Easiest way to get some grass growing on a bike shed?
September 25, 2016 5:40 AM   Subscribe

I'm building a bike shed, and I'd like to have grass growing on the roof. I've googled green roofs for hours, but most of it is sales information for various membranes/products and installation companies - nothing simple or even close to DIY.

Now, my requirements are pretty simple. It'll be a bike shed, so i don't care about insulation or if isn't 100% waterproof. The roof is a sloped 3/4" of plywood (roughly 4'x8') - so what's the easiest way to get some grass growing up there?

I know I'll need a side to hold the soil - so how much soil is needed? Do i need some gravel under the sod to help with drainage? What's an inexpensive waterproof membrane? (I've seen some blue membrane used on roofs, but don't know what it's called.)

Anyone attempted something like this?
posted by kamelhoecker to Home & Garden (13 answers total) 5 users marked this as a favorite
The Norwegians do it with several layers of birch bark and then sod on top. Or you could use this method from Mother Earth News.
posted by RedEmma at 6:12 AM on September 25, 2016

Wet soil is very heavy. Before you build the green roof, make sure the structure below it can handle that.
posted by jon1270 at 6:29 AM on September 25, 2016 [9 favorites]

The weight may be an issue. In addition some cities have permits and regulations about these things ( looking at you chicago)
posted by AlexiaSky at 6:35 AM on September 25, 2016

Grass is hard enough to maintain on a lawn, let alone a roof. Use Sedum Tiles. We have this plan for a dog house. They're approx. $24 at Home Depot-type stores or Garden Centres in season.

The dog house is cedar with regular house shingles. We plan to put a frame around the roof edges to hold the tiles in. The garden centre we bought them from has one as an example (Sheridan Nurseries, here in Toronto.) They don't need much water, so I'm not as worried about rot. I have a patch of the tiles on the ground in my garden and in an old sign letter, and they're doing beautifully perennially.
posted by peagood at 6:42 AM on September 25, 2016 [9 favorites]

I found a resources section with engineering info on this plant selling site (also: cool diagram). It has a link to a book on green roof design and promising web resources, like an ASTM standards page.

Also has a link to greenroofs.com's greenroofs 101 page.

Seconding that grass isn't a good choice (at least according to this and one other site). They recommend succulents like sedum, but have other possibilities listed too.

Grass is ecologically problematic in general; it's made for cool wet climates, like the UK and northern Europe; most of it required constant watering in most climates, and if it were on a roof it wouldn't even have the moisture buffer of deep soil. Put it on a roof and it's doomed.

It looks like that site has some good ideas, though.

I found it by searching for [good plants for green roof]

I also had luck with [how to build a green roof]
posted by amtho at 7:26 AM on September 25, 2016

I would use a corrugated fiberglas (like the corrugated metal used to build sheds) as a base layer (also will help with drainage) and be sure to put liquid nails or similar over every nail/screw (to stop drainage through the roof). How sloped is the roof? If anything more than "almost flat", you're going to need to put in some cross boards to keep the dirt from just sliding or washing off.
posted by sexyrobot at 8:09 AM on September 25, 2016

I did this, basically following these instructions: Create your own green roof

Reinforcing the structure is really important, don't skip that bit! I lined the roof with PVC pond liner, over the top of the roof felt.

It's not a habitat many plants can grow in well, it's thin, dry soil, so choose plants that suit that in your climate. I got some sedum matting, but didn't use a sedum soil and I've suffered for it, and a lot died off, but it's starting to grow back now.
posted by Helga-woo at 8:22 AM on September 25, 2016 [1 favorite]

How much soil? Generally 4-6" for a shallow greenroof on a house, I believe, but it seems like you can get away with less if you just use sedums. Some people go with 12" so they can put larger plants in.

What membrane to use? The other day one of my housemates (who also happens to be a general contractor) said thermoplastic polyolefin (TPO) is what he recommends.

