I agree completely with your method, but is this even needed in the model? Genuine question, not being devil’s advocate. I would assume apart from the stem walls and foundations, you would just specify the starting plane and the desired grade for the concrete contractor to use. Maybe there’s another element I’m missing?
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Click in sequence on the Scenes tabs of this SU file for ideas.
Garage slab with sloped floor.skp (133.8 KB)
Notice that this is only one possibility. I t all depends on the topo of the terrain around the house and its garage, the the street sewer elevation or the septic tank, etc.
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What happens if there is a fuel spill?
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I wouldn’t want to drain to my septic system, in case it would wreck it, and I’d find out from the agency before connecting to the sanitary sewer. Otherwise all garages are just required to drain to “outside”, and the (liquid) fuel spill goes downhill.
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Yes, fuel cannot be dumped in a septic system.
Where I live, all home garages are drained as the rest of the house toward the sewers in the streets.
If you want to be really environmentally friendly, you may envision installing an oil separator but, for the few occasions and the relatively small oil or fuel quantity that may be spilled, it is probably overkill. Check with local authorities to see what they impose for other houses like yours.
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You can of course describe it only by notation. But for something that takes literally 5 minutes to model, why not. I always end up with 3-4 sections that go through the garage. It makes it very clear in the drawings when that slope is accurately portrayed.
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Correct. If you do an internal drain, you must have an oil separator for each drain. I always thought it was odd considering that same residue is allowed to simply run out the front. It adds a cost of nearly $2,000 / drain depending on the interceptor used and how large a drywell is proposed.
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Yes, the IRC says to drain toward the door or a drain, and @Sonder highlighted the caveat of what you might be faced with putting in a floor drain. It also doesn’t say how much pitch is required. Flat roofs, decks and the like usually have at least 1/4" per foot, but everyone I’ve dealt with seems to agree that’s too much pitch for a garage, so I’ve mostly seen less, more like 1/8" per foot. Over 24’ that’s something like 3" which doesn’t seem like much, but it makes a difference when determining how many risers you need up into a house.
I typically shoot for no less than 1.5% with a garage slab. At 1% you will get ponding with dirty water.
For flat roofs with PVC single ply or 3 ply MB torch down, no less than 1/4:12 which is just over 2%.
Interesting, where are you located where this isn’t the norm anymore?
We have gone about looking at others’ job sites on occasion and I don’t think I have ever seen a garage floor that isn’t slopped here in Tulsa, OK.
Definitely don’t want to spend more money than necessary,
Since I and my brothers both design and build the project from start to finish, we don’t usually have to deal with hardly any contractors.
Indeed! Thanks for the insight>
In old times, people would perhaps opt to wash their car in the garage. Driving in a wet car wouldn’t probably create a situation with a lot of water to drain away, even coming in from a snowstorm.
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Whether I like it or agree with it or not, the International Residential Code adopted with modifications in CT tells me too, specifically paragraph R309.1. If you’re working where there is no code or Building Official to call you on it, then I guess you can do as you like, but no concrete guy I’ve ever dealt with complained about sloping a garage slab other than 1/4"/ft being a bit more than needed.
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Just as an aside: Which are the nations involved? AFAIK building codes in the world are broken into nationwide regions, or smaller. Not even the Eurozone is very standardized 
(I understand that football matches (soccer for the Americans) between England and Wales are called “international”).
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Yes, it’s USA centric thinking:
International Building Code = USA
World Series = USA + Canada (and Canada only has one baseball team left.)
Building codes are implemented on a state level, so the model code is only a template, so to speak. Each state that adopts it can modify it, and not every state adopts it.
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As rare as it might be, the house is actually being built in an area where there is “no code”, we couldn’t hire an inspector for this job if we tried. But we are trying to go above and beyond all IRC regulations anyways. Tis’ our goal.
We were actually going for 1/8"/ft.
I’ve seen too many inspectors walk in for about 5 minutes and say “cool beans, see ya!” and not measure or level a single thing.
smack goes the sticker
No offense intended, I meant what I said about how it matters where you’re working, and I’m sure your experience is what it is where you are. I did one project in Ohio, and they built something I know my local inspector wouldn’t pass, and when I said their inspector might have a problem with it, the said, “What building inspector?” So it’s there to this day. Local conventions and enforcement really do matter, and I’m just saying, where I am, pitching a garage slab is just normal everyday practice.
I’ve designed hundreds of homes at this point. Not one of them has a flat garage slab. Not sure where you are but any jurisdiction under the IRC requires it…at least all the states where I am licensed.
Sometimes they just want some pictures.
Who doesn’t spend a lot of time around contractors? Draftsmen likely, but most architects are involved in the construction administration at some point.
I agree there is a general lack in detailing in the industry, but some of us do put that effort in.
I do think every architect should spend some time building to understand what goes on, but I simply don’t agree on a flat garage- at least where the car sits. I live in snow country and this would be a nightmare with puddles from snow melting off the wheel wells and undercarriage, especially if it ever freezes.