Garden Room - 4m flat roof span timber size issue

[Alex]

Hi All, 

Am just planning a Garden Room / Gym which is planned to be 6m x 4m internal. The issue I'm having is that to keep the height within permitted development I'm struggling with keeping the roof make up thin enough. Based on the 4m span from what I understand 2x8 or 2x10 at 400 centres would be fine. However this takes me over the max height limit and I don't really want to drop ceiling height inside.

Would using doubled up 2x5 or 2x6 timbers work instead of the bigger timbers? I can't find span tables that show what are basically 4x5 or 4x6 roof joists.

 

[markc]

6x2 timbers will be fine over 4m. If you want to go even thinner then 100mm zinc purlins (very light weight) will be ideal.

As long as you didnt get heavy snow then 4x2 timbers could be used dependent on roof make-up and weight

[Conor]

I had the same issue and ended up glueing and bolting 4x2 timbers together (was only a 2.7m span). Glueing and bolting timbers together increases strength far more than double.

[Alex]

thanks, that would also be my plan, glue and bolt them together with m10 or m12 bolts and probably add some nails for good measure!

It's just trying to figure out the smallest timber size that can be joined that will work for a 4m span.

 

Edited March 9, 2021 by Alex

[Temp]

The old Trada tables I have say that for 4m span use somewhere around 175 * 63 on 400mm centres (Access for maintenance only). The next size down in the table is 145 * 63 which cant quite go to 4m. So 150*50 doubled should be plenty.   

[Alex]

many thanks that's good to hear. 2x 6x2's it is then. ?