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How Roofs Are Made

Updated on November 9, 2012

It is best to start with a knowledge of roof construction so you will understand how your roof is made and therefore will be better able to carry out repairs on it yourself or get someone in to do the work for you.

Roofs basically fall into two types - flat or pitched - and it is quite common to find both on a property. For example, the main house may have a pitched roof and the garage or a one-storey addition a flat built-up felted roof.

Pitched roofs

This type slopes at varying angles. The sim­plest is the lean-to, which is widely used on single-storey additions and lean-tos built on to a house. It is constructed with rafters resting on a wall plate at one side and supported by a wall plate bolted to the house wall or by joist hangers, in a similar fashion to a flat roof, but at a much steeper angle.

Pitched roofs may be covered with glass or plastic where it is necessary to let in plenty of light. They can, of course, also be covered with any type of traditional pitched roof covering. The roof slopes at each side from a central ridge. Traditionally, the rafters rest against a central ridge board at the apex, while at the eaves they are notched into a wall plate and tied together with ceiling joists. The rafters are supported with purlins, which are in turn stiffened with cross-braces. This type of roof is constructed on site and is normally fairly easy to adapt if you decide you would like to have an attic conversion in the future.

The modern roof is often constructed with prefabricated trussed rafters, in which case there is no ridge board or purlins. Diagonal bracing timbers are nailed to the undersides of the rafters and plywood sheathing is fixed to their upper surface. This type is often of much lower pitch than the traditional one. Because of this - and its prefabricated method of construc­tion - it is not suitable for conversion into additional attic space.

Three types of covering are commonly found on older pitched roofs - slates, plain tiles and shingles of various types.

Because of their age, slate roofs are often unlined, the slates being nailed directly to the roof sheathing. You will find some lined with felt underlayment beneath the slates.

If a slate roof has been replaced in the last 20 years or so, it is likely that the roof may have been lined with felt. This makes the attic space warmer, drier and cleaner. An examination of the attic area will tell you J whether felt is present or not.

To make the roof waterproof, alternate rows of slates are staggered and each row overlaps the one below by about half its length. At any particular point, there are at least two - and perhaps three - thicknesses of slate.

It is also possible for a plain-tiled roof to be unlined, in which case the attic space will be dirty and draughty. You might be lucky and find a roof that is lined with felt.

Plain tiles are hung overlapping, so that at any particular point there is a minimum of two tile thicknesses to ensure the roof is watertight. Alternate rows are staggered so that the joints between tiles do not line up.

Interlocking tiles are also used for re-roofing and sometimes replace an old slate roof. Usually made of concrete, but sometimes of clay, they are usually found on relatively at eaves modern houses. And because these roofs tend to be newer, they are often lined with felt. The sides of the tiles, and sometimes the heads, are specially shaped so that the adjacent tiles interlock with one another along their edges to form a watertight roof surface.

Shingles and shakes may be laid over spaced or solid sheathing, with an exposure that depends on the roof slope, the shingle size and the grade of wood used. Underlayment is not normally used with shingles except where additional insulation is needed or blizzard condi­tions are common, but interlayment is essential with shakes because their textured surface prevents them from fitting closely together.

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