Tips on how to build a landscaped roof
jonathanb
| Posted in Green Building Techniques on
We’re building a single-story house in CZ3C with a 12″ deep green roof atop a low-slope roof with an encapsulated attic for HVAC ducts.
Any good references for what the structural engineering of the roof should be? Ideally it will include a drawing, showing framing and insulation and waterproofing and drainage.
We have a structural engineer, and I have no doubt they can draw up a plan the city will approve. I’m less confident that it will follow the best practices that people here on GBA would recommend, or that it won’t have problems after a few years.
Thanks in advance!
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Do you have an installer for the green roof? Find them, and get them to tell you what they want to work over.
Good tip. I have selected a landscape designer, and they will help me get bids from installers. They might be willing to share some advice before I submit the structural plans to the city.
The 50s promised us energy to cheap to meter.
The 60s promised us cities under glass domes.
The 70s promised us free passive solar to heated our homes.
Ect
About 20 years ago they promised us the green roof would be the answer to every problem they failed in the real world.
The people that built them found too expensive to build and under preformed in addition to being totally unrepairable. Some ideas are best forgotten.
Do yourself a favor don’t put your ductwork in your attic you will lose 20-50% less heat thru a low-cost flat ceiling with an empty R60 attic. Simply because no one can afford R60 at the roof line that has almost twice the surface area losing energy.
Note the words “encapsulated attic” are very deceiving they imply that zero energy and equipment are required keep the humidity of the air in the attic from condensing on the walls making them moldy and rotten. Consider calling it a “conditioned attic” in that way you have admitted to yourself there is no free lunch and that you are prepared to buy and operate the equipment necessary to keep the attic at more or less the same conditions as the rest of the home.
Walta
You make a good point about the encapsulated attic... There's insulation at the roof joists, and then a drop ceiling with an unvented space of about 1 ft for ducts. I hadn't thought about it as conditioned, but makes sense.
I guess that means there needs to be air interchange with the house, and the heat will rise up there in the winter, meaning the HVAC will need to be able to handle it.
Maybe this should fork off into another thread. After the ducts are all in there, would it be ok to completely fill that space with batts or blow-in insulation and then seal it up and have no air exchange with the conditioned space of the house?
I found a somewhat related question: https://www.greenbuildingadvisor.com/question/waterproofing-both-exterior-and-interior-of-foundation-wall
From Martin's excellent response, I gather that if the roof is a concrete slab, it's ok and maybe even advisable to waterproof both sides.
Waterproofing underneath the slab seems kind of like adding a membrane for the pre-slope in a shower, directing water to a drain or downspout. Then the top of the slab can be waterproofed with a roll-on liquid product, again like you might use in a shower, but made for outdoors. And finally, add another membrane layer. To really go wild, maybe use an integral additive in the concrete mix so that the slab itself is waterproof. Maybe with triple or quadruple protection, the green roof will be as leak-free as a well-installed shower?
I'd love to hear from anyone who installed a green roof a decade ago and has had no leaks, and how they did it.
There are several good books on the subject, I bought one that is far more detailed than I can use, up to the level of planting trees above. A good book will give you the language to speak with the really good architect and really good installer you will need
foot thick?
pounds per square foot, yow
I came to the conclusion that if I built one[appease local authorities] I would use a tray system or somesuch and not attempt a true integrated roof.
Too many things to go wrong
Can you share the title of the book? I'd love a recommendation. Thanks!
green roof systems
weiler /scholz-barth
Best drawings I've found so far:
http://www.newdesignfile.com/post_green-roof-design-details_185028/
I'm leaning towards a concrete slab for the roof deck. I like the drain details in a couple of the drawings.
The books I've been able to find on Amazon seem very light on the construction details, based on the reviews and the free previews.
Best of luck on your green roof. I'd like to hear how it goes. So please post an update when the time comes.
I proposed one to a client in Portland, Oregon, who was building a triplex. He loved it. We were going to go with a simple tray system. But the design review board nixed it because we were on the edge of a historic district. They were perfectly fine with asphalt shingles though. They wanted something pseud0-Craftsman. Go figure.
This project is a California Senate Bill 9 (SB9) lot split, with a new single family home in the back yard of the original house, which was built several decades ago. The yard is sloped enough that the floor of the new house will be about 14 ft below the floor level of the main house. The green roof will provide a level backyard for the main house above. It's intended as a useable space where kids and dogs can play, and also to look out at from the windows of the main house.
