Concrete-Free Slab for Garage
With the greener building practices turning up around concrete-free slabs, I have not found anything around using the same practice for a detached garage.
To be clear (and if you’re not familiar), I’m referring to a general floor assembly of crushed/compacted rock, then foam, then poly, then staggered sheets of plywood subfloor.
The immediate concerns that come to mind are:
– the finish layer: what would you lay on top of the plywood to protect it from the elements? tile? epoxy?
– the compressive strength of the foam and plywood: would it hold up with a vehicle driving and parking on it?
– garage door transition: would you use the same one you’d use for any other door opening?
What do you think?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
I don't think you can as wood won't meet non-combustible requirement unless you get expensive wood. At that point, might as well pay for the concrete.
You also have a pretty large point load you have to deal with, works out to around 100PSI, which is well above the capacity of most rigid insulation.
The concrete topping again helps with this as it spreads the point load out a fair bit. You can do the same thing with wood by adding more layers, but now you are adding even more of the expensive fire rated wood.
I think concrete free detail is best left for houses.
Excellent points. I hadn't thought of non-combustible requirements.
obenwalkenobi,
Not a particularly good alternative, but maybe consider pavers instead - much as you would use on a driveway? However I have no idea how their embodied carbon compares to a slab.
As the alternatives represent such compromises, for the small amount of concrete used in a garage, perhaps the best strategy is to offset the carbon footprint in other ways.
Pavers at least are reusable and can be taken up and put back for repairs. Concrete is single use. So it's not quite a fair comparison.
I wanted to try pavers for a garage, the issue I ran into was they were quite expensive.
Pavers are definitely one alternative. And as pointed out, reusability/repair is one benefit.
But due to higher cost, questionable embodied carbon savings (if any), and loss of a smooth, easily sealable surface, this alternative is less attractive than just sticking with concrete.
You also need to consider the impact of both flammable and non-flammable liquids. Garage slabs are pitched to the door for this reason. Consider what would happen if gasoline spilled and soaked your floor sandwich. Or pesticides. Or just the constant dripping of meltwater from the vehicles. Go with concrete here and save carbon elsewhere.
I guess I think of concrete as porous - so it would need to be sealed to prevent gasoline and other liquids from soaking into it as well, right? If I can seal concrete, why not other surfaces?
If we're not sealing the concrete, then we're in the same categorical design as the gravel / paver routes in my mind.
There is many a garage out there with no finished floor, just compacted gravel/stone.
Many basements in this area from the 1800's are floored with pavers.
It's a bad idea for a multitude of reasons.
What is, pavers or no floor?
I'm really late to this question but I've been thinking about it and thought I'd revive the discussion.
As far as just a gravel floor, my concern would be how to manage moisture. With no vapor barrier underneath, moisture from the ground would make the garage humid all the time, leading to rust, mold, etc. With a vapor barrier underneath, moisture from wet cars would pool Perhaps it's possible to have clean coarse gravel that allows that water to flow through easily, and have a layer underneath it to shed the water, that's still above grade so it can run out. Maybe an EPDM roof membrane.
If you can source cobblestones locally, that's an option that's lower CO2 than concrete paver blocks. Either reclaimed or newly quarried. I got to wondering about the CO2 impact of shipping rock long distances, and calculated that on North American diesel-powered freight railroads, you could ship rocks 41 thousand miles before the CO2 impact would reach that of concrete. Or maybe 4000 miles by truck. So that's not a bad option from a carbon perspective even if there's some trucking involved, but from a cost and climate perspective local sourcing is of course best.
But is that really better than just gravel? If you could grout the seams, you could, to some extent, allow water to run off instead of infiltrating below, and you could use a vapor barrier underneath without water pooling on it. So that's what I like about it. And maybe cutting blocks uses less energy than crushing gravel. You'd want blocks cut as square as economically feasible.
