What size timbers do I need for my rafters or joists?

The size of rafters or joists will depend on several factors including the span and spacing of the timbers. The following is a guide for the most common sizes.

COMMON TIMBER SIZES FOR A GIVEN SPAN

 

Joist Span(m) Floor Joist(mm) Ceiling Joist  (mm) Pitch Rafters 22.5o – 30o (mm) Flat Roof (max 10o pitch) (mm)
2 47×120 @ 600c/c  38×120 @ 400c/c 38×125 @   600c/c 38×120 @ 400c/c
2.5 47×145 @   600c/c 38×145 @ 400c/c 38×150 @   600c/c 38×145 @    400cc
3 47×170 @  400c/c 47×170 @ 600c/c 47×150 @   400c/c 47×170 @    600c/c
3.5 47×195 @   400c/c 47×170 @ 400c/c 47×195 @    600c/c 47×195 @    600c/c
4 47×220 @  400c/c 47×195 @ 400c/c Purlin needed 47×195 @   400c/c
4.5 75×195 @   400c/c Binder Needed Purlin needed 47×220 @  400c/c
5 75×220 @  400c/c Binder Needed Purlin needed 75×220 @ 400c/c

If timber sizes to be used or the pitch of the roof differs to that indicated then further professional advice should be sort.

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