Roof Top Garden

I was teaching an Intro class at the local community college this evening and one of my students is a teacher at a very large Catholic school.

One of the things they are considering is a "green roof" and she wants to introduce the idea of an SFG there. I'm "assuming" (yeh, I know what they say about that) they have already had the structure inspected, but she thinks they may want to consider an SFG up there!

I would love some thoughts here. What things do you think I need to ask or make sure they are taking into consideration?

Presumably a structural engineer could give a lbs. per square foot (or some such measurement) that the roof would hold. Of course I could find out how much a raised bed with a bottom would weigh. I could soak 8 cf of Mel's Mix and see how much it weights sopping wet.

BUT how would I estimate the weight of mature plants? For instance watermelons or a full tomato plant?

HELP! LOL

Also, they have a group from the school that goes to Haiti every year and she thinks they'll be interested in introducing SFG there.

I'm not sure if it was Urban Homesteading or Mother Earth News but one of the two talked about roof top gardening and suggested self watering containers. It's difficult to get water up to the roof and it helps minimize run off.

I have no personal experience.

ksroman wrote:...Presumably a structural engineer could give a lbs. per square foot (or some such measurement) that the roof would hold. Of course I could find out how much a raised bed with a bottom would weigh. I could soak 8 cf of Mel's Mix and see how much it weights sopping wet.

BUT how would I estimate the weight of mature plants? For instance watermelons or a full tomato plant?

Early this year, Furbalsmom did the work for you. Extrapolating from her extra deep box, a 4x4x6 inch box needs 19 gallons of water to saturate the MM. (water is 8 pounds per gallon *closies count in sfg*). Heaviest case is using 2x boards and 3/4 ply. Round up for safety, and call it 250 pounds for a 4x4x6 box of saturated MM ready to plant. Can the roof support two adult men? It will support the box. Realistically, the weight of the veggies doesn't matter.

I would look for reasonably easy access, a water source on the roof, and easy disposal of plant debris at harvest time.

Something to watch for: rooftops can be considerably hotter than standing on the ground. It depends on the type of construction, and whether the space under the roof is insulated or vented.

If you're worried about weight you can distribute it over a wider area. For example if you had 250 lbs and used four 4"x4" posts to support it, there would be 62.5 lbs of weight on each post (approximately 15 lbs/sq in). However, if the posts rested on a plywood platform then it would distribute the weight over much of the platform. Even if you put it on a frame made of 2x4's that would help distribute the weight.



As Boffer said, if the roof can support 2 men, it can support a box. However, you also need to account for the weight of the person or people working on the box(es).