STRUCTURAL DESIGN FOR ARCHITECTURE

Table 6.13 Approximate span ranges for

different types of timber pitched-roof structures

Span

Spanning across Timber joist (flat), 2.5 m to 6 m

building plain rafter, close

couple

Tied rafter 3 m to 7 m

Trussed rafter 6 m to 20 m

Truss and purlin 10 m to 30 m

Spanning between Sawn-timber purlin 2.5 m to 3.5 m

cross-walls Plyweb, laminated or 2.5 m to 8 m

steel purlin

Timber structures

Fig. 6.46 Purlin roof supported on cross-walls. This
arrangement leaves the roof space free of structure. The
purlins carry a relatively large area of roof, however, and
therefore significant load. Built-up-beam sections are
required to achieve spans greater than 3 m. Fig. 6.47 Typical cross-section through a purlin roof.

Cross-wall spacing is normally wider (5 m to
6 m). The trusses which are used in truss-and-
purlin construction are much stronger and
heavier than trussed rafters and usually have
sub-elements which consist of several boards
jointed by timber connectors. Alternatively, a
truss can be made by bolting together a
number of trussed rafters.
Purlin construction has the advantage of
leaving the roof space relatively clear of struc-
ture. It also allows the longitudinal walls of a
building to be non-loadbearing, which can be
an advantage with certain types of planning.
The purlins themselves tend to be fairly heavily
loaded, however, and must normally be
timbers of large cross-section. The size of
cross-section which is required depends on the
purlin spacing and on the weight of the roof
which is carried, but the largest sizes of sawn-
timber elements are rarely capable of spanning
further than around 2.5 m when acting as
purlins. Where larger spans are required, built-
up timber beams or lightweight steel elements
are used. The depth of built-up timber purlins
is likely to be large (800 mm for spans in the
region of 5 m to 6 m and provision for this
must be made in the planning of the building.

In truss-and-purlin structures it is common

practice for the top element of the truss to be

located in the same plane as the rafters as this

simplifies the detailing of the roof. The arrange-

ment requires that purlins be suspended from

the top elements of the trusses rather than

located on top of them (Fig. 6.45) as was the

normal practice in traditional roofs (see Section

6.2). Span ranges for the types of timber roof

described here are given in Table 6.13.

221

Jack rafters birdsmouthed

over purlins and wall-plate

Floor joists

Purlin

Non-loadbearing

partition

External

loadbearing

wall

Internal loadbearing wall

or beam to support

intermediate floor

Ridge purlin