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Single-Storey Frames, Shells and Lightweight Coverings 199
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Photograph 4.11 (Continued)
too large a deflection of the flat roof structure under load, thus leading to ponding of water
on the surface of the roof. The advantage of a flat roof is that there is little unused roof
space to be heated. Solid web I-section steel beams supported by steel columns may be
used for industrial applications where the main beams are used to support lifting gear, but
the most common form of framed flat roof construction is with lattice beam or with space
frames.
Lattice beam flat roof construction
The terms beam and girder are used in a general sense to describe lattice construction. The
term ‘beam’ is used to describe small depths associated with most roof construction and
‘girder’ for deeper depths associated with, for example, bridge construction. For flat and
low-pitch roofs, it is convenient to fabricate the top boom to provide a fall for the roof
decking or sheeting. Lattice beams are either hot dip galvanised, stove enamel primed or
spray primed after manufacture.
Short-span beams that support relatively light loads may be constructed from coldformed
steel strip top and bottom booms with a lattice of steel rods welded between them,
as illustrated in Figure 4.41. The top and bottom booms are formed as ‘top hat’ sections
designed to take timber inserts for fixing roof decking and ceiling finishes.