Green Roof (FLAT) – mid-span point-loading on concrete-steel composite deck
rbpgba
| Posted in General Questions on
One of the ironies of greenbuilding is that living roof-gardens require quite a bit of structure. And to meet code requires, usually, quite a bit of concrete.
We are in the design stage of a complicated residential build on Cape Cod, flat concrete slab-on-corrugated-steel roof, need help with mid-span loading on multiple levels.
Is there an engineering section of this community, or any recommendations where to go for concrete-steel deck engineering questions?
Thanks,
Rolf in MA
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I have a green roof on my house. During the design process I contracted with a green roof installer, that's actually a trade. (He was in fact one of my favorite subs to work with.) He gave specifications to my structural engineer and my roofer.
The specification he gave the structural engineer was 100 pounds per square foot. We were able to achieve that with 12" TJI joists on 24" centers. The house is 34' wide with a bearing wall down the center so maximum span was 17'. So it really wasn't anything super-exotic.
So find a green roof guy and start with him.
Why do you need a corrugated steel and concrete roof? I have never found it necessary to use concrete on a roof, and I minimize concrete anywhere I can because of its extremely high embodied carbon emissions.
If you're using a system like that, in my opinion you need a licensed structural engineer involved. I often answer structural engineering questions here, as I have a BS in it and almost 30 years of experience using it on the projects I design and sometimes build, but I'm not licensed and I'm not comfortable providing advice on this type of system.
I will say that a current project of mine in Maine will have a living roof over a porch roof with an 11' span. Our structural engineer had us add 50 psf to the load, in addition to a 50 psf snow load and 10 psf dead load, which resulted in 11 7/8" LVL rafters 12" o.c..
Michael, may I ask what your expected sheathing, insulation, and waterproofing solutions are for that Maine green roof? It's one thing to be able to carry the expected loads, of course, and quite another to weather many freeze-thaw cycles, ice dams, torrential rains, and all the things that real houses have to endure, especially in coastal climates like ours.
Also:
We are ALL, of course, deeply concerned with embodied carbon, and attempting to minimize it everywhere in our buildings, as well as offsetting it with maximum green cover, etc etc.
And, of course, in our projects, we always consider every possible structural modality that code allows - stick, timber, SIP, ICF, etc - to minimize ecological impacts. We are all on the same site here.
But such calculations are not always so simple or obvious as they might appear superficially.
Wood rots, especially in humid environments like Cape Cod, especially in applications like flat roofs. And having to replace massive structural elements or whole buildings 40 years later is not particularly 'green.' So building to last is a major concern.
Also, the only thing bad about concrete is the binder. Sand and stone are local resources, and among the LEAST impactful building materials we can use. If cement were not made of limestone pulverized and baked in a furnace, but were instead as renewable as clay or lime, then concrete would be one of the greenest materials on earth.
Electric cars are incentivized NOW, even though the energy to power them still comes significantly from coal. The betting is that EVENTUALLY electricity will be much greener than fossil fuels.
Similarly, many farsighted greenbuilders envision tensile materials that are greener than steel (bamboo fibers in a natural resin matrix, for example), and binder materials that are greener than cement (hemp-based adhesives, etc). So advancing the *technology* of composite slabs is not inherently irresponsible, and more than driving electric cars now is.
Anyway, thanks for the responses! Onward.
Rolf in MA
Wood doesn't rot unless it gets wet.
The most important thing about a roof is that it not leak. If your roof leaks, it doesn't matter if the structure is concrete, that's a problem for your whole building. If you're not sure you can keep it from leaking, don't build it, because building it out of concrete solves nothing.
In the broadest sense, that's true. But leaks are not the only way wood gets wet. On Cape Cod, in Florida, other places around the world, AIR is wet. And wood is a moisture magnet. It will absorb water freely from the air, as my son and I know very well, both being guitar builders. No leak necessary. The problem happens when the moisture can't get out of the wood. Which happens in modern, very tightly-constructed, sealed, and heavily-insulated homes. If the wood has no way to breathe, moisture it absorbs can't get out. Rot is inevitable, which is why we're seeing so many cases of black mold and deteriorated structure in buildings that are barely a few years old. Which is one of the reasons that we've been thinking about concrete as a building material for this structure. Modern regulations and standards are having a counterproductive effect from their intended goal. Deeply unfortunate!
Put four inches of polyiso over the deck. Then put sloped polyiso over that to give the base of the green roof some pitch, that's how commercial roofs are build. Fill between the i-joists with fluffy insulation. You'll get your R-60 and it will dry to the interior, lots of vapor drive to the interior when the sun beats down on that roof.
