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.

< Back to FAQ's

20/09/24 LABC Trainee Building Control Surveyor

Looking for an opportunity to earn while you learn and gain the qualifications needed to start a successful career in the construction industry?   Following a successful initial recruitment campaign resulting in over 140 job offers, LABC, the organisation that represents all local authority building control teams in England and Wales, is recruiting additional Trainees... > Read More

02/11/23 Staff Training Day – Wednesday, 3rd October 2024

At CNC Building Control we are committed to ensuring our staff are trained to the highest standards and this is done in a number of ways including internal & external training courses, CPD, day release and mentoring. As part our staff training programme we have initiated ‘all staff training days’.  Our next ‘all staff training... > Read More

03/05/23 New Building Regulations/Approved Documents – Transitional Arrangements Update

There were substantial changes to the Building Regulations in June 2022 in respect of Part F (Ventilation), Part L (Conservation of Fuel & Power), new Part O (Overheating Mitigation) and new Part S (Infrastructure for Charging Electric Vehicles). Any application submitted prior to 15 June 2022, where the work does not commence before 15 June... > Read More

18/01/23 Leading the Way into 2023

Following the Grenfell tragedy and subsequent enquiry, there are reforms being introduced by the new Building Safety Regulator and regulatory changes passed by parliament.  One of these changes will see a requirement for all building control professionals to be accredited by April 2024.  The level of accreditation will correspond to the type of work they... > Read More

03/11/22 We’re Moving – Beyond the Horizon

We are delighted to announce that CNC Building Control’s Eastern Team will be relocating to The Horizon Centre, Broadland Business Park, Peachman Way, Norwich NR7 0WF from Tuesday, 30th May 2023.   Our phone number, email address and website will remain unchanged.   If you have any questions please do not hesitate to contact us. > Read More