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.