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Is there any way for a semi-skilled DIY-er to build a rooftop garden on a three car garage roof??

It's got a fairly low pitch, and a fairly new roof.  It's 20' x 30'-- much bigger than it looks in the photo.  I figure  That 600 sq ft is a lot of room to be just wasted, so I'd love to do something "green" with it.  Any suggestions and ideas would be so appreciated!  Thank you!

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As others have already pointed out, you'd need to get a structural engineer to determine how much weight can be supported and if you'd need to reinforce the structure.  There are also potential issues with permits and such although it's less likely with a structure that isn't a dwelling.

HOWEVER!  That doesn't mean the roof has to "go to waste".  You have a fair amount of surface area that can be used for rain water collection which is something you can do.  You'll need gutters, pipes, and a reservoir, and rain water is MUCH better for plants (especially bog plants) than city water and sometimes well water.  If you have city water, it'll also reduce your water bill when it comes to watering.  Definitely have a look into it because it's something green that you can do with that roof.  It also means you won't have to be climbing up there all the time to inspect plants and water them and do all the necessary work required when it comes to rooftop gardening.  You can save all that energy for increasing planting beds on the ground where it's a quite a bit easier to garden.
 
There are some really awesome ideas here and I know this isn't the question you asked, but to avoid forestalling the seemingly obvious....
how attached are you to the dead brown lawn? betcha could grow a bunch of food in all that wasted dirt! if you don't want to break ground(lawn chemicals pesticides, etc), you could do it on wheeled containers...
just sayin'.....
crazypantsjones (author) 7 years ago
Thank you to all of you who put so much thought into my answer!  I want to be green, but I also want to have a garage that still stands, and of course, I never once thought about my own safety during this project (maybe that would explain the amount of stitches I've gotten during my life, hmm?)

I will not be building a garden up there, but rainwater collection is a fantastic idea!  I bet 600 sq ft can collect a LOT during an Oklahoma downpour!

Thank you, thank you, thank you!
paganwonder7 years ago
If the garage was built with engineered trusses for the roof framing they will hold quite a lot before failing.  That being said- anything you do up on the roof runs the risk of compromising the integrity of the protective covering (the shingles) which can lead to pre-mature building deterioration.   There have been some good ideas presented here for how to do what you are thinking- but....

Any activity which requires ladders puts you at high risk for bad injury-  especially when it is a routine and repetitive activity since complacency tends to creep in with familiarity.   Most of the ladder injuries I have seen happened on the last step before the ground- as in- you are nearly done so you let your guard down.
jeff-o7 years ago
I suggest using that roof for rainwater collection, and plant either a raised bed or traditional garden in a portion of that huge (seemingly unused) backyard you've got there.
aeray7 years ago
Do you have the truss plans for the garage? What area of the country are you in? Do you know what the design (snow) load there is? Is there one?
seandogue7 years ago
You will need to find out how much weight the roof can sustain, in order to do anything substantial.

However, it would be very easy to put a few planters up top without worrying about compromising the integrity of the roof to grow tomatoes, herbs, etc., using a simple slope-foot ed platform to hold each pot, as long as you put the planters close to the wall line on the two sides that do not have gutter lines. (picture a set of terraced steps up each side of the roof...

The trick would be in keeping storm winds from blowing the pots over, (there are alot of ways you can avoid that if you use your cranium) and of course, not thinking that by planters, I mean the sort of planter that could hold a small tree...that simply would not be practical without consultation with a structural engineer and possibly alot of work.

Obviously, the roof will hold more weight than just the roof and a bit of rain or snow, since several people can walk around on such a roof without it collapsing, but I really hope you don't mean covering the roof with a heavy layer of soil. That simply would not be a realistic expectation.
BobS7 years ago
Get some PVC gutters, drill some holes in them and put them parallel (horizontal of course), with a spacing of 2- 3 ft. I would put rock garden plants in them, and or annuals. Of course it would need a drip watering system
NachoMahma7 years ago
.  Depends on what you mean by a "rooftop garden." Put too much load on the roof and you will end up with three crushed cars. Unless you know much more about designing buildings than your question would indicate, I'd leave it alone.
crazypantsjones (author)  NachoMahma7 years ago
Nope, my skill set is one that would be best leaving it alone.  I don't have a ton of cash for engineers, repairs and then building, you know?  Still, I wish I could do SOMETHING up there... it seems such a waste of space, especially because we have only one car.  Thank you for your answer!
As others have said, the roof is almost definitely not designed to support the kind of load that a real green roof requires, but that doesn't mean that the space can't be useful in other "green" ways. It looks like it might be an ideal spot for a solar cell array. If you had a swimming pool, it could a good place for a solar pool heater, too. You could set up a rainwater collection system for watering a (ground based) garden or your lawn. I'm sure there are other ways to use the space as well that don't involve loading up the roof with super-heavy stuff.
mikeasaurus7 years ago
Modern definition of growing medium on a roof surface:

Intensive Green Roof:
The most popular conception of what a green roof should look like.
An intensive system with deep growing medium, able to sustain larger and diverse plant variations due to deeper soil depth (16"+).
Can accomodate light/moderate human traffic.
Engineered design complete with water retention/handling assembly under growing medium.

Extensive Green Roof:
What most green roofs actually are.
System is designed with a shallow growing medium, able to sustain small plants and low ground cover to cover a larger surface area with a consistant growing medium.
Not designed for human traffic.
Engineered design complete with water retention/handling assembly under growing medium.

It's worth mentioning that most green roofs are typified by a flat, or undulating hill design. For a sloped roof like the one shown in the picture you will need to put a lot of work into engineering the existing surface to accept growing medium and water handling.

However, all is not lost! Consider that some of the oldest (and rudimentary) 'green' roofs out there are from looking to mother nature and the most simple materials; Assuming you don't intent on walking on your roof, you can very easily coerce moss and friends to find a home up there without compromising structural integrity and still achieving that 'green' look.

Good luck!

Re-design7 years ago
You need to get an engineer to look at the roof and determine if it is built to support the extra weight of the garden structure, soil and water.  A building inspector not builder, nor framer, nor roofer can not tell you whether it will work or not.  If it fails then everything above the ceiling will come down on top of anything and everybody that happens to be in the garage at the time.  You must use a structural engineer.

If it is, and I'll bet it's not, then build a platform to level a garden area. and fill it with dirt, plant seeds and water them in.

If it is not ready to handle the extra weight then the engineer can tell you what is needed to make it so.
crazypantsjones (author)  Re-design7 years ago
*sigh*.  I'm sure you're right, and although we're only a one car family, I doubt smashing one would be any happier of a feeling than smashing three.... let alone all of hubby's beloved tools and all of my gardening stuff.

Still I wish there was something I could do with that space up there.  Thank you for your answer.
Burf Re-design7 years ago
+1
Exactly what I was going to say.