If it were just an eco-friendly statement, it would be really expensive...
jonathanb,
Sounds like a really interesting project. If you remember once it is built, it would be great if you could post photos here.
That's really asking for a lot.
I have a green roof, it's mostly covered with sedums because they are hardy enough to handle the environment, it's often hot, and usually either very dry or very wet. We get sub-freezing weather in the winter so whatever we plant has to be cold hardy with shallow roots.
The tradeoff is they don't stand up to much foot traffic. I don't see how you could get grass to grow on a roof without an irrigation system, which kind of defeats a lot of the purpose.
My roof has 4" of dirt and is engineered to 100 pounds per square foot. For 12" of dirt I imagine you'd have to go with 175 pounds per square foot or so.
Oh wow. I'm not sure what the live load will be. I am not a structural engineer. I was thinking of a roof of 2x10s 16" OC with a 4" concrete slab, and expecting that would hold enough weight, and the structural would include enough 2x6s or posts in the walls to hold it up.
In California, it's a mild climate and we'll use drought tolerant plants and some empty space with just DG, can be quite a bit less than 12" in those areas since it's heavy. It actually fits the look of the rest of the yard, which has no turf at all.
That said, yes, some irrigation will be necessary, with basically no rain for half the year.
I think that rather than focusing on how it's built you'd be better off thinking about what you want, then communicating that to a team of professionals and letting them figure out how it's built.
In post #10 you said, " The green roof will provide a level backyard for the main house above. It's intended as a useable space where kids and dogs can play, and also to look out at from the windows of the main house."
That's good description. Start with that. I'd first talk to the landscape architect.
I think the biggest design challenge is going to be keeping the weight down. Weight on the roof makes the whole building from the foundation up have to be stronger. My engineer decided with that much weight that high the building needed supplemental bracing.
My roof is basically built like a floor -- it's flat, horizontal I-joists with 3/4" Advantech over it. Over that is 4" of polyiso insulation, then tapered polyiso to give it slope: https://www.gaf.com/en-us/products/energyguard-tapered-polylso-foam-roof-insulation
The foam is peaked in the middle and tapers toward he sides. Over that is a TPO membrane, and then a root barrier and 4" of dirt. At the edges is a 4" aluminum curb, and about a foot of gravel, 4" deep.
This is significantly less beefy that what you're imagining, and it's still 100 pounds per square foot. The whole rest of the house is about 50 pounds per square foot, so it triples the load on the foundation.
Deleted
Thanks, @dccontrarian . That's some good info.
I like to understand the details pretty deeply.
The landscape architect has done lighter weight green roofs, more like yours. The structural engineer can ensure it supports whatever weight is needed, and the insulation and waterproofing is there.
People on GBA tend to have more expertise than anyone I might find locally. I wanted to learn enough to be sure the structural plans make sense, and be able to ask informed questions. Maybe I'll post again when I have the drawings, to get a second opinion that it looks reasonable.
Hi,
This may be a good application for Insulated Concrete Forms (ICF) with an ICF formed concrete roof - kid play slab. The main reason would be the seismic loading from the planted roof, the ICF concrete roof slab would tie the walls together for seismic loads easier than a wood wall wood roof system. The concrete will creep less with the long term heavy roof load like wood might. If concrete it won't rot when it leaks.
Interesting suggestion! I'll ask the structural engineer about it. Thanks for the idea.
Your concept is cool and I'm a big supporter of bio reclamation of a buildings footprint.
I have done a green roof on my last build that will involve a rooftop terrace. I used a very light system of 2" of rockwool and 2" of pregrown sedum mats. Next liveroof I'll do, I will push for 2-4" of engineered soil and will hand plant plugs of specific sedums, mostly to have full control on plant type and aesthetics.
I will say some things regarding a live vegetative system to take into consideration:
-The deeper the soil, the more ability for robust plants that can handle foot traffic (like small trees & grass), the heavier the system, the more robust/expensive the structural components of the building.
-the shallower the soil, the less robust plants for foot traffic (sedums), the lighter the system, the less robust engineering and structural, they easier/cheaper the build and can stay within conventional wood frame building that framers can handle.
- roof waterproofing design must be well designed, well executed and will need to be tested.
