Question about new concrete slab adjacent to existing slab.

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Old 06-14-16, 12:17 AM
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Question about new concrete slab adjacent to existing slab.

OK, so I'm making drawings for a patio cover with new slab, to submit to the city for permit. This will probably be a monolithic slab with a thickened edge or full perimeter footing (4" slab plus 16" footing depth). I think this is called a "slab on grade". The question I have is regarding where the new slab will butt up against an existing slab.

See my attached pics.

The existing porch slab is 4" and there is a pier footing (i.e. spot footing) on the left side which supports the porch roof, and the house footing starts on the right side. The left footing has what I'll call a "bump" and the right house footing has what I'll call a "step". I could probably cut off the bump on the left footing, but the step in the right footing appears to continue all the way down to the bottom of the footing so cannot be cut off. My question is, are the bump and step going to cause problems? Of course I will specify expansion joint material to be placed between the edge of the existing slab/footings and the new slab.

So, my questions are:
1. Is it a problem for the new slab to bear vertically on the "bump" and the "step".
2. Should I ask for 1/2" or 1" rigid isolation material where the new slab bears vertically on the bump & step?

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  #2  
Old 06-14-16, 06:36 AM
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Why you are installing a full perimeter footing for a patio? Will a building be constructed on top of the slab?

In my area a plain/simple 4" slab for a patio would be no problem. It's just a simple patio slab and doesn't attract much attention. Pouring footers makes it a whole other can of worms implying that a building will be constructed and all the codes come to bear. Adding footers up next to an existing structure would require exavating down to virgin/undisturbed soil which is about the depth of the bottom of your house's existing footings. The alternative is to hire an engineer to test your soil to insure it can support a footer.

If you're pouring your new slab on undisturbed/compacted soil then you should not have settling and the step and bump on your house.
 
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Old 06-14-16, 05:31 PM
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Thanks for the reply Pilot Dane.

Frost depth line is 16" here.
Presumptive soil bearing capacity is 1500 psf.

I'll be using the presumptive soil bearing capacity so I don't have to hire an engineer to do a soils test. I talked to the city guys and that's what they recommended for me to do. I'd rather spend the money I save from not hiring an engineer, on the slab.

There will be a patio cover over this slab. One side of the cover will be supported by a ledger on the house. I've already checked the loads from the ledger on the house with the city guys so that's not a problem. The other side of the cover will be supported on two posts bearing on footings which will not be on the perimeter. I didn't show these footings in the sketch. The city requires these footings to be 16" for a patio cover. I could go with sonotubes for those, but I figured maybe if I used a full 16" perimeter footing this thing ain't gonna move much and I could just include the footings in the monolith, and I'll have a single pour.

I don't know for sure what the footings on the house are, but it's a monolithic slab so the house footing probably goes down to the frost line.

I'd probably want to do a thickened edge of at least 8" or so. The full perimeter footing down 16" may be overkill, but the house is only a few years old so the soil near the house may be loose so I'd at least want to go down 16" on that side. The soil away from the house is probably undisturbed, at least beyond 4 inches or so.

I don't want this thing to move. So how about a thickened edge of 16" depth adjacent to the house, and a 8" thickened edge around the sides and back?

And back to my question about the "bump" and "step" shown in the sketches, are those a problem?

What sort of isolation should I specify?
 
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Old 06-15-16, 06:28 AM
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It sounds like you've done your research. Going down to 16" or to the bottom of the house's footer to make sure you get to solid/virgin ground is a good idea. And, maybe taper on the sides away from the house to account for loose backfill close to the house. When you think about it most/all walkways and driveways don't have perimeter footers so you're going above and beyond and doing some thikening at the edges certainly won't hurt (unless you're hand digging).
 
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Old 06-15-16, 09:55 AM
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Thanks again PD. I don't mind spending a little more for a better slab, if it makes sense. I intend to stay in this house for a long time, so that's a factor too.

Yes I've done some digging (pun intended), but one thing I don't understand is why my existing porch, which is actually one corner of the house, is just a 4" slab with NO frost protection whatsoever. