283757893275

Single-Storey Frames, Shells and Lightweight Coverings 229
Minimum 150
overlap
50 × 25
timber spacer
Zed purlin
Minimum 150
overlap for
adjoining sheets 20 × 100 Nylon
spacer follows the
roof profile (provides
ventilation gap)
Zed purlin
(a)
(b)
Figure 4.67 (a) Fibre-based agricultural roof: breathing roof with timber spacer. (b) Fibrebased
agricultural roof: breathing roof with nylon mesh.
1000 mm wide sheet
15–25 mm gap
Small gap left between each sheet allows for ventilation
and minimises weather ingress
Fixing should be placed in
first corrugation from edge
Figure 4.68 Fibre-based agricultural roof: spaced breathing roof.
nylon mesh to form a spacer between the courses of profiled sheets, thus providing a simple,
cheap and effective means of ventilation (Figure 4.67). A spaced roof is used for buildings
that house high unit intensive rearing, which require high levels of natural ventilation. In
this roof, the profiled sheets are positioned to create a gap of between 15 and 25 mm
between the sheets; this provides excellent ventilation but also allows some rain and snow
penetration (Figure 4.68).
Decking
Decking is the general term used for the material or materials used and fixed across roofs
to serve as a flat surface on to which one of the flat roof weathering membranes is laid. The
decking is also used to support the thermal insulation, thus creating a warm roof construction.
The decking is designed to support the weight of the materials of the roof and imposed
loads of wind and snow, and is laid to a shallow fall to encourage rainwater run-off. Decking
is sometimes applied to low-pitch lattice beam and portal frames. The most common form
of decking is constructed from profiled steel sheeting. Decking can also be made of timber
(for timber structures) or lightweight concrete slabs (for steel or concrete frames).