Most greenroofs use soil with something like pumice or vermiculite mixed in to reduce the weight. Even so, houses need to be engineered to support the extra load of a greenroof. At the very least, you should substantially overbuild your shed.
posted by sibilatorix at 9:51 AM on September 25, 2016 [1 favorite]

I can't tell where you are but I highly recommend growing a sempervivum roof instead of a grass roof. The will thrive on a sloping roof (which will drain too well for grass to grow without very regular watering). Alpines and stonecrops have evolved for precisely this growing situation. They are also reasonably winter hardy as long as they avoid standing water situations - my parents had them grow in Toronto around the edge of their house with no problem. They also thrived in my garden in England.

Sempervivums that you buy in America will suffer at first - they are almost always overgrown overfertalized hothoused plants and will suffer when adapting to new conditions. But perserve and once they harden off they multiply faster than rabbits.

Something like sempervivum arachnoidium is ideal.

You probably also want to use very little soil and instead use a gravelly grit substrate - but due to weight you are better off using a pumice(lava rock) based mix - which means you will need some sort of lattice structure to hold it in place so it doesn't all just roll down the sloped roof. Interspersing pumice/soil with a couple of layers of chicken wire would probably do. You need less than an inch of mix to grow semps and sedums.
posted by srboisvert at 10:02 AM on September 25, 2016 [1 favorite]

I'd be afraid of the weight of that. Googling around, a cubic foot of top soil weighs about 96 pounds. Say you only put six inches depth, that's 48 pounds per square foot. In my city, we account for 25 pounds/square foot for snow. 73 psf live load is pretty heavy but probably won't break the rafters. The thing I'd be more worried about is that 73 psf over 4x8 is over 2000 pounds, over my head sometimes. That better be one sturdy-ass cross braced shed to not collapse sideways.

For whatever that's worth. I've been known to be overly concerned about shed engineering. :)
posted by ctmf at 10:19 AM on September 25, 2016 [2 favorites]

First question: Where do you live?

There is a heck of a big difference between living in, say, the California Bay Area, and Northern Alberta, or the soggy wet Maritime littoral. The first two factors to consider would be how much rain you get, and how cold it gets. If you live in a desert your plants will need protection from drying out. If you live somewhere soggy your roof will need reinforcement to prevent collapsing under the weight of all the water that the roots absorb.

Similarly, if you expect to have your roof solidly frozen from mid December through to about April your roof will have a slightly different eco-system than if the coldest it will get is about 72* F at 2 AM year around.
posted by Jane the Brown at 4:17 AM on September 26, 2016

Response by poster: Great info, thanks all! Yes, sedums and sempervivums seem far more realistic than grass. And yes, supporting 12" of wet earth would require a much more massive shed than i want to build, but supporting 1-2" of porous soil sounds doable. I'm in Toronto so I probably missed the planting season this year, but I can build the roof before winter sans-succulents, and then plant next spring.
posted by kamelhoecker at 6:48 PM on September 27, 2016 [1 favorite]

May not be useful to you now, kamelhoecker, but in preparing for our own greenroof I ran across the City of Portland's guide to building a DIY greenroof on a shed.

I also have a list of layers as recommended by an architect for a 4" deep roof on a house:
- Soil (25% compost, 75% pumice, with river rock at the bottom edge for drainage)
- "landscape fabric" (aka "geotextile," usually sold at Home Despot or the like)
- 40mil EPDM root barrier (sold as pond liner, but way cheaper if you can convince a roofing supplier to sell you a partial roll)
- 60mil TPO waterproofing membrane (we had this installed by professional roofers because it's on a house)

You could probably get away with skipping the TPO, especially if you find a sheet of 60mil EPDM that's big enough for your entire roof. The standard "square" of EPDM seems to be 10x10, but you can get larger contiguous sheets sold as pond liner.

In a month or two we'll be installing all the layers above the TPO on our own roof. I'll try to drop by with an update...
posted by sibilatorix at 9:10 PM on January 9, 2017 [1 favorite]

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