Engineering Design Ltd.
Consulting Structural Engineers
Tel: 01768 890261
often asked to look at the structure of roofs. Both to
check out suspected defects in existing roofs and to
design new roofs or alterations. See checklist below.
|Roof structures can be formed in a variety of ways. Each system has its own advantages and disadvantages.|
Usually comprises timber rafters supported by timber purlins which, in turn, are carried by the walls of the building and/or timber trusses. The purlins are often tilted (canted) over to line up with the rafters rather than being set vertical. Canted purlins are illustrated above.
Timber trusses can take a variety of forms. Typical examples are shown to the right and below.
|A King Post Truss in a recent project:|
structures can be formed very simply using trussed
rafters. These are made from small section timber
and comprise the rafters on the roof slopes, a horizontal
to support the ceiling, and various combinations of cross
members to form a frame or truss. The members are usually
fixed together using metal plate connectors. The trusses
are designed and produced by the manufacturer to suit the
span and profile of a roof. They are a very cost
effective way of forming roofs of houses and are often
seen on building sites. Their major drawback is that they
fill the loft and reduce the useful space available.
Long span steel, glulam or plyweb purlins
purlins can span further than timber ones and are often
used to support roofs, spanning between the walls of the
building without the need for intermediate trusses.
Glulam and plyweb purlins are engineered forms of timber and are factory made. Their advantage is that the imperfections in natural timber are removed and large sizes can be manufactured. This allows long lengths to be used similar to steel purlins.
building is a frame structure the roof is often formed as
an integral part of the frame. Typical of this roof form
are portal frame industrial buildings and out of town
supermarkets and stores.
|The shape of roofs can create complications. Hipped ends, where the end of the roof slopes down to the eaves, require careful design. This roof form can produce a horizontal thrust at the top of the supporting walls and over time, cause the walls to move and crack and the roof to spread. This is a common problem with some house types built earlier this century.|
|JS Engineering Design Ltd., The Office, Mardale Rd., Penrith, Cumbria, U.K. CA11 9EH.|
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