Integrating a Green Roof with a Rainwater Collection System

Albertino

Hey everyone,

I'm kicking off a project in a coastal area where we're incorporating a green roof for environmental and insulation benefits. The twist is, the client also wants to integrate a rainwater harvesting system into this setup. The challenge lies in ensuring the green roof's soil and plants don't interfere with water collection and that the system is efficient in channeling rainwater from the roof to the storage tanks.

The roof will have layers of soil and vegetation, and I'm pondering the best way to set up drainage that allows water to pass through without losing too much to absorption, while still keeping the green roof healthy. I've done bits and pieces of both systems separately but never combined on one roof.

Would love to hear if anyone has tackled something similar or has any insights on balancing these two eco-friendly systems on a single rooftop.

Albertino

Albertino said:

Hey everyone,
I'm kicking off a project in a coastal area where we're incorporating a green roof for environmental and insulation benefits. The twist is, the client also wants to integrate a rainwater harvesting system into this setup. The challenge lies in ensuring the green roof's soil and plants don't interfere with water collection and that the system is efficient in channeling rainwater from the roof to the storage tanks.

The roof will have layers of soil and vegetation, and I'm pondering the best way to set up drainage that allows water to pass through without losing too much to absorption, while still keeping the green roof healthy. I've done bits and pieces of both systems separately but never combined on one roof.

Would love to hear if anyone has tackled something similar or has any insights on balancing these two eco-friendly systems on a single rooftop. Roofing company in Arlington VA

Thanks so much for any help you can offer!

Larry Weingarten

Hi, What sort of slope does the roof have, if any? If sloped, it seems you could use filter fabric (maybe multiple layers) at the lower end, to keep the soil where it should be, and collect excess rainwater in a normal gutter system. Another thought is to have rain go first into a drum, which can act as a settling basin for any silt, then to the main storage tank. It sounds like an interesting project!

Yours, Larry

Erin Holohan Haskell

@Albertino, the University of Maryland had something similar on their WaterShed home that they presented in the U.S. Department of Energy Solar Decathlon in 2011. Here is more info. If you click on Team Deliverables, you can find construction drawings. And more on the green roof here and here.

Jamie Hall

You have somewhat conflicting aims here -- on the one hand rainfall to keep your roof nice and green, and on the other rainfall to your collecting tank.

First question: for what purpose do you intend the water in the rainfall collecting tank? If it is just for irrigation elsewhere on the property, you won't have to concern yourself (much) with water quality. If it's for sanitary conveyance -- toilers and the like -- similarly, though silt may be more of a concern. It the intent is drinking water, you will need to filter it and disinfect it to meet any sort of public health or local code.

Now. As to the actual mechanism for collection. Not difficult, but heavy (VERY heavy -- get your structural engineer engaged in this project early on! You are looking at a live load of at least 150 pounds per square foot, plus snow load if any -- which is well over any standard roof load). What you need, from the bottom up, is a good waterproof membrane -- I'd suggest heavy EPDM but there are other materials; whatever you use must be waterproof and flexible. Then 3 inches or so of stone (I'm presuming a flat roof here, or rather not flat, but relatively low pitch -- not more than 1 rise on 6 horizontal, and preferably more like 1 on 12)(steeper you will have problems with the soil and plant cover slumping). I'd use half inch to thee quarter inch or one inch, laid and graded but NOT compacted. Then a filter fabric suitable to your soil material above (I'd probably go with Mirafi 140 NL, but get an engineer on this). Then a foot of good clean sandy loam (not a slit loam or clay loam), spread and rolled but not compacted, then three inches of good topsoil.

On the low edge us a slightly thicker layer of gravel to form a slight swale a foot and a half or so from the edge, and in the gravel between that and the edge should be a perforated collection pipe which drains to whatever your rainwater treatment and storage facility is going to be. You'll need a low structural curb, and bring the EPDM up the curb.

