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Green Roof

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)?

RE: Green Roof

I am curious to see if other replier will consider garder roof 40 psf as additionnal dead + Live load of 100 psf as reunion location.

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

Here in canada, roof are designed for maximum load between snow and live. Check your local code for both load combination for roof !

RE: Green Roof

(OP)
This is a new structure. The systems doesn't really look like it would take well to foot traffic. To me, the 100psf seems high as the area is not accessible to the general public but I'm not entirely sure the two loads do not add up either.

RE: Green Roof

Be very clear with the client - will this be a public area? If so 100 psf LL, otherwise 20 psf.

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

(OP)
As of right now there isn't even a stair located up to the roof so it is not accessible to the general public.......yet. Even if there was a stair, I don't see him allowing the public on the roof. This seems more like a way for the client to get some tax breaks..... maybe the green roof is allowing him to build the building? In any event I will need to clearly be define the design loading on the roof drawings.

RE: Green Roof

Is the client aware of the cost realities of this? We have watched this design feature increase the cost of the structural steel by 2 - 3 times only to be deleted in the end. If this is for LEED points, there are more cost effective points. A white roof will get you the same points and is far more cost effective considering the torch on membrane is only about 15% more in white. That said we have done one roof this way, and have seen other systems. They do vary. We had one incredibly wealthy client that wanted to use 7" of soil on the roof. Others we have seen use an extruded insulation product that significantly reduces the dead weight. Another issue when they start growing things on the roof is how much they need to water it. Most opt for a low maintenance option, but this type of design tends to bring out some interesting characters that have wildly varying ideas that they never tend to think to mention to people like us.

RE: Green Roof

Then there is the Spring Garden Party attended by lots politicians. Maybe just use 5 psf and get rid of the trash on the roof. that said. the owners will eventually use this as a gathering place for company functions. Add people, landscaping(paths and chairs/tables/food service). I would really go with the live load.

Richard A. Cornelius, P.E.
WWW.amlinereast.com

RE: Green Roof

(OP)
We had a meeting with the architect yesterday and he agreed with the 100psf live load (although he seemed to think it should be higher based upon a code reference I can't seem to find). As of right now, the only access to the roof is through a hatch so no parties will be held up there. I'm not sure if this warrants using lighter loads but we are in a preliminary phase here so I don't think we will back off the 100psf requirement.

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

So, what load per square foot happens when one 250 gardener plus two guests go on the roof between the 7" dirt roof and start playing in the dirt? You've easily got 700 lbs of people up there in a four foot safe-walking area long area between the gardens?

Or is the dirt right on top of the panels? Big corrosion and root damage if that happens!

RE: Green Roof

(OP)
racookpe,

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

That will certainly help control the problem...

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

(OP)
I agree, but as of now,t here are no trees planned for the roof..... only what appear to be short grasses and plants (plants less than 4" in height.

RE: Green Roof

Adding a green roof does not automatically trigger a 100 PSF live load requirement. This is only if it is accessible to the public. From what you described, I would not consider your roof accessible to the public or designed for assembly.

RE: Green Roof

(OP)
steellion,

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.

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