Roof form and framing: original details

Most houses had relatively shallow gable roofs.

Roof form

Roofs were generally gabled, including Dutch gables. Hipped roofs were less common.

Roof pitch was typically 10 -15° for metal roof cladding, although some had a lower slope (below 10°). Wide eaves were common.

Internally, roofs could be:

  • a timber frame of rafters, collar ties and ceiling joists with an enclosed roof space and flush ceiling
  • a skillion roof with exposed (Figures 1 and 2) or concealed rafters supported on an exposed ridge beam (Figures 3 and 4) and sometimes additional beam (Figure 5) at mid span
  • a prefabricated timber truss with an accessible roof space and flat ceilings.

 

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Roof framing

The roof could be constructed on- or off-site. Off-site roof framing consisted of prefabricated timber trusses, while on-site roof framing consisted of a framework of rafters, ridgeboard or a ridge beam, underpurlins and underpurlin struts, and collar ties and cleats.

Skillion or exposed rafter roofs were framed up on-site.

As with wall framing, roof framing timber was often green or wet, which could lead to shrinkage problems and sagging where long spans were not supported until the timber dried.

The introduction of NZS 3604 in 1978 gave specific requirements for rafter, purlin and ceiling joist sizes and spacing.

On-site framing of gabled roofs generally used couple-close construction - that is, where each pair of rafters are tied at their bases by ceiling joists to prevent spreading.

Rafters were typically (ex.) 100, 125 or 150 x 50 mm depending on span, and spaced at 900 or 450 mm centres.

Ridge boards were 25 mm (1") wide with a depth dependent on the rafter size. Rafters were birdsmouthed over the top plate and extended beyond the wall framing to create the eaves.

Purlins, typically 75 x 50 mm and fixed across the rafters, were spaced at 760-900 mm centres depending on the roof cladding.

Roof bracing

Hipped roof construction does not require additional bracing but gabled roofs require both roof plane and roof space bracing. This was provided by diagonal braces that were either 150 x 25 mm solid bracing cut in between rafters, or continuous bracing fixed below the rafters for roof plane bracing, and bracing from the ridge board at the gable ends to an internal wall top plate for the roof space bracing.

The early 1970s saw the introduction of 25 mm wide galvanised metal strap to provide roof plane bracing.

For exposed rafter skillion roof construction, the installation of sheet plasterboard or plywood ceilings provided the roof plane bracing - plasterboard was also used to provide bracing at ceiling level.

Sarked roofs

Sheet sarking (plywood and, for a short period, moisture-resistant particleboard) was used to support butyl rubber, or asbestos/bitumen (Nuralite), and occasionally, due to the brittle nature of the sheets, was used under asbestos-cement roofing.

Tile battens

Concrete and metal tiled roofs required battens fixed to rafters to support and secure the tiles. See roof cladding.