Green roof - types and weight

[ashthecat]

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?

[Conor]

Need to add snow loading as well and live load for maintenance. 

 

When I researched green roofs, figure I saw most commonly to allow was 1kN in addition to the roof deck. That's obviously worse case but that's what you should design for. If you double up every other joist and glue and screw, I'm sure you'll bring it to 1.5kN/m² which would be standard.

[SteamyTea]

3 hours ago, Conor said:

1kN

Is 102 kg/m²

 

[saveasteading]

Don't forget its saturated weight, plus snow.

Also note that the waterproofing layer has to be root resistant, and that some of these that are, are not UV resistant, so a challenge where it wraps over eaves or parapets.

I don't know about grp, but have a feeling it isn't enough on its own.

[SteamyTea]

1 hour ago, saveasteading said:

don't know about grp, but have a feeling it isn't enough on its own

It can be.