Roof History: Skylights and Conversions

Roof History and Conversions

Velux Diagram of Roof and Loft

So you may understand loft conversions, roofing, skylight and dormer installation better this provides a little information about roofs development over time.

For a comprehensive guide to roofing materials and services see: K-Roofing.co.uk

The Basic Roof

The purpose of a roof is to protect a living area from rain. Basic and early roof designs used what is called a cruck frame which consists of 2 timbers secured where they meet at the top. To make the roof you align several crucks' and tie them or secure with horizontal members and then fix weather and waterproof covering onto them.

The Coupled Roof

Rafter and Purlin Roof

Because housed started to have masonry walls and a number of storey, timber roofs needed to be built to manage lateral tensions that forced the walls apart.

To prevent sagging of the roof designers and experts installed a collar or wind beam across the rafters (crucks). Crucks/rafters were evenly spaced, placed a foot apart, and wall plates were pegged onto the top of the walls to secure the roof.

The collars act as a tie or strut holding rafters together. Collars manage the outward thrust force at the base of the rafters and the sag caused by the weight of roof coverings such as slate. As home and roof size grew, so did the forces and more sophisticated collars and couplings were developed to stabilise the roof.

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Roof Stability

To increase roof stability in high winds more members had to be added to larger higher roofs, so collar braces or sous-laces added.

On the masonry walls vertical struts or ashlars are added to stiffen to lower sections of the couple.

Principal rafters were introduced which are more substantial and the main roof members.

Between the main load baring Principles, simple rafters increase roof support to prevent sagging.

Intermediate supports, 'purlin', longitudinal members horizontally run the length of the roof. Between the house walls a timber beam is used to tie the roof. The tie-beam lends itself to the name of roof type - Tied or 'Trussed' roof.

Trussed Roof

Truss Frame

Tying the roof together is the tie beam and truss, a long timber spanning the house and frame attachment.

The tie-beam is fitted to corbels (stone brackets protruding form the wall below the wall plate) to create the major structural items in a house that supports the Principles. Historically each truss was individually designed and made but prefabrication took off in the 1950's.

Further developments were the king post, which from the centre of the tie-beam to the ridge purlin (marmamant) supports the roof. Across the struts or beams were placed planks to seal the roof, i.e. a ceiling. These 'bastard' roofs provided limited space and often had weak floors.

Whether your loft is spacious or small we can turn your ideas for storage or living space into reality.

Top Permission Truss

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