Green Roof over detached residential garage
Green Roof over detached residential garage
(OP)
Owner wants a flat green roof on their garage. Roof structure: LVL joists/rafters and plywood sheathing. 8" masonry parapet.
My gut says:
My gut says:
- "design for bathtub/rooftop swimming pool" (i.e. assume the/any roof drainage fails).
- I plan to tighten up the joist spacing (probably 12" o/c), and go with thicker, pressure-treated plywood.
- 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.
- 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






RE: Green Roof over detached residential garage
RE: Green Roof over detached residential garage
RE: Green Roof over detached residential garage
RE: Green Roof over detached residential garage
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.
RE: Green Roof over detached residential garage
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.
RE: Green Roof over detached residential garage
RE: Green Roof over detached residential garage
RE: Green Roof over detached residential garage
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.
RE: Green Roof over detached residential garage
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)
RE: Green Roof over detached residential garage
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.
RE: Green Roof over detached residential garage
RE: Green Roof over detached residential garage
RE: Green Roof over detached residential garage
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)
RE: Green Roof over detached residential garage
RE: Green Roof over detached residential garage
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!!
RE: Green Roof over detached residential garage
RE: Green Roof over detached residential garage
RE: Green Roof over detached residential garage