Any hints and tips regarding new sedum roof for garage

[Modernista]

We've just demolished the (unsafe) reinforced concrete roof on a 60s garage which is recessed into a bank (so the roof is easily accessible from the adjacent garden). Approx size 3m x 6.5m. We plan to replace with a lightweight, pebble-edged sedum room on a fairly standard timber roof (44x170 C16/ 400cc, 18mm EN636:2 ply deck and EPDM base membrane). 

 

Be interested on any hints / tips / trusted supplier suggestions (we are in Scotland). Also any experience of creating a drainage fall across the width - do we need more of a slope than a standard flat roof? Any suggestions for best way to have water run-off points at the pebble edges? The previous roof had been covered over with a pitched roof to slow the concrete decay and that discharged straight to the ground without any problems so we can do that again - or we can add a gutter if necessary - or even an integral drain point as there is a garage floor gully and we will only be using the garage for electric bike storage and charging, not a car.

[Iceverge]

Are you planning on using the roof as part of your garden? 

 

Any pictures would help 

[Modernista]

Thanks for the interest - here's a couple of pics. The steel you can see inside the garage will be going (it had been put there for safety by a previous owner because of instability of the concrete roof). In terms of using as part of a garden probably not, we don't need extra garden and would have to guard it too - so we will stick with standard sedum species mix for vegetation.

[Thorfun]

we have a sedum roof and a wild flower roof but I paid someone to do it for me as I got a good warranty for all the work and it wasn't something I wanted leaking! but ours is over habitable spaces and not a garage so I might've had a go if it was over a garage.

 

I'm not really sure I can offer much advise but I put bits about it in our latest blog post:

 

 

which might offer a few bits of insight. I would probably suggest to get the roof designed (or at least signed off) by some form of structural engineer as the weight of the sedum, growing medium and water can be quite substantial. I found this on the web when I was researching green roofs:

 

“To support lightweight sedum green roof construction, your roof needs a loading capacity of at least 120 Kg per square metre. For a wildflower roof with a 150mm depth of the substrate, work on loading capacity of 250Kg per square metre.”

 

Source: https://www.turfonline.co.uk/blog/green-roof-construction/

 

 

[Modernista]

Thanks - yes I was thinking of doing that on the engineering front. Interestingly, the previous owner clearly had a living roof idea in the 70s as there was a 200mm layer of earth on top of the concrete (with no membrane under!!) and I think that must have contributed to the demise of the concrete over many years, even though he later covered it (earth and all) with a pitched roof and added the steels under with ply panels to catch the fist sized concrete chunks that started to fall from the underside of the roof!

 

As it is just a garage we think we will DIY the sedum roof on a pro-built roof deck and membrane.

 

Looks like an amazing project that you have! Presumably OSB is ok as the decking if you have a sealed membrane on top? I was thinking we'd have to use plywood of at least class 2. Are the drains to your living roof integral? 

[Thorfun]

1 hour ago, Modernista said:

Looks like an amazing project that you have! Presumably OSB is ok as the decking if you have a sealed membrane on top? I was thinking we'd have to use plywood of at least class 2. Are the drains to your living roof integral? 

thanks. we're getting there! 

 

our flat roof makeup from inside to out is:

 

posi-joists

18mm OSB

VCL

190mm PIR

11mm OSB

Alwitra VSK single-ply membrane

 

then the green roof is simply laid on top of the single-ply. not sure on the exact make-up for the green roof though sorry. I wasn't really paying too much attention at that point and they laid it so quickly that I missed most of it!

 

we have 2 drains on each green roof which are part of the waterproofing layer, so built in when the single-ply got laid. then the green roof drains down to that layer and the out of the outlet. you can see them on the far side of this photo.