D

esigners frequently rely

on framing manuals for

the sizing of timber

components in a house. The

selection of rafters is one

example.

The rafter span is commonly

taken as the distance between

understood which can lead the

unwary designer to specify an

unsafe structure.

This can happen when the

design involves a vaulted

ceiling formed by a collar tie

across opposite rafters. This

profile is often also achieved

with trusses, using what is

commonly termed as overhang

tables for the situation in

Figure 1. The reason lies in

understanding the structural

systems involved.

Put simply, the timber

framed structural system

usually relies on internal load

bearing walls to support the

ceiling joists and underpurlins.

That is usually how spans

smaller.

However, the examples we

have in Figures 1 & 2 do not

use any internal walls for

support. Hence, the roof

structure has to span from wall

to wall, whether it is raftered

or trussed.

It is incorrect to size the

rafter based on a span from

should more correctly be

based on a span from wall to

wall.

This brings us back to the

basis for the traditional timber

framed roof covered by

framing tables. The traditional

approach assumed involves

rafters used in conjunction

with ceiling joists that act as a

and ceiling joist (horizontally)

and at the other end by the

opposing rafter.

The ridge board merely acts

as a longitudinal tie and to

maintain a level ridgeline. It

does not have the ability to

support the rafters.

Another common traditional

approach that does not have a

rafters span from wall to ridge

beam.

As the ridge beam supports

the roof, they are naturally

much larger than ridge boards

in size.

boards, the ends of the beam

should also be properly

supported.

uplift as the joints are more

reliably connected.

identically to ensure a

consistent profile and roof

shape. Trusses are certainly

cheaper, easier and quicker to

install than framing timbers.