- irrigation systems on the roof are easy to accomplish. But they are more important for thinner/lighter systems, because they have less moisture capacity. I've been told you could lose a 2" roof in a week of drought. Deeper systems have more moisture resilience. But since you're doing a "rooftop at grade" thing with walk-on access, you can consider watering manually during a drought, just like any other lawn.
- a well designed live vegetative roof system will last for years becasue there's no UV damage on the waterproofing membrane.
To answer your original question:
1. There are companies that consult and sell greenroof systems. like Zinco or even commercial roofing suppliers have special divisions that do this as well, like Soprema or Tremco. Please do not use a landscape company or designer that doesn't know anything about live vegetative roofs. The liveroof profession is its own entity (for good reason), it might as well be its own trade.
2. Consult with them and pick the system that you want (plants, medium, depth, etc)
3. They will tell you the weight of the system.
4. You take that weight to your structural engineer.
5. Your structural engineer will specify the structural system needed to handle that load along with the liveloads and deadloads of your region.
6. You add the structural components to your drawing set for city approval.
7. Keep in mind that every AHJ is different, so you might need a separate permit and drawing set, declaration letter from the eng, maintenance plan, etc specifically just for the liveroof.
Hope this helps,
Jamie
Thank you, Jamie! I didn't realize I needed to talk to a specialist in landscaped roofs. It's incredibly helpful to have specific names of companies to talk to, and to have advice from someone who's done it before.
The steps you suggest make a lot of sense, and will help me to ask the right questions when I work with the structural engineer.
By the way, I went on a long and deep dive into span tables for lumber and steel, and live loads, and ... building for more weight gets really expensive really fast. It's much better to find out if I can get by with less weight, or can localize the heavier load to certain areas that have interior walls that can be reinforced.
Really appreciate the tips!
Jonathan
Looks like Zinco only does commercial projects over 5,000 sq ft. Tremco is responsive and I'm following up with them.
My experience so far: The structural engineer says, I'll design a roof that supports whatever live load you need. The green roof company says, I'll sell you a roof system that stays below whatever live load your roof supports. The landscape designer says, I'll design you a beautiful landscape for whatever green roof system and live load your roof supports.
Given so far no one is willing to make a specific suggestion, I'm going to start with what seems sensible and probably not crazy expensive: 2x12s at 16 OC, doubled up in a couple areas where I'd like deeper soil for taller plantings. Sloped foam board and OSB. EDMA or PVC. Green roof system on top.
I'll ask the structural engineer to do the load calculations, and then I'll take it to the green roof company and the landscape designer and ask what they can do within those parameters. If it turns out that we'd like larger, heavier plantings than what the roof supports, I'll go back to the structural engineer and ask for some reinforcement wherever a higher load is needed.
2x12 on 16 seems skimpy. How wide is the largest unsupported span?
Most of the spans are 12' to 14'. Over the bathrooms, it's 5'.
You mentioned that you framed yours like a floor. Do you remember what size lumber and spacing and spans?
Span is 17', house is 34' wide with a bearing wall down the middle. We used I-joists.
I found a building code! In California, section 1607.1 requires a minimum of 100 psf for an occupied green roof and only 20 psf for an unoccupied one.
The structural engineer didn't volunteer this information... I had to find it. Likely I will go up higher than 100 psf for select areas. Now I just have to figure out how high to go.
I'm guessing the code is at 20 psf for an unoccupied roof, assuming it's 3-4" of perlite and sedums, the basic lightweight green roof. For the occupied version, subtract 60 psf for human occupancy, since that's what a deck requires. That means they're budgeting a dead load of 40 psf for the landscaping. A perlite mix with drought tolerant plants like lavendar and rosemary could probably go to a depth of 1 ft with that weight budget.
Saturated potting soil can weigh 100 psf. Waterlogged plants can increase that further. Probably 6" of potting soil and plants would be fine on a code-minimum 100 psf roof, especially if it's localized to certain areas. If any heavier plants are placed above structural walls or shorter spans, maybe nothing more would be needed.
I'd like some variety of height for the plantings. Basic non-walkable ground cover can be stonecrop. That's fine. Some taller plantings. Along parts of the perimeter, I'd like to have a few 5' tall bushes, with enough soil depth. And to really make it feel like a yard rather than a roof, it would be nice to have one or two small trees, either in a large planter or a mound.
All of that is to say, your original recommendation is probably pretty accurate, @dccontrarian.
