Green roof drainage plane
I'm planning to build a green roof over a 120 sq ft structure and I have sources for all the components except the drainage plane.
I got a quote from a company offering a solution; $35 per square foot! Way too pricey
Gravel would work and is cheap but I'd rather not have that kind of weight on the roof. Wiffle balls would work and they're light but they're not cheap (in the quantity I'd need)
The systems are designed together for a reason. Proper drain mat is critical and designed to ensure proper water flow while being able to support the growth medium and vegetation.
I do entire systems for less than $35/sf. Never heard of any type of drain mat for that price.
I'm not in the office but I'll see if I can't get you information on the two mats I've used. It's not cheap but I guarantee you it's not $35/sf.
Even though it's only one square, four grand does not sound unreasonable for a membrane roof and slip sheet that someone is going to grow plants on top of... If I have to provide any kind of warranty, that is.
The system we use is prevegetated and uses lightweight grow medium in a 1x2 tray system. Even with the lightweight aggregate the structure still has to support a lot of extra dead load when it's wet/snowy (140lbs/sf IIRC). Never heard of the wiffle ball method.
We use EPDM as our drainage plane, but my honest advice is call a pro. As noted above some contractors may be able to install the whole system for that price.
I have the soil media, I have the plants, I have the filter fabric, I have the waterproof liner (EPDM); all of these were cheap ($200) I just need a drainage plane
140psf is crazy heavy. Fully saturated a 4" soil depth of sedums on a lightweight drainage plane (commercial) is around 25psf
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