Green Roof
Green Roof
(OP)
I have a client who is looking to place a green roof on top of a proposed single-story commercial building. Currently the building is being proposed using a modular roof system. The proposed depth of the system is 5". Being the structural engineer I need to account for the weight imposed on the roof by the system. We are investigating the feasibility of the system and not much information is available at the moment other than the proposed depth.
Web searches have turned up some information saying that the system should weigh about 7 to 8 pounds per square foot per inch of depth (yielding about 40psf in my instance). ASCE 7 requires a roof load of 100psf for roofs used as gardens so it seems like this would control over the 40psf requirement. Does this seem correct?
Also how does this impact snow loads on roofs (I don't think I will find any information on this in any of the building codes)?
Web searches have turned up some information saying that the system should weigh about 7 to 8 pounds per square foot per inch of depth (yielding about 40psf in my instance). ASCE 7 requires a roof load of 100psf for roofs used as gardens so it seems like this would control over the 40psf requirement. Does this seem correct?
Also how does this impact snow loads on roofs (I don't think I will find any information on this in any of the building codes)?






RE: Green Roof
Changing roof mass (depending of the green roof type and size) can trigger seismic rehabilitation clause depending of your local code.
Personally, I always consider that the garden users might change the low density soil to normal soil !
RE: Green Roof
RE: Green Roof
RE: Green Roof
Add a little extra for the green roof area. 5" will turn into 7" and more when they add some new soil and plants. There are modular systems that are basically trays that drop in. These are ideal.
RE: Green Roof
RE: Green Roof
RE: Green Roof
Richard A. Cornelius, P.E.
WWW.amlinereast.com
RE: Green Roof
Is the owner aware of the costs? He soon will be as he is "putting the numbers together" meaning we are working with a fabricator to put the budget together on the major structural components. Also adding to the problems is the fact that the proposed building is on bad soil which will require a pile system (we have done work in the area before and the soil was not good). Knowing the client the building will be constructed.... whether or not it has a green roof is a different story.
RE: Green Roof
Or is the dirt right on top of the panels? Big corrosion and root damage if that happens!
RE: Green Roof
What is being proposed is a special engineered plant trays that are only 5" deep and hold the medium for the plants to grow but also allow for drainage for the roof so you will not end up with 7" of soil being placed on the deck. The system is currently proposed to rest directly on top of the roof membrane so water tightness is not a concern either.
RE: Green Roof
But tree/plant roots are devious, mean, nasty, and very - very pervasive. Perverse too. Look for the root tips to come poking out through those drain holes.
RE: Green Roof
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RE: Green Roof
While I agree that 100psf does seem high and probably isn't warranted in this case however, I can't find anything in ASCE 7 that limits the load based upon accessibility. In this instance the client is going to have to climb up a 22' tall ladder and go through a tiny hatch to access the roof but there is nothing I can see in the code that lowers the load in this instance.