Correct Answer - Option 4 :

\(\frac{1}{3}{\rm{\;to}}\frac{1}{5}\)of span

__Concept:__

For economical spacing of truss,

The cost of the truss should be equal to twice the cost of purlins plus the cost of roof covering.

t = 2p + r

where,

t = cost of truss, p = cost of purlin, r = cost of the roof.

However, the above expression is used to check the spacing of the roof truss.

It can not be used to design the spacing as spacing does not occur in the equation.

As a guide, the spacing of roof trusses can be kept 1 / 4 of the span up to 15 m and 1 / 5 of span for 15 - 30 m spans of roof trusses.

∴ Option 4 is most appropriate