STRUCTURAL DESIGN FOR ARCHITECTURE

Fig. 6.42 Traditional pitched roof arrangement of closely
spaced rafters and ridge board. The rafters exert a horizon-
tal load on the tops of the walls.

must therefore be long as well as of large
cross-section. This method of construction has
now been largely superseded by the trussed-
rafter system in which rafters are incorporated
into a fully triangulated arrangement (Fig.
6.27). Very small timber sections may be
employed with this arrangement and savings
are possible of up to 30% in the volume of
timber which is used, as compared with trad-
itional methods. Trussed rafters can be built
up on site using bolts or nails but more
commonly are proprietary components jointed
with punched-metal-plate fasteners. They are
most suited to spans in the range 5 m to 11 m.
A disadvantage of the trussed-rafter roof
system is that the structure occupies the whole
of the roof space.

Fig. 6.43 Semi-trussed arrangements reduce or eliminate
the horizontal force on the wallhead but require long sub-
elements.
(a) The tie at eaves level eliminates horizontal thrust at the
wallhead. The tie is here spanning the entire width of the
building, which limits the span.
(b) The collared roof leaves some horizontal thrust at
eaves level but requires a shorter length of tie.
(c) A longer span is possible if the tie is supported from
the ridge.
(d) This semi-trussed arrangement is dependent on con-
tinuity of elements through joints for stability.

Timber structures

219

(d)

Binder

Ceiling joists

acting as ties

Wall plate

(c)

Ridge board

Hanger

Rafters

(b)

(a)

Ridge board

Wall-plate

Rafters