What Will it Cost? - Roofing

Your choice of roof structure and covering will have a big effect on the overall cost of your house, explains Mark Brinkley.

The cost matrix for roofing looks remark­ably similar to that for external walls. Both walls and roofs tend to be sub-divided into structural elements and clad­dings. The claddings are the easiest to under­stand because they break down easily into unit area rates, though you have to undertake a little basic geometry to convert a flat plan area into a three-dimensional roof shape.

The table below confirms what you would instinctively anticipate: that the huge concrete roof tiles, known as inter­lock­ing tiles, seen across the country on every­thing from developer housing to super­markets, are by far the cheapest way of cover­­ing a roof. As the coverings become smaller and more interest­ing, they increase in price. There is in fact a huge difference between the very cheapest roof coverings and the most expensive – some­thing like a 1:5 ratio – far more than you see in any other aspect of building the super­structure.

Bear in mind that our five bedroom bench­mark house has 213m2 of roof covers: you can see how the choice of roof cover can alter the budget from as little as £4,500 to well over £20,000. In fact, it uses a popular choice of tile, Forticrete’s Gemini, which has many of the advantages of a large format interlocking tile but looks to all intents and purposes like a small format clay tile.

Roof Structure

There are three methods commonly used to construct the roof carcass:

• Traditional or cut roofs
• Trussed rafters, either closed or open (attic trusses)
• Beam and panelised roofs

The choice as to which you use is largely concerned with buildability and how you want to use your roof space; cost is usually only a secondary consideration here. The simplest (and cheapest) solution for most roofs is to use closed roof trusses, made in a factory and delivered to site, in a similar manner to timber framed homes. The major downside with these trusses is that the roof space is effectively closed-off for further use. If you want to live in your roof space, you will have to use one of the other construction systems, being the traditional cut roof, the attic truss or a panel roof, commonly used in Holland and now being rapidly taken up over here — at a higher, although not substantially so, price. Panel roofs are very quick to install but work best when the roof detailing is very simple: if you have a complex roof shape, it is best to stick with traditional carpentry skills and to build the roof on site.

Whichever roof structure you choose, you still have to plan in the roof edge details, notably how you want to build the eaves (overhanging or closed off with fascias and soffits) and the verge on gabled walls.

Roof Shape

One of the key factors driving housebuilding costs is the shape of the house. It might seem strange to look at house shape in a feature on roofing costs but there is a direct connection: the complexity of the roof is a pretty accurate indicator of the additional costs incurred from the form of the house. What is more difficult is to cost this complex­ity: every extra junction will add to costs but the extra cost may be as little as 2% or as much as 10%.

Our benchmark house (see below) is a useful indicator in this respect. It is designed to be pretty simple and easy to build but its roof shape is actually quite complex. It has three distinct roofed areas. The main aisle of the house, where the chimney is located, is a rectangular box, which has a hipped roof over it. Abutting this at the front is a lower roof with dormer windows protruding through it: this part is the master bedroom, built with attic trusses. Finally, behind the chimney, out of view, is a single storey addition with a hipped roof, which makes up the sun lounge. The entire roof, including carpentry, tiling and fascias and guttering, cost just over £15,000 — 11% of the total build cost. Had the house been one (much larger) rectangular box, I estimate that the cost would have fallen to around £12,000, maybe as little as £10,000.

Hips or Gables?

The two common roof shapes, hipped roofs and gabled roofs (see diagram), both come with their own set of problems. From a roofer’s point of view, a gabled roof is far simpler and easier to build than a hipped roof, as there are only two planes to consider, rather than four. However, the work involved in building up gables in brick and blockwork is painstaking and slow. In terms of overall cost, there is very little difference, except where the superstructure is factory built, as it would be with a timber framed house: here, the gable option is much simpler and there­fore cheaper, especially when the outer wall cladding is not brick or stone.

ROOFING JARGON EXPLAINED:

About These Articles

This series is based on a typically constructed, 4/5 bedroom house, with an internal floor area of 200m2 (2,150 sq.ft.) plus an integral garage. Its raw build cost (that is only labour and materials) is £135,000. Professional fees, contractors’ overheads and profit, insurances and warranties would all be in addition to this. The raw build cost presumes a straightforward job finished to a fairly basic standard; the sort of finish you would expect from a professional housebuilder. In this series we will be looking at how the costs break down for all the various component parts of a house, and looking at the cost implications of choosing alternatives. This house is also featured in Mark Brinkley’s latest book, The Housebuilder's Bible (6th edition).

OTHER COST GUIDES:

• Getting out of the ground
• Insulation
• External Joinery
• Constructing Walls
• Installing Services
• Internal Finishes