ROOM IN ROOF (‘RIR’) TRUSSED RAFTERS - Product Data Sheet

Oct 29, 2026

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PRODUCT DATA SHEET Sheet No.1 May 2007 covers Room in Roof (‘RiR’) trussed rafters, explaining how a simple component can provide both structural roof and attic floor. It highlights advantages versus other living roof systems, including computer design and factory assembly, fewer complex site joints, faster erection, earlier weathertight enclosure, and freedom to plan within roof space. It compares member sizing versus standard trusses by increased load capacity and reduced triangulation, and provides construction guidelines for bearing arrangements, safe transportation dimensions, service routing, planning of openings, and bracing requirements.

LanguageEnglish

CategoryResearch & Report

FormatPDF

Pages4

Reading Time10 min

AccessFree to download

Three-Bearing ‘RiR’ Trussed Rafters

Size of ‘RiR’ Trussed Rafters

Some Typical ‘RiR’ Configurations

Services in ‘RiR’ Trussed Rafters

Layout and Planning Position of Openings

Bracing of ‘RiR’ Trussed Rafters

PRODUCT DATA SHEET Sheet No.1 May 2007 ROOM IN ROOF (‘RIR’) TRUSSED RAFTERS The ‘Room-in-Roof’ (‘RiR’) 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. Fig. 1 Typical 8 metre span conventional trussed rafter 35 x 97 rafter 40° Weight of truss approx 55kg 35 x 97 ceiling tie Fig. 2 Typical 8 metre span ‘Room in Roof’ trussed rafter Comparing an 8 metre span standard trussed rafter (see opposite) with an equivalent 8 metre span ‘RiR’ truss, 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. Some basic guidelines to the construction of roofs from ‘Room in Roof’ trusses are as follows: Three-Bearing ‘RiR’ Trussed Rafters 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 they 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 ‘RiR’ Trussed Rafters 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. 5 shows a typical two-part truss arrangement Some Typical ‘RiR’ Configurations Fig. 3 Three bearing‘RiR’ trussed rafter 0.4L 0.2L 0.4L To be most effective the third support to be located in this zone and as near to centre line as possible Fig. 5 Two-part ‘RiR’trussed rafters Connection detail provided by the trussed rafter designer with each truss design Fig. 4 gives some ideas on the size of loft rooms available from differing configurations of span and pitch of ‘RiR’ trusses (all room widths shown in metres) . These sketches are intended to show geometry of roofs at various spans and pitches and not structural details. In some cases extra intermediate supports may be necessary to achieve these spans. Internal, intermediate members may be needed within the nonhabitable spaces of the trussed rafters on very large trusses and in some cases trusses may need to be produced in two parts. For clarity such details have been omitted from the sketches. PAGE 2 Services i

FAQs

What are the key advantages of ‘Room in Roof’ (‘RiR’) trussed rafters over other living roof construction methods?

‘RiR’ trussed rafters provide structural roof and floor within the same component, enable flexible lower floor layouts, benefit from computer design and factory assembly for quality control, reduce complex site joints, and can be erected quickly to create a weathertight shell earlier.

Why do ‘RiR’ truss members tend to be larger than standard trussed rafter members for the same span and pitch?

The ‘RiR’ truss supports about 60% more load than a standard truss of the same span and pitch, and increased member sizes are also attributed to lack of triangulation.

How should services and bracing be handled in ‘RiR’ trussed rafters?

Services should be routed in the lower void area between bottom chords, accessed via a small hatch, and the floor joist should never be notched or drilled for services. ‘RiR’ trussed rafters require bracing like other trussed rafter roofs, with special attention to diagonal bracing.

Oct 29, 2026

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