Hi, I’m installing a sedum roof onto a 20 sq m garden room GRP flat roof.
A common system seems to be: drainage layer on the bottom, then substrate, then sedum roll on top.
But some companies sell a lightweight substrate roll e.g. https://thegreenroofcompany.co.uk/product/sempergreen-lightweight-system/ while others use a loose growing medium e.g. https://www.sedumsupply.co.uk/green-roof/growing-mediums/ instead.
Is either one better?
And I’m concerned about the weight if using a loose growing medium. My calculations are:
Joists - 21.5cm deep x 5cm width
40cm between joists centre to centre
span 4m (front to back of garden room)
https://www.timberbeamcalculator.co.uk/en-gb/span-table/floor-joists?load=1.5&class=C24 says with up to 1.25 kN/sq m, ie 127 Kg / sq m weight, joists can be 4.414m long
Total weight per sq m:
Sedum roll 400kg (I bought it, that’s what it weighs!) / 20 sq m = 20 Kg / sq m
Growing medium (recommended 1.5 - 2 bags / sq m) say 1.5 of the 25Kg bags: 37.5 Kg / sq m or when wet multiply by 1.5? So 56 Kg / sq m
GRP 14 Kg / sq m
Plywood 4 Kg / sq m
TOTAL: 94 Kg / sq m
So it is under the 127 Kg / sq m maximum, so should be OK?
Can anyone check this and say if they agree?