I've also thought about putting steel diamond-plate sheets on top of crushed gravel, with welded or even just bolted seams. It's hard to know how thick it would need to be. You'd want the load to transfer to the gravel underneath, and if it flexes a little that's fine, but you won't want it to crumple and rip. It might be that a sandwich of steel on top of plywood would be more robust with thinner steel and still meet requirements to be non-flammable.
For rust protection of the steel, galvanizing would probably do the trick. I might add tung oil and/or active galvanic protection. Or use aluminum instead of steel--these thin diamond-plate aluminum sheets are only $4/sq ft. https://www.elitegaragefloors.com/aluminum-tread-brite-sheet-3003-48-x-96-17-ga/ They might not be thick enough over crushed gravel, but would probably work with a plywood layer underneath.
I'd probably use "glavel" instead of crushed gravel to get some insulation and avoid needing the foam your design has--with either the steel or the cobblestones.
Finally, I've thought about reverting to a car port with a gravel floor, so it's open and the humidity is just the humidity outdoors. But that allows mice and squirrels easier access to the car. Perhaps a car port with 1/4" wire mesh walls?
I don't understand what you are trying to achieve by not putting in a concrete garage slab. Just do it in concrete and be done with it. What do you think you will "save" in terms of environmental impact? The answer is NOTHING. Your attempt to go "concrete free" to "help" the environment is all but futile and a waste of time, money and energy. Not to mention, you will end up with some horrible garage floor that will need to be constantly maintained and pose other issues.
China is producing 2,100,000 metric tons per year of concrete
India = 370,000 metric tons per year
Vietnam = 110,000 metric tons per year
United States = 95,000 metric tons per year
and the list goes on and on...
So your garage slab is meaningless in the grand scheme of things. Especially with China and other nations building with concrete. Wait until they have to rebuild Gaza. There are ZERO wood structures there, it is 100% masonry. The concrete plants will not be able to keep up with the demand.
Hi Jack,
I think you want to add three zeros to each of your figures for tons of concrete produced.
So should Taunton Press shut down this site? If what we do doesn't matter, why are any of us here?
No TP should not close this site because we want efficient and durable houses.
That said, I agree with Jackofall trades that the attempts to save the planet by doing without concrete are futile. 1+ billion people in Africa want the same standard of living as us and you are kidding yourself that they are going to do it on wind and solar. Fossil fuels are here until they run out which is likely never. If you want a carbon free world, start advocating for nuclear or your not a serious person.
Nuclear option? Start arguing for point of use cold fusion or you're not a serious person! ---pfffffft!
There's a lot of tools in the toolbox and wind and solar and hydro all have a place.
Since the dawn of the environmental movement the leaders of polluting industries have pushed a two-prong PR strategy to defeat environmental regulation:
Part 1: Regulation isn't the answer, personal responsibility is.
Part 2: The problem is so big and you're just one person, so nothing you do on an individual difference will make a difference.
Any time you hear someone talking about how we shouldn't regulate pollutants in this country because they're so much worse in other parts of the world, wittingly or not they're echoing Big Oil talking points.
Now, I actually believe that Part 1 does more damage. The environment is too important to be left to personal virtue, environmental regulations have a long record of being both effective and enforceable. It's always good to have an informed citizenry but we shouldn't count on people to make choices counter to their immediate self-interest to solve environmental problems.
Yes, and part of the reason for finding ways to do things in your own projects better--cheap and with low emissions--is to have examples to show that the regulations we need won't make everything ultra expensive.
As much as I agree with the practical notion of just using the concrete inside of a garage, the rest of the argument is a little off. Yes, there are larger impacts globally, but we still don't need to just throw up our hands and give up.
The question raised is a good one...are there alternatives to using concrete? And the answer is that yes, but it also means not building a conventional garage but a simpler carport with gravel. So if the OP did that they'd be reducing their carbon impact with the build. It won't change the world by itself, but if more people consider the option and we end up with fewer concrete slabbed garages and more carports then it starts to have a real impact.
Any thoughts on the mouse proofing issue for a carport? Do you think my idea of 1/4" hardware cloth for carport walls is reasonable?