Choosing a Roof Type

ABOVE: The design of this self build is dominated by the full height vaulted ceiling. It has a steeply pitched roof of interlocking grey Redland fibre slates which sweeps down with a wide overhang over the 4.3m high eaves. Read more about this project.

What is the best way to build a roof? It’s a question not many selfbuilders think about when they set out on their journey, but it can have a significant effect on the budget and on how the house can be used and developed in the future.

Until the 1960s, almost all domestic roofs were what we now call ‘cut roofs’. That is to say, they were assembled by the carpenters from individual timbers delivered to site. Building a roof was one of the more exacting parts of a carpenter’s skill, involving some detailed angle calculations and some complex cutting. Or, put another way, it was time consuming and, therefore, expensive.

In the 1960s, a simplified, industrial method of roof building arrived on our shores from America — the prefabricated fink roof truss. It caught on like wildfire and quickly took over as the main method of roofing. All a carpenter was required to do was unload the roof trusses off a lorry, haul them into place and then secure and brace them altogether. Typically, a detached house would require between 12 and 20 roof trusses. By the 1990s, only people building very complex roof shapes seemed to bother with the old roof carpentry skills.

But the cheapness and simplicity of roof trusses came at a price. Unlike cut roofs, you couldn’t easily adapt a roof built with trusses. If you wanted to have a loft conversion, you were stuck. By the end of the 1990s, builders started to worry more about the value of the wasted space in the lofts, and the fink roof truss stopped looking like such a bargain. The options are explained below.

The Options:

ABOVE: The fink roof truss: 1. Binder; 2. Web

The fink roof truss is still the cheapest way of constructing a roof. It’s lightweight, uses relatively small timber sections, and can be erected on most roofs in a day. However, it’s not a great idea if you want to use the roof space for anything more than a little light storage. It’s also worth bearing in mind that the fink truss roof is at its best where the shape of the roof is very simple. The more junctions, hips and valleys, and dormer windows you add to the roof, the more complex the underlying carpentry and the less advantageous a fink roof becomes.

ABOVE: The attic truss: 1. Rafter; 2. Joist

Truss manufacturers are not confined to fink trusses. One of particular interest to self-builders is the attic truss which, as its name suggests, 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.

ABOVE: The traditional cut roof: 1. Collar bean; 2. Purlin; 3. Strut; 4. Hanger

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 — i.e. whether it’s via a prefabricated truss or built on site. The cut roof tends to come into its own when the roof shapes are complex.

ABOVE: Panelised roofing: 1. Ridge boars; 2. Purlin

Panelised roofing uses large pre-insulated sheets, laid across roof beams. The beams will typically be at the ridge, at the eaves and halfway between (known as purlins). Like trusses, panelised roofing 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.

Insulation requirements have increased so much that the typical rafter depth is no longer enough, so builders using cut roofs and trusses have had to look at various insulation options such as warm roofs and hybrid roofs, where the insulation sits between and under the rafters. But on a panel roof, insulation is already taken care of at the manufacturing stage.

The Decision:

So which method to choose? 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 — in truth, there may not be that much difference in cost between them, and it might 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.

Further reading:

• A Guide to Roofs and Roofing
• A Guide to Green Roofs
• A Guide to Flat Roofs