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Attic Trusses

With the ever increasing demand to optimise building land, more and more builders are looking to trussed rafters to provide the solution for additional living areas within the roof space. A significant proportion of all trussed rafters now produced are RiR trusses which offer up to 65% more living space using a method of construction that has been in common use within the construction industry for some 40 years or so. No outdated or untried methods of construction are needed.

 

The Room-in-Roof or attic trussed rafter is a simple means of providing the structural roof and floor in the same component. This offers considerable advantages over other forms of living roof construction:

  • There need be no restrictions on lower floor layouts since the trusses can clear span on to external walls although greater spans and room widths can be achieved by utilising internal loadbearing walls.
  • ‘RiR’ trussed rafters are computer designed and factory assembled units, resulting in better quality control.
  • Complex, labour intensive site joints are not required.
  • ‘RiR’ trussed rafters can be erected quickly, offering cost savings and providing a weathertight shell earlier.
  • Freedom to plan the room layout within the roof space.
  • A complete structure is provided, ready to receive roof
    finishes, plaster board and floorboarding.
Comparing an 8 metre span standard trussed rafter (see Fig 1) with an equivalent 8 metre span ‘RiR’ truss, (see Fig 2) the external members will increase in width and depth. There are two reasons for this: The ‘RiR’ truss supports approximately 60% more load than a standard truss of the same span and pitch. This difference in load is made up of plasterboard ceilings and wall construction, full superimposed floor loading and floor boarding. Lack of triangulation in a ‘RiR’ truss is the second reason for increased member sizes. Predominantly 47mm thick timber is used, with member depths ranging from 145mm to 245mm.

 

For most purposes 'RiR' trussed rafters can be designed to clear span between the front and rear walls of a dwelling thus avoiding the need for building loadbearing walls and foundations on lower storeys. However, if loadbearing walls exist or can easily be added then they can be used to good effect to provide additional support to the 'RiR' trusses. In this way greater room sizes are possible but to be effective the walls should occur within the centre 20% of the truss span and are most effective when placed near the mid-span of the truss. See Fig. 3

 

Size of Attic Trusses

 

Where possible keep the size of 'RiR' trussed rafters within the limits dictated by safe transportation. There may be local conditions that affect this but generally an overall height of truss of 4 metres is easy to transport. If greater height is required then trusses may be constructed in two parts. The two-part trusses will be structurally joined on site and instructions for this will normally be provided by the trussed rafter fabricator. This joint is often made with a proprietary connector plate. Fig. 4 shows a typical two-part truss arrangement.

 

Top Hat Attic 

 

 

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