I lived in Massachusetts for over 20 years. Yeah, it's humid in the summer but the heating season is pretty long, an appropriately-ventilated house will dry out in the winter.
Oh, and in the town I used to live in the oldest house was from the 1630's. Made entirely of wood. Somehow they managed to keep it from rotting for almost 400 years.
The roof on my project is over a screened porch, no insulation, so it's not a typical situation. We will use Advantech roof sheathing and boards of some sort, probably white cedar, on the ceiling. I have no concerns about longevity.
I was a remodeler for three years on Nantucket and for several years in the Cambridge area, and for the last 20 years I have worked mostly in coastal southern Maine, with many projects directly on the ocean, so I understand durability concerns pretty well.
One way that I minimize my carbon impact is by avoiding flat (technically, "low-slope") roofs. I like how they look but I don't like the details and materials necessary to build them in a durable way so I don't design them into projects, except occasionally as porch roofs, but even then I try to stick with pitched roofs.
I design for an estimated 100-year lifespan. Few houses make it longer than that, and I expect that as climate change accelerates, houses built challenging areas will be either completely renovated, torn down, or simply abandoned.
Regarding concrete, sand and stone actually require a fair amount of energy to crush, sort and transport, but the amount is so variable that carbon-accounting software ignores it. The carbon impact from Portland cement is only half from the energy required to burn the limestone; the other half is via a chemical reaction, and again the mining is ignored. I also minimize concrete when possible, including being the first to publish my details for a slabless slab here several years ago. There are folks such as Bruce King working diligently on low-carbon concrete, so there is hope, but hoping for change doesn't affect actual emissions today.
Using materials like SIPs, concrete or spray foam may solve long-term problems, but the climate crisis is here now--we are currently in the worst time in human history to be loading the atmosphere with more carbon. Hopefully in the not-too-distant future there will be ways to mitigate the damage and we can go back to imagining our buildings will last 500 years and using easy materials like plastic foam. But it is irresponsible to build that way today.
My personal thought regarding electric cars is that I probably won't buy one until I have enough PV to charge it, but even when powered by coal plants, their emissions are well below what a typical ICE vehicle emits.
All that said, I also believe that we as individuals should not be carrying the load of reducing worldwide carbon emissions when there are so many corporations and governmental organizations emitting ridiculous amounts. I pessimistically think it's too late to solve the problem effectively and I'm gradually shifting my focus to designing homes and eventually communities that can weather climate extremes and various disasters. I'll still choose the greenest materials I can, and won't create designs that unnecessarily require high-carbon materials.
Yup. I've been doing the most sutainable design and building I can for over 40 years, so I'm no stranger to these concerns. And I agree it's gonna be more about riding out the wind and waves than preventing them at this point.
Much of my work over the decades has been about maximizing structural properties of the most minimal materials. Even have a few patents to my name.
AND.
In this case, where we are required to have R60 in roofs and R30 in walls, as well as meet height restrictions, plus wanting to build homes that are more than 100% green-covered, and can't do anything crazy b/c we still have to meet code, our choices get restricted pretty quickly.
I love building with native timber, for example - in fact, just cut up a huge black locust today from my sister's back yard - but it's hard to meet the HERS numbers, and the blower test, and the height limit etc etc with materials that move so much and need to breathe to stay strong.
I'd be curious what you've found that is very low carbon but ALSO built to withstand superstorms?
I have been designing homes with R-60 to R-140 roofs and R-30 to R-60 walls for over ten years, with a decade of historically-oriented design work and a decade of hands-on construction experience before that, after earning a BS in structural engineering. I generally don't use or recommend "green roofs" because they are 100% greenwashing, if attractive when designed and maintained properly. I typically use double stud walls and pitched roofs, either stick-framed or trussed, solidly engineered. I have not found it necessary to use foam above grade except on a few very specific situations, and on renovations when space is limited. My projects typically blower door test between 0.5 and 1.0 ACH50, with a few around 0.1-.02 ACH50, air-sealed with European products made for high-performance wood-framed buildings.
I agree with almost everything you said, but I'm going to take one bit of exception.
I live in the city, and stormwater management is a huge issue, the city is spending billions of dollars right now to upgrade its storm sewer system. As part of that, all new single-family construction in the city is required to catch and handle all rainfall on the property. Usually this means that you have to have a dispersal tank, which is basically like a septic system, in the back yard.
A green roof allows you to have a smaller dispersal tank. In a crowded city neighborhood it actually pays for itself.
We also have floor area to lot area ratios and height restrictions. The price of real estate is high enough right now that virtually every house is built to the maximum size that the zoning will allow, the floor area ratio drives people toward boxy designs and the height restriction incentivizes flat roofs. If you're going to have a flat roof anyway making it a green roof makes it a nice usable space.
Granted, these are all special cases. But I believe that the greenest way for people to live is at the greatest density possible, so we should be thinking about ways to pack people into smaller spaces as efficiently as possible.
Reply to DC #14: good points, and I should have hesitated longer before claiming 100% of anything.
That's interesting to hear about on-site stormwater storage; that's not something I've had to worry about. If you have to use a low-slope roof for zoning reasons, I can see where a green roof would make some sense. I agree that dense housing is in many ways "greener" than alternatives, and if we're talking about a multi-story, multi-family structure, the advantages of a low-slope roof probably do outweigh the many downsides of a low-slope roof.
But in my world of mostly single family homes, which in some situations are a better choice regarding emissions than being in a densely packed city, using a low-slope roof is almost always due to architectural vanity, not any practical reason.
Question for you Michael, although this may merit its own thread:
I spend part of the year in Buzzards Bay, near the OP. In the summer there it is cool but quite humid, it's not uncommon to have condensing fog that sets off smoke detectors. Plus there's a steady breeze off of the ocean. The traditional way to live is all windows open all summer.
What's the best way to design for those conditions, especially in a house that you then want to close up and heat in the winter? Minimizing solar gain is one thing, you really don't want the house to heat up under these conditions. You also need to ventilate the bathrooms extra-well. It seems like humidity is otherwise not a problem so long as you can avoid cold spots, the house is humid but it's not unpleasant when it's cool and breezy.
So basements are problematic, if they're uninsulated they have all sorts of condensing surfaces and if they're insulated they tend to stay cooler than the rest of the house all summer. It seems the only solution that kind of works is to seal the basement off from the rest of the house and run a dehumidifier.
Any other thoughts?
Response to DC #16: that would indeed make a good thread. A current project of mine is a 3-level new home including a finished walkout basement, directly on the ocean. (I'll attach a photo--the row of trees are at the top of a 40' bluff, with the beach below.) My clients currently live next door and complain about squishy carpets when it's foggy. And I know the effect well from formerly living on Nantucket, where no matter which way the wind blows, it's a sea breeze.
On this project, my clients are outdoorsy and will have the windows open often. The heating system is two air source heat pumps; the one covering the second floor is rated to be good at dehumidification.
At the lower two levels, the ducted heat pump system is not rated as highly for removing moisture. We included a Santa Fe ducted heat pump to dehumidify those areas.
We also have a professionally designed Zehder ERV system that includes bathroom ventilation.
The basement level is open to the main level but we used my typical detail at the walls: rigid polyiso against the concrete, all seams taped, then a framed wall with cellulose insulation, and painted drywall at the interior. The sub-slab insulation connects with the wall insulation. The heating system is well-distributed, so I don't think we'll have any cold spots.
We have also educated our clients that if they leave windows open on foggy or muggy days and then close the house up, that they will need to turn on the dehumidification system, and/or set the heating/cooling systems to "dry."
Other than that, I do my best on every project to make the building air-tight and vapor-controlled; to use natural materials that can safely absorb and release moisture instead of trapping it; to vent behind cladding and use vented roofs; to super-insulate and use triple-glazed glass to avoid cold spots where moisture will accumulate.
BTW, thanks for the Bruce King tip. I knew about mycocrete and recycling emissions, but nice to see it summed up in one place: https://youtu.be/a1Pw1fhK2yQ?si=GiTFhnWVwgFMNWWN&t=624
AS a side note, Mass uses no coal to power its grid, and non carbon sources account for twice the amount of electricity as coal nationally, per the EIA
Yes, I've been part of that movement. Although the significant number is really the 60% fossil fuel generation (nationwide) vs the 20% 'renewables.' (Last 20% is nuke, of course.)
Burning anything makes CO2. That's literally the definition of burning. And unless we are capturing and sequestering 100% of that CO2 (turn it back into wood?), we can't really call any of it clean.
Luckily, there is a technology available now that can absorb atmospheric carbon and convert it into a renewable building material. It even CREATES fresh oxygen as a byproduct! What was it called...? Oh yeah, a 'TREE!'