The Roof: Cost Guide

This month’s look at roof structures costs up a prefabricated attic truss roof, but there are other options available, as detailed below. Our benchmark house has a pitch of 40°, meaning that if occupation is required, there is quite a bit of work involved. There are two options. The first and cheapest, but only by a few hundred pounds on the material costs, is to occupy just the centre section of the roof beneath the short ridge and between the two hips. The second is to occupy more of this quite large roof by extending the attic into the hips — and that seems to make more sense.

No windows to the roof space are on the front elevation and natural lighting is obtained via two Velux windows to the rear plane of the roof. The roof space is to be left boarded, with plasterboard walls concealing the insulation. Access is via a loft trap, but it is entirely possible that a full staircase, above the proposed staircase to the first floor, could be installed and the attic space used for habitable purposes.

The rear extension to the family room, at 35°, does not provide sufficient space for anything but storage and is therefore constructed using simple fink trusses.

This roof could have been built using sawn lumber, which would have been cheaper as far as materials are concerned. But the labour costs would have risen proportionately to cancel out any benefit. So these costings assume provision of a complete prefabricated roof.

The Roof Structure

The wall plates have already been costed (labour and materials) within Part Four: Superstructure because their bedding is the responsibility of the bricklayers. However, it’s quite likely that many roof suppliers will include this item, and even those that don’t will probably include the wall plate straps and lateral restraint straps, which can be retrofitted by the carpenters. With a roof of this type with attic, double, treble and girder trusses, it would be necessary to have a crane on site. Insulation is costed as rigid foam, 100mm between the rafters and 50mm beneath, right into the eaves to create a warm roof. Nevertheless, although belt and braces, 100mm insulation is set between the studs of the attic walls and mineral wool insulation is laid in the eaves void. The rear projection is a cold roof with mineral wool insulation between and over the ceiling joists.

The Roof Covering

As the main roof is a warm roof with the insulation between and down the slope of the rafters, the roofing underlay is a breathable one and there is no need, therefore, to install eaves ventilation. However, the rear single storey roof projection and the small monopitch roof to the side of the house are both cold roofs with mineral wool insulation laid in two layers, between and over the ceiling joists. This roof would, therefore, require eaves ventilation. The chosen roofing cover is natural slate and the most cost-effective and consistent at the present time seems to be Spanish.

How to reduce these costs:

• You could cut down on the insulation in the stud walls to the attic floor. Strictly speaking, as the roof is a warm roof there is no need for this insulation or that on the eaves floor. The saving on rigid foam alone would be £571.61 and 18 fewer rolls of mineral wool would save another £305.10 plus, in both cases, the labour, which would amount to around £500. But what is the point of heating the eaves space?

• 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 amounting to £1,310.

• Plain clay tiles would cost roughly the same but if one swapped to concrete interlocking, the tiling costs would almost half.

The main roof 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.

More Build Costs:

• Buying a Plot
• Building Foundations
• Floor Structure
• Superstructure
• Roofing
• Drainage
• Electrics
• Heating & Plumbing
• Plastering
• Internal Carpentry & Joinery
• Finishing Trades

Further reading:

• A Guide to Roofs and Roofing
• Loft Conversions
• Roof Tiles and Slates