Green Roof over detached residential garage 1

[FoxSE14]

Owner wants a flat green roof on their garage. Roof structure: LVL joists/rafters and plywood sheathing. 8" masonry parapet.

My gut says:[ol 1]
[li]"design for bathtub/rooftop swimming pool" (i.e. assume the/any roof drainage fails). [li]I plan to tighten up the joist spacing (probably 12" o/c), and go with thicker, pressure-treated plywood.[/li][li]I'll plan to design with some pretty stringent deflection criterion as well. L/600 - is that unreasonable? Joist span is 20' so total deflection of 0.4" is a value I can't quite get decisive on....I'll be discussing joist slope with the architect to assist with this.[/li][li]I am told there is no plans for roof access via permanent stairs/ladders...but I keep thinking I should have a contingency/safety factor built in for pavers[/li][/li]
[/ol]

I have not yet dived into design/detailing this one, but any words of wisdom/caution from those who have been down this "green roof road" before? See the attached Architect detail. Happy Weekending.

 

[oldestguy]

Don't forget that nice lawn would make a great place for a big party some day down the road, perhaps teen agers unsupervised. I'd also look at side walls in case the dancers sway back and forth to the rock band rhythm. Saturated earth weights a lot more than plain water.

 

[FoxSE14]

oldestguy - thanks. yes I should clarify a bit more...my intention is to design for 115 pcf (saturated mud) for the soil layer, and standing water above the soil to the top of parapet.

 

[oldestguy]

More: Will the insurer of automobiles under there cover them for possible failures that then affect the cars? How about home owner's insurance, are they covering this is case of some problem associated directly with the earth there? Then too, what about yours? Let's say some driver hits the gas pedal instead of brakes and takes out some of a wall? Without that load up there maybe no problem, but now the situation is different.

 

[dhengr]

FoxSE14:
Get the Arch. to line up the ‘Others’ in the note “GREEN ROOF (BY OTHERS)” and have him/her provide their designs and details for roof waterproofing system and the drainage system, and complete details on entire green roof system. Watch out for ponding and long term deflections. I would slope the entire roof structure to assure some positive roof drainage to scuppers or a drainage pipe at one side. But, the drainage system should be their design. The flashing, counter flashing and roof waterproofing system do not look well thought out at the parapet and parapet cap. The flashing and waterproofing system should go completely over the top of the parapet and end up with a outside drip lip on the top flashing. The parapet is not strong enough as shown and will probably have to include a handrail system or fencing system for the goats or teenagers.

 

[Brad805]

I would try to get a sense from the owner what they intend to grow. There are some lighter systems on the market for green roofs that are aiming for LEED points, but if they want a nice lush lawn and planters around a roof living space this will become more challenging for you. I would be tempted to suggest a more durable roof system than a wood structure.

I hope someone has cautioned them of the cost of such a design feature. We have seen green roofs increase the cost of the roof structure by a factor of 2 to 3 when one considers the impact upon the foundations. If they are not aware of that reality this can be a wild goose chase.

 

[Ron]

DO NOT FORGET THE WATERPROOFING AND HOW IT AFFECTS THE STRUCTURE!! Yes, I'm shouting!

 

[XR250]

For 20' span, I assume you are using i-joists or trusses. I would be very careful doing this as, in my experience, they tend to creep alot more than they should under sustained loads. I would try to use dimensional lumber and steel i-beams if possible.

 

[FoxSE14]

Thanks all for the input. The new information I've received in last few days shows that (thankfully) this is one of the lower profile systems, only 4" soil. (It's still vegetation, so I'll be designing for clogged drains & saturated soil).

Met with Arch yesterday and have successfully steered away from wood and toward steel. Roof will be sloped, as expected. Much of this is still in motion, but we're tip-toeing in the right direction. Thanks again to all.

 

[msquared48]

And who is going to mow this green roof? Will this be a putting green for the executives? If so, I guess the flag cup can serve as a roof drain too to save money.

Needless to say, structurally speaking, I absolutely detest green roofs.

In the long run, it would be much cheaper to hire an artist to paint grass and shrubs on the roof.

Mike McCann, PE, SE (WA)

 

[molibden]

I don't know about US but in Europe these light green roof systems are common and well developed. I have a lot of modern timber homes where people are looking from windows on a flat roof below and they want a green roof. These are basically low maintenance vegetation mats.
As for material, if your roof is leaking you have a problem in either case. So as long as you can see the bearing structure from below and inspect it from time to time, you can have a safe and reliable structure made of timber. It's done all the time around here.

 

[FoxSE14]

molibden - thank you for the valuable input. That is good information to have.

 

[spieng89]

OP you appear to have considered the important design aspects. There is nothing wrong with green roofs, and as structural engineers we should expect to see more of them pop-up in the future as architects and general population consider them pleasurable and environmentally friendly. A green roof is no different than a regular roof if the drainage is compromised except for the additional vegetative/soil load...no dynamic loading consideration at all if access is maintenance only. Will it cost more than a conventional roof, sure. Should you care as an engineer, no, if the client is well aware of the cost up front.

 

[msquared48]

Can't agree. Never have. Never will.

I am in a high seismic area, and the extra roof load will add to the seismic load significantly. The roof will be a problem.

Mike McCann, PE, SE (WA)

 

[molibden]

Green roofs I'm dealing with weight around 60 kg/m^2 full of water. On the other hand if they choose gravel, then it's 90-100 kg/m^2.

 

[FoxSE14]

We will be keeping the underside exposed to facilitate leak detection. There comes a point where execution of the waterproofing installation is out of our hands as engineers, so I have been trying to let go of that worry (it's happening slowly). I informed the Architect that he will need to convince me that the w.p. system has been detailed properly before we submit for permit and the project potentially flies away from me.

For those interested, we are going back to Wood LVLs (versa-lam or similar) spaced at 12" o/c due to cost and availability of materials. Exterior rated plywood. Fortunately, we are not in a high seismic zone here (I have the same concerns msquared and I agree with you). If I remember to, I'll share the updated detail in the coming days.

raspivey - I'm with you...just took a little time to let the mind explore this and get comfortable with the idea. If we're going to elevate the profession, we've got to keep an open mind!!

 

[molibden]

Thumbs up for wood

 

[FoxSE14]

The more information I receive from the Arch, the more our system sounds similar to your low profile mats, molibden. In a previous life, I had designed structural repairs for green roofs in a major city with many feet(!) of soil (parks/gardens over occupied space). I became used to serious structure (concrete flat/waffle slabs) whenever you see vegetation above grade.

 

[molibden]

Fox, check out Xeroflor green roof system.