No, repeat NO, penetrations permitted in this roof. I don't care where you vent the plumbing or heating or whatever, but NOT through the roof.

And yes, I have built a couple of these, in central Vermont, and I wasn't sued (I'm retired now) and the clients paid me, so they can't have been too bad...

Albertino

Jamie Hall said:

You have somewhat conflicting aims here -- on the one hand rainfall to keep your roof nice and green, and on the other rainfall to your collecting tank.

First question: for what purpose do you intend the water in the rainfall collecting tank? If it is just for irrigation elsewhere on the property, you won't have to concern yourself (much) with water quality. If it's for sanitary conveyance -- toilers and the like -- similarly, though silt may be more of a concern. It the intent is drinking water, you will need to filter it and disinfect it to meet any sort of public health or local code.

Now. As to the actual mechanism for collection. Not difficult, but heavy (VERY heavy -- get your structural engineer engaged in this project early on! You are looking at a live load of at least 150 pounds per square foot, plus snow load if any -- which is well over any standard roof load). What you need, from the bottom up, is a good waterproof membrane -- I'd suggest heavy EPDM but there are other materials; whatever you use must be waterproof and flexible. Then 3 inches or so of stone (I'm presuming a flat roof here, or rather not flat, but relatively low pitch -- not more than 1 rise on 6 horizontal, and preferably more like 1 on 12)(steeper you will have problems with the soil and plant cover slumping). I'd use half inch to thee quarter inch or one inch, laid and graded but NOT compacted. Then a filter fabric suitable to your soil material above (I'd probably go with Mirafi 140 NL, but get an engineer on this). Then a foot of good clean sandy loam (not a slit loam or clay loam), spread and rolled but not compacted, then three inches of good topsoil.

On the low edge us a slightly thicker layer of gravel to form a slight swale a foot and a half or so from the edge, and in the gravel between that and the edge should be a perforated collection pipe which drains to whatever your rainwater treatment and storage facility is going to be. You'll need a low structural curb, and bring the EPDM up the curb.

No, repeat NO, penetrations permitted in this roof. I don't care where you vent the plumbing or heating or whatever, but NOT through the roof.

And yes, I have built a couple of these, in central Vermont, and I wasn't sued (I'm retired now) and the clients paid me, so they can't have been too bad...

Thanks for all the great info! I'm thinking of using the water for watering plants, so I'm not too worried about it being super clean. Definitely going to chat with an engineer because of all the weight we're talking about. Your tips on layering from the bottom to the top and the idea to catch water at the edge are awesome. Super thankful for your advice and hearing about your projects. Thanks a ton!

Jamie Hall

Couple of additional items occur to me -- there is such a thing as too flat. The roof membrane does want to be able to drain -- I'd say 1 on 12 is about as flat as you want it. There's no reason, though, why the roof can't drain two or even four ways, just have collection on all the ediges.

The other is access. Things will grow, and lugging a lawnmower up a ladder is not fun...

jumper

Even if you don't want to harvest water roof needs drainage. Consider perforated pipes in gravel trenches.

Jamie Hall

The drainage from the stone (that's your gravel) bottom layer should be sufficient -- but it does have to have the fabric overlay to prevent clogging. Then the perf. pipe at the edge to pick up the water and take it where you want it.

hot_rod

Any limitation to rain water harvesting in your state?
Here in Utah you are allowed 200 gallons of rain barrels.

If you register with the State Division of Water Resources, you are allowed 2500 gallons

I have a 2500 gallon tank that I fill once a week from an irrigation cancel behind the house, mainly for a small lawn and large garden. The biggest issue is algae growth in the warm summer months. If I were to do it again I would get a bury type tank, keep it out of the direct sun. Those plastic septic tanks are fairly affordable and can be direct buried.

Plenty of filtration systems out there from basic irrigation backwash type particle filters, to multi filtration systems that could give you drinking quality water. Any of the irrigation and sprinkler suppliers have filter options.