Ah, I'm learning about i-joists. Yes, looks like a 12 in deep i-joist can support enough weight at 16 OC. This looks like the right solution, without adding too much thickness to the roof.
Example span tables:
https://sherwoodlumber.com/wp-content/uploads/pdf/Wood-Beam-Joists-Specs.pdf
@jonathanb
Regarding your 3-way Chicken & Egg & Nest problem:
Go back to step 2 in my last post. Pick the liveroof system you want first, then go to your structural engineer.
If cost or build logistics is an issue, then its the reverse. Pick your structural component's first, know the load capacity from your structural eng, then take that to your liveroof supplier. (this is usually how I design, I think of the build cost/logistics at first principals)
Zinco Canada works with resi builders & I like my rep. Keep pushing, or consider bringing on a consultant. They're out there.
If you're truly going to use it as just a lawn (seeeding the grass, mowing the lawn, higher traffic, no trees or veg gardens), I suggest Looking into getting a 6" intensive system. Zinco calls this their "lightweight lawn" system, at 34psf saturated weight, which is easily done and not crazy to achieve. I'm not a structure eng, but I am assuming you will easily meet this load along with your live loads, with 11.7/8" I joists at 16"OC and 3/4" ply decking.
Please don't use solid sawn lumber to make life easier for your build. Also please don't use OSB for your decking, I suggest sticking with ply, just personal preference, you're going to spend a lot of money on a liveroof, give it the best shot. I don't have access to things like advantech, so I won't comment on that.
You will need to detail your parapets, as well as consider the height of the sloped insulation, as well as your drainage plan (ie eavestrophs, scuppers or integrated drain.)
I highly suggest using a liquid applied membrane for the waterproofing (just my personal preference to eliminate any potential of seam failure)
hope this helps,
Jamie
Thanks, Jamie! Great tips.
I'm starting from code requirements: it's minimum 100 psf for an occupied green roof. Then I'm picking a couple of areas to support a higher load. I'll see what the structural engineer comes back with.
I would not use EDMA or PVC, spend the extra and go with Kemper. Another tip drainage is so important, make sure you get that water off faster the better. Don't do the deeper soil either, keep it simple.
I would definately not try a residential intensive roof. I have mostly pavers over the medium, with like green areas, with seedums and flowers whatever. Its super nice but i just got the roofs right 8 yrs after the house was done. Serious nightmare, I would do a concrete flat roof, if i did it again.
@PeterAAA Can you share more about what kinds of problems you had for those 8 years, and what you had to fix?
Thanks for mentioning Kemper. There are so many liquid waterproofing options, I wasn't sure which to consider. It's great to have a specific recommendation.
and please do not forget the barriers that kids/dogs/people will not fall down the cliff..
It's true that all the photos online show these landscaped roofs with no fences. I see those and think, really? What city would allow that?
Here, a 3'6" perimeter fence is required. I'll maybe go with a wire fence to keep the look open.
I suggest you do a Google image search for "Germany green roof". You will see that most of them are on commercial buildings and have guardrails. These are roofs meant for common everyday use and will be on the heavy side, requiring significant structural support. In the industry these are referred to as "intensive" green roofs.
You will also see a few roofs that have no guardrails and are not meant for common access. They are lightly planted with sedums and are much lighter. In the industry they are referred to as "extensive".
This website provides a bit of background on different approaches.
https://greenrooftechnology.com/green-roof-finder/extensive-green-roof/
It seems like your project is straddling between the two, semi-intensive. You're wanting a useful outdoor space, which will bear significant foot traffic and have plant loads heavier than just some sedum mats a few inches thick. This is certainly possible with standard residential construction, but will require some structural upgrades. A fully intensive roof is more likely to be built using commercial construction techniques in steel and/or concrete.
You understand exactly, @jollygreenshortguy.
Hi,
A wood roof structure built to 100 psf is no big deal. In Mammoth Lakes and Tahoe snow loads are in excess of 200 up to 350 psf. Decks have to be built for the ground snow load. It just takes a lot of wood. The architectural design needs to follow the structural requirements, so some iteration of the seismic load & shear wall lengths & placement are needed to get the economical solution. Steel moment frames can be the brute force approach. Simpson has off the shelf solutions.
I've seen photos of the snow in Tahoe piled as high as the house. I never thought about how much it must weight in psf.