Sloping concrete slabs for drainage

For the concrete patio of a covered breezeway for a dogtrot home design, should the concrete slab have a slope or can it be flat? It is entirely covered by the roof with 2’ overhangs on the exposed sides.
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You always want impervious surfaces to slope away from your structures so that water drains away. The slope doesn't have to be drastic. It doesn't necessarily even need to be noticeable but you want to be sure it slopes away from the structures. The www says a 2% slope is the minimum but I would say anything above 1/2% is sufficient for this application.
I would slope it. I recently had an owner/builder client decide to ignore my plans which show 1/4":1' slope on the concrete floor of their large screened porch and placed a level floor instead. Their reasoning was that the roof and screening should keep most water out. On my last site visit, they were embarrassed to show me standing water in several places, two days after it had rained. Granted the screens weren't installed yet, but that will only block some stormwater.
Even if you think the floor is level, there will be low spots, and because of surface tension, water will stay put even with a very slight slope on something grippy like concrete.
Mike,
The same thing became popular with garage slabs here, especially among owners who wanted to use them as shops. Even with a diligent finisher concrete is never completely flat, and the standing water on those slabs pooled everywhere.
I agree that the slope is important. I also agree that it sucks from a comfort standpoint, especially (this is a new term to me and I'm going to use it forever now) in a Dogtrot.
So I'm my propensity to overfix everything. Maybe consider a more complex approach. Like a linear drain in the middle of the Dogtrot. There are some linear drains that are very minimalistic these days too, like this:
https://www.slotdrainsystems.com/products/7000-series/
Linear drains are game changers for slopage because they're easier to make monoslops towards them instead of a conical/funnel that's required of a point drain. Then, put that linear drain in the middle of the field and you can cut the total height of the slope at the top end.
If you're going to seal and/or densify the slab, I'd be more comfortable to go with a ⅛" slope, but that's just me. ⅛" slopes way more comfortable/less noticable to stand on compared to a ¼".
So to answer your question, good practice would be to slope it towards the front or back out of your Dogtrot. Even better and more expensive, cooler and more complicated practice would be to have a linear drain system.
Jamie
Funny, I've only heard it called a dogtrot style, so when the original title only referenced "breezeway house" I had to click to see what that was!
Thank you. I’ll ask our contractor about the linear drain. Our contractors had two ideas when we asked for a slope… 1. About 2’ of a flat surface centered horizontally east to west, sloping out to the weather exposed north and south sides for a total of a 1” drop on each end. 2. Choose an exposed end on the north or south and slope the entire slab 1”.
Am I right base on the answers so far that this isn’t enough of a slope? Also, since the slope would not slope it away from the enclosed parts of the home, but rather along side them, is that also not ideal? I attached a quick sketch of the idea #1 for rough visual also showing where the doors will be.
Lots of good advice above.
Carefully done with a 1/8" slope per foot your option 1 should be fine, and relatively easy to do for your contractor. That would minimize the sensation of being on a sloped surface.
A couple of off-topic thoughts::
Is there a step up from the breezeway slab to the inside of the house? Give some thought to the threshold and the step height.
If you are in an area that experiences very strong winds and you have very little step up you may have issues with water intrusion under the door threshold. Also the amount you step up can be tricky. I suggest you look at some exterior doors around you and see how you feel about height differences between inside and out. Is wheelchair access a consideration? That's a whole different kettle of fish. I encourage anybody building their own home to consider designing at least 1 exterior door for wheelchair access.
Second thought - If I understand correctly, the house is a slab on grade throughout, not just at the breezeway. I don't know your climate zone. But having no thermal break between the breezeway slab and interior slab may result in the first few feet of interior slab getting uncomfortably cold in winter, even to the point of getting moisture condensing on them. If you are in IECC climate zones 1-3 it's likely not a problem, however Code does require slab edge insulation for climate zone 3. For climate zones 4 and up I would definitely be concerned about this thermal bridge.
Best of luck with your project.
These are all really great thoughts. Thank you. Here’s a really rough picture of what we’re planning. The breezeway drops 1” below the house so it’s a 1” step. The closest door to the end of the overhangs is 5’ inward. We’re hoping to mitigate the thermal break by bringing the 2” of exterior foam up to the top of the stem wall like the rest of the home’s exterior. Would this eliminate the #2 concern you had about thermal bridging in our cold climate?
For the texture of the patio, our contractor had planned to mag trowel the surface and not seal it. I’m not sure if they plan on using a densifier.
I would go for option 2, a constant slope across the entire space. Otherwise furniture will wobble if you move it to where the legs are sitting on different slopes.
I would consider pavers with a permeable joint sand. Handle drainage below grade. Will be more costly than concrete.