Turf water demand varies with the climatic conditions where your project will be located. Your location extension agent should be able to supply you with the local weather conditions.
Here is an example from Minnesota for the summer:
The graphs infigure 1, for an average year, indicate that lawns use about 4 to 6 inches of water per summer month during June, July, and August. The precipitation during each of those months is usually 3 to 4 inches, so it is appropriate to schedule lawn irrigations to add about 1 to 1 1/2 inches per week minus any rainfall received during the hot summer months.
Your demand will be somewhat less that typical horticultural or agricultural demand because you have no vertical drainage. If you put the same amount of water into your system as would be required on drainable ground, you'll end up with root rot and other plant diseases.
Find out which plants will be used and research the nutritional and transpiration expectations of those plants.
Extensive – Yes, occasionally during the first year of establishment just like any landscape. Drip irrigation is ideal for large projects plus it is inexpensive and delivers the right amount of water to the best area – the base of the plants. But then the answer should be no, if you have chosen the correct drought tolerant plants wisely for your area, except in extreme periods of drought. Then the plants would certainly benefit from occasional watering during extreme periods of duress – that’s one reason a water source should be close by.
Intensive – Yes, since an intensive greenroof can accommodate a large variety of plants, shrubs and trees, their watering requirements are higher than succulents and herbs. Treat an intensive greenroof like any garden or landscape at ground level, but take into account that high winds can be very drying. Usually large intensive greenroofs have an irrigation system installed.
So what about irrigation or supplemental water – can it still be environmentally friendly?
Yes. You can install a traditional active irrigation system or a solar powered system. Pair this with a recycled rainwater collection system, harvested in cisterns at roof deck or at ground level, and you’ve got the ideal self-sustainable answer to supplemental water and how to power it.
Many states in the U.S.have extension web sites with regionally calculated Evapotranspiration estimates for different vegetation type. Examples are as follows:
If you can find this resource in your region it should get you in the ball park (Demand may be higher due to a microclimate that may be more windy at rooftop and less humid in urban setting). You probably want to design system for peak month use.
Drainage system may be advisable to prevent accumulation of salts in soil which could cause issues in the future.
Depending on area and climate drought tolerant plants may be the way to go--and may prevent the need for irrigation system.
One thing that MUST be done, though, is to make sure you have a very good roof membrane system on the roof, including full time, in process inspection during construction.
This is such a stupid idea anyway. When the roof eventually rots, the garden will fall to the ground anyway, where it should have been in the first place.
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
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