GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Roof membrane and live vegetative roof

Jamie B | Posted in Energy Efficiency and Durability on

Hey all,

So after Material quotes and city grants etc, it looks like its going to be viable go me to do a live vegatative roof (green roof) on the flat roof I’ve recently built.

now I’m thinking about what roofing membrane to put on, to go under the green roof. I am considering things like cost, longevity, ease of install, potential for leaks etc.

I’m in climate zone 6. My potential roof assembly is as follows:

9.5″ I joists with R31 rockwool batting
3/4 plywood decking
6″ polyiso at R34
1/2″ roofboard
Roofing membrane ***
3″ green roof assembly

For the roofing membrane I’d like to pick your brains.

If I go with TPO, its about $1.50sqft but I’d have to bring in a guy to install it so there are unknown labour Costs there on top. I worry about things like bad workmanship that I might miss which could lead to moisture issues later. Do I still have to replace the roof every so years if its buried under a freewheel roof?

To go full pendulum swing, the other thing I’m considering is the Tremco Puma membrane, which is a cold applied liquid membrane, comes in at $8/sqft, but its easy to apply and the rep will be there on site with me basically run the job and make sure I apply it correctly. To my understanding, I can be pretty confident with this roof system as it will be a monolithic membrane and is suposedly guaranteed to hold water for years.

Anyone have any experience or suggestions for this?

Thanks!

Jamie

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.

Replies

  1. Steve Knapp CZ 3A Georgia | | #1

    Jamie,

    Remind us again where you are located.

  2. Andrew C | | #2

    Protected membrane roofs or "inverted" roof membrane assemblies always make more sense to me. The air/water/vapor control layer is protected from UV, heat and cold, and physical damage. Joe L. has at least one article on inverted membrane roofs (eg BSI-052) that may give you something to think about...you may want to skip the first several paragraphs, as he isn't exactly a fan of "living" roofs, but he gets into some useful details farther down in the article.
    I know this doesn't address your membrane question, but it's perhaps something to think about, depending on where you are in design stage.

  3. Stephen Sheehy | | #3

    Does putting soil and plants over the membrane void the roof warranty?

  4. Expert Member
    Malcolm Taylor | | #4

    Stephen,

    My feeling is that all roof warranties are next to useless. They generally only cover replacement of the defective materials, and are often further pro-rated to take into account the age of the roof. Probably more important with a vegetative roof is whether your home insurance would cover any damages, as any leak is going to affect a lot more than the membrane.

  5. GBA Editor
    Martin Holladay | | #5

    Jamie,
    I don't think it's a good idea for a homeowner to select the components used to build a vegetated roof. At a minimum, you need to locate a company that sells components specifically intended for this type of roof, or to hire a consultant experienced with these installations.

    You are rightly concerned with roofing longevity, which is a high priority with this type of roof. You also need to specify layers which provide excellent drainage; these layers are installed between the roofing and the soil-like medium that holds the plant roots. All of these details need to be perfect, all of these details need to be long-lived, and all of these details are expensive.

    For more information, see Vegetated Roofs.

  6. Jamie B | | #6

    Thanks for all your responses

    Steve: I am in Toronto. the city had a standard for green roofs but it looks like it was designed specifically for commercial buildings (a bylaw requires new commercial buildings to have green roofs). I might have to adhere to this for the city grant but would change things for better aesthetics after inspection.

    Andrew: that Big Joe article is one of my favourites of his. He really let loose with sarcasm. Nonetheless, a good read for me an this issue at hand. My takeaway is I should add secondary drainage capability underneath my insulation. Although, Joe says to only use EPS, and my plan is to use polyiso to meet the insulation plan. Joe says to use eps on the premise that the roof will leak and the insulation will get wet. Of that ends up being my case, is there Any major issue with using poluiso instead ?

    Stephen and Malcolm: the live roof assembly I'm looking at comes from Tremco, who is primarily a sealant company. They promote their live roof assemblies in conjunction with their products as the water membrane, so the warranty should be inclusive. however, the both of you bring up a good point with warranty and that reminds me to look into it as its a serious consideration in my decision process. I'll look into this further.

    Martin:

    In terms of the live roof assembly, it will all come pre selected for me, ie root barrier, drainage mat, growing medium etc. Basically it all gets delevered in rolls and where I place them down and then unroll them (including the growing medium and plants, they arrive in rolls like sod) for the spec of the system I'm looking at see: http://www.tremcoroofing.com/media/285429/vr_lite_brochure_1_.pdf

    Where the issue I suppose lies is that I'm building my roof as per normal, and it could end there. But now I'm considering dropping a live roof on top at some point in the future. so picking a good membrane now is important to not necessitate inefficiencies of cost and work later on.

    That being said, in a non live roof situation, do torch down or heat welded membranes develop leaks easily? From your experience how long do they last compare to a liquid applied roof? My thoughts being in terms of value engineering, perhaps it might be worth the extra expense just do the liquid membrane of it does in fact provide longevity and significantly less prone to leaks. Thoughts?

  7. Expert Member
    Malcolm Taylor | | #7

    Jamie,

    My experience with flat roofs comes primarily from working in Montreal and Ottawa, where a large proportion of the houses have traditionally been built that way.

    The ones built for most of the 20th century were quite resilient. They used tar and gravel and were isolated from the space below by a short attic space, which provided a buffer from leaks, good drying potential and a way to visibly inspect the roof sheathing from below. If they failed, the damage was typically limited to the sheathing. Their Achilles heel was ice. If a thick layer built up over the whole roof area, it could push on the parapet walls causing damage. They also had to be be replaced at regular intervals.

    More recent designs have done away with a number of the features that gave these roofs their resiliency. No attic space, better insulation meant the drain pipes stay colder and can freeze, less slope, lower parapets, and more roof penetrations. Moving the insulation layer from the floor of the attic to above the roof sheathing means that any leaks do a lot more damage, and to more components of the roof than they used to.

    While more predictably water tight, membranes are now often not protected from physical damage by ballast (or in your case a vegetative cover). They are also more susceptible to flaws in the substrate, like errant sheathing nails even plywood splinters.

    I guess that's along way of saying that I don't think the choice of roofing membrane is as important as the surrounding details. Membranes need to be protected, and the roof design should anticipate its eventual failure and repair.

    Good luck with your build!

  8. GBA Editor
    Martin Holladay | | #8

    Jamie,
    I'm sorry to say that I don't have enough experience to provide you advice on which roofing approach to use. Suffice it to say that I am not a fan of vegetated roofs. If you have a leak, repairs won't be fun.

Log in or create an account to post an answer.

Community

Recent Questions and Replies

  • |
  • |
  • |
  • |