The roof structure isn’t just there to provide a lid on the ‘box’. It has to be able to carry the heavy load of the roof covering in all kinds of weather — wet, dry or covered in snow. It provides lateral support for the walls and it helps turn, what would be an enclosure, into a home.
Roof structures have evolved over the centuries from simple cross supports to complicated crucks, hammer beams, principal and common rafters. Up until the mid-20th century, most roofs were created on site using loose lumber. But then came the advent of the prefabricated trussed roof and today most new homes employ roof systems that are factory manufactured, brought to site for erection.
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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. That is one reason why the trussed roof has gained such popularity. Roof truss manufacturers can design and prove their systems by testing them to destruction, whereas a cut roof has to overemploy timber to create a greater degree of certainty.
Not that the cut or purlin and spar roof has gone away — far from it. Many people still prefer this type of roof and, in many other cases, roofs are a hybrid mixture of roof trusses and cut sections. A roof truss relies on being in company with other trusses and each has to maintain complete integrity and be bound to each other in a prescribed format. You can’t cut a truss around obstacles — if you do it loses all strength.
With most pitched roofs it is the rafters, from ridge to eaves, that carry the weight, and the tie beams – that form the ceiling of the floor below – which hold them together and stop them spreading apart. And it is this principle that governs the three main forms of roof seen on the homes of today. The costs outlined below apply to a 10x8m rectangular roof.
Fink Trussed Roof
Undoubtedly the cheapest form of roofing for most situations, a ‘fink’ truss (pictured at the top) 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.
Fink Trussed Roof Cost
|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.
Attic Trussed Roof Cost
|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.
Purlin and Spar Roof Cost
|Labour (carpenter plus mate) for seven days||£2,156.00|
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.
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.
- 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.
Roof trusses: What are the options?
1. 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.
2. 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.
3. 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.
4. 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.
Room in the roof…While the cheapest way of building a roof is with prefabricated fink roof trusses, they inherently have a webbed post structure extending the width of the roof, meaning any available space would be unliveable. However, an attic truss is designed especially to maximise the space available, leaving an open expanse. On more complex roof, a cut roof is the best option.