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Truss Roofs and Conversions

Loft Conversions, Dormer Extensions, Truss Replacement

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Truss Roofs - Designs and Development

A Modern Trussed Roof
Velux Diagram of Trussed Roof

A truss roof is (usually) a W shaped timber roof structure, efficient for roofs but trusses prevent further use of a loft.

Trussed rafter roofs are built to conventional designs that lock together 3-inch timbers into a strong and load baring truss frame.

If your roof uses a truss frame (as many post 1965 houses do), when you look in your loft you should see:

  • The distinctive W shape design
  • A shallow pitched roof
  • Flat, rectangular, metal connector plates joining the timbers
Truss Roof Timber Frame
Truss Frame

Trussed roofs have a more technical solution when converted because timber frames are substituted with steel girders, and often the roof-line is raised, trussed loft conversions always require planning permission.

Truss Roof Development

The truss roof is a 1950 and 60's response to the demand for lower pitched roofs.

Housing technological development borrowed and integrated the latest solutions from other construction industries such as aviation.

The technological solution - trussed roofs - balances roof stresses of roof and ceilings and allows lower roof pitches.

However, the design significantly limits further use of the roof for storage or habitation

In 2000 Britain, the need for housing space has become increasingly important as land and new build costs increase.

Once again, modern materials and techniques have come to the rescue, so today we use steel in trussed roof loft conversions to replace bulky timber structures and free up loft space.

Standard Roof Trusses

Prefab Truss Roof Frame
Prefab Roof Trusses

During rationing and war year scarcity, building work was licensed because it was a large volume user of scarce timber.

Control allowed enforcement of more economic solutions in timber use.

The Timber Development Association (TDA) recognised that roof structures were wasteful and began to influence the pattern and design of roof trusses with free roof truss plans. This created savings in timber use of up to 30%.

Domestic housing development began to use principal trusses made from small timber sections either bolted or using metal connector plates. The purlins and common rafters were known as TDA Trusses and they remain in use today.

The roof timbers for the truss systems used connector plates or gang-nails mechanically pressed during factory prefabrication or onsite fabrication. The resulting prefab roof trusses could carry direct loads on them and are kept vertical by diagonal bracing and tie batons for stability.

20th Century Roof Design

Post War pitched roofs were built with economic, lightweight, wooden, premanufactured / prefabricated roof trusses designed for shallower pitches - 35 to 40 degrees depending on roof span.

1950's and 1960's housing technologies included roof truss designs for lower pitches - 22 - 30 degrees.

From the mid 1960's shallower pitches still - down to 15 degrees for up to 12 meters.

Loss of the Loft

A consequence or flatter roof design is lower lofts and less roof space.

Modern lofts became less useful as roof voids shrank, especially after water tank and pipes are included.

Loft floors have not been designed to take loads and generally are less suited for storage conversion than pre war loft attics because of the costs involved.

Modern 1970' and 80's detached house loft conversions may require elevation of the roofline to achieve the required roof height for living space and to meet building regulations.

Minimum Height Rules - New

Minimum Loft Height
© Crown copyright

Internal loft height requirement states that loft height should be a minimum of 1.8 m minimum at the edge of a stair with 1.9 m at the centre line if the ceiling is sloping in order to meet building regulations approval for height.

See page 7 of Approved Document Part K. Low lofts can now be developed for habitable use, adding significantly to the use and value of your home.

The 1965 move to 'W' shaped trussed rafters means that loft conversions, skylight installation and dormers require a structural modification to the roof and supports and planning permission.

Conversions

Converting trussed roofs is possible. Conversion requires replacement of trussed roof rafters with steel roof trusses, raising of roofline and strengthening of the loft floor. A trussed roof need not limit your dormer and loft design plans and ideas.

Truss Design

The picture shows precision premanufacture of roof trusses at Truss Form Ltd. (part of Alpine Automation (U.K) Ltd. - a Truss Design Software Company). Timbers are positioned in electric vices and connector plates are mechanically pressed.

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