Roofs - tips from structural engineers

		JS Engineering Design Ltd. Consulting Structural Engineers *Roof Structures*
	We are 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. Existing roofs We do inspections and reports on major roof defects (see also Structural inspections) and will suggest any repairs and alterations that are needed. Extensions into the loft space usually require some alterations to the roof structure and we can advise on the alterations needed and provide design and drawings for them. New roofs All but very simple roof structures need to be structurally designed and a significant volume of our work is producing designs for new roof structures.
	Roof structures can be formed in a variety of ways. Each system has its own advantages and disadvantages.

	Traditional construction 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:


	Trusses
	Roof 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
	Steel 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.
	Frame structures
	When a 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.
	Hipped roofs
	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. can:

Check out existing roof structures.

Advise on repairs to roofs.

Design alterations to roof structures, i.e. loft conversions.

Design and detail roofs with traditional timber construction.

Design roofs using modern steel or engineered timber beams e.g. glulam or plyweb beams.

	JS Engineering Design Ltd., The Office, Mardale Rd., Penrith, Cumbria, U.K. CA11 9EH.
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Roof Structures

Trusses

Long span steel, glulam or plyweb purlins

Frame structures

Hipped roofs