Green Roof Joist sizes

[Doug_Kent]

I am putting a new flat roof on a brick built garage, stage 1 is to get the roof watertight and stage 2 next year is to make it into a green roof, probably sedum.

The garage is approx 5.25m x 3m with joists to run across the width (3m) I have been looking at some of the weight considerations for a green roof and they vary from 125kg - 250kg per sqM.
The joists are going to be 40cm apart but I am unsure what size joists to use, different advice some say 5"x2" other 6"x3". The joists will be attached to timber bolted on top of brick wall probably 4"x2".
The other concern is should i use noggins to eliminate timber bending.

Any advice would be appreciated planning to use EPDM for making watertight (?).

Thanks

 

[oldbutnotdead]

Noggins prevent racking rather than bending but yes fit 2 at 1/3rd span. Bare minimum 6 x 2 for that load and span, if you add snow load 8 x 2 might be sensible

 

[Bouy]

Running the calcs
50 x 125 - no
Taking a min load of 125kg and snow load of 75kg, 50 x 150 (c16 or c24) would suffice
Taking max load of 250kg and snow load of 75kg, 50 x 200 or 75 x 150 (c16 or c24) would suffice

Snow load could be increased to 100kg if you are in Scotland or high ground

 

[catlad]

6x3" & 16" centres sounds right for that type of roof.