creates an open space along the length of the roof with sloping ceilings to the occupation section, together with eaves storage.
Not unnaturally, the timbers have to be bigger. And the result is a much heavier truss, which can’t easily be manhandled on to the roof and may need a crane or some other form of lifting assistance. Once they are up on the wall plate their erection proceeds in much the same format as for the fink trussed roof.
An average cost for each attic truss, assuming a span of around 8m, with a 45° pitch (necessary to provide sufficient headroom in the proposed attic), including supply of the necessary binding, bracing and truss clips, would be £163.
|17 trusses with bracings, bindings and truss clips||£2,769.74|
|Labour (carpenter plus mate) for two days||£616.00|
Purlin and Spar Roof (or Cut and Pitch)
A traditional cut roof is constructed on site using loose sawn lumber with the timber sizes determined by a set of tables or by the recommendation of a structural engineer. Because the rafters do not have support struts within the triangle formed by them and the ceiling/floor joists, they require some form of support at their mid span between the eaves and the ridge. This is provided by heavy timbers, on each side of the roof, running counter to the rafters and built in, and supported by the end and cross walls of the house.
The pile of lumber that arrives on site can be quite daunting and the labour involved will be counted in weeks rather than days, which tends to put the cost up.
(MORE: Ultimate Guide to Roofs)
|Labour (carpenter plus mate) for seven days||£2,156.00|
How to Reduce Roof Costs
You can’t skimp on the materials. The roof trusses have to be what the manufacturers say they must be and the timber sizes have to be what the architect or engineer specifies.
What you can do is shorten the time that labour will be on site. With light trusses, getting them up on to the wall plate yourself can save at least half a day’s labour. Sorting out the piles of lumber into sizes and, if possible, uses such as rafters, joists, purlins, hips etc., will save at least a day and possibly more of an expensive carpenter’s time.
You can then make savings on the roof covering, and insulation:
- Getting the slates onto the roof is a long job and one that the self builder could do. But it’s not easy, although one could save around five days of labour
- Plain clay tiles would cost roughly the same as slate but if you swapped to concrete interlocking, the tiling costs would almost half
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