Planning Your Roof

Find out how to get things right up top with Duncan Hayes' roofing walk through
Articles by Build It magazine
by Build It
26th November 2012

The roof of your house will be one of its most dominant features, and the shape and style will be principally dictated by three things: the planners, your budget and what you want to use it for.

Your package company or architect will suggest roofing materials, so decide early on if you want a specific look or finish. Planning will dictate an awful lot about what sort of roof you have. “Ridge height seems to be of particular interest to planning authorities,” says Robin Batchelder, building systems advisor at Buildstore, “and the pitch of the roof will be decided as much by them as anyone.”

Planners may also insist on a design and roof covering that reflects the local style, especially if the vernacular is dominant, for example in an area such as the Cotswolds. They may well want to have a say on the shape and finish of your roof, so inform them early on if you want a specific covering, such as a certain tile colour and profile. Be prepared to compromise and dig deep if they ask for a more expensive material.

Construction methods

The two most common methods for building a roof are either ‘cut’ or ‘truss’roof timbers. With a truss system the structural element is made off site to specific calculations. They arrive ready-made and are quickly erected, with diagonal and longitudinal bracing to avoid wind deflections. Steel straps to connect the timber structure to the gable ends. Trusses are quick, cheap and easy to erect, but are not good for complicated shapes such as dormers. However, there are plenty of different styles of truss on the market, including designs for flat roofs or vaulted ceilings.

The other (and far older) build method is the cut roof, where the timbers are made up on site using a complex set of engineering and traditional joinery skills. This custom-made approach is more expensive and more time consuming, but can easily deal with awkward shapes and details. If you want your roof structure to offer internal design features, then a cut roof can produce handsome detailing, such as queen post, king post or cruck frames. Some builds, such as a one-and-a-half storey house with dormers where the first floor has reduced-height walls, will have to have a cut roof, raised-tie trusses or attic trusses, as the bedrooms use the attic space.

Fit for function

Together with budget, what you plan to do with your attic space will also influence how you build your roof. For a conventional house, most volume builders opt for a truss construction using the distinctive W-shaped Fink truss. These are cheap to produce and require a shallower pitch, which in turn is cheaper to cover. The disadvantage is that the roof can be used only for storage space, unless altered by specialists after installation.

If you plan to convert the loft into habitable space in the future then it’s advisable to go for an attic or raised-tie truss that is designed to provide useable space. Attic trusses are more expensive than Fink types, are heavier and usually require a steeper pitched roof with greater overall roof height, which planners may not allow. Raised-tie trusses might get round this issue, but generally won’t give as much headroom.

Another consideration is whether you want a ‘cold’ or ‘warm’ roof. Traditionally, roofs are cold – they are insulated over the joists at ceiling level, and the ‘cold’ timbers above must be kept ventilated to avoid condensation. Building Regulations requires continuous ventilation along the eaves, and there are plenty of products on the market that do this without letting in birds and insects. If you’re planning to live in the roof then ventilation becomes more complicated.

With a warm roof, the insulation is fitted between and on top of the rafters, directly underneath the roof fabric. Solid insulation board, such as Kingspan or Celotex, is laid over the rafters, with battens attached on top that are fixed down through the insulation and into the rafters. In this way the whole of the roof structure becomes part of the heated envelope of the house and doesn’t need ventilation in the same way.

“Consideration will have to be given to fire regulations and access into the roof if it is to be habitable, and these aspects should be factored in at the initial design stage,” says Robin. “Spending a few thousand pounds during your build can give you a very cheap ‘extension’ to your accommodation in the future.”

Roofing materials

Different roofing materials require different pitches, with metal (such as lead, copper or zinc) being the best for extremely shallow pitches, although modern single ply-membrane coverings are also suitable and offer at least a 15-year warranty. A shallow pitch of 22.5° will require a heavier interlocking tile, such as Sandtoft’s 20/20 large tile. More conventional pitches are suitable for a range of tiles, including traditional slates that rely on considerable overlapping to deliver weatherproofing. This style of tile is referred to as ‘plain tile’, and a typical roof requires around 60 tiles per square metre.

The alternative is large-format tiles, which are usually interlocking, meaning you need to use less per metre square. It’s important to specify your roof covering at the design stage of the project since different coverings have different weights, and roofs must be structurally engineered to deal with a heavier load. Your architect, structural engineer or roofing supplier should be able to advise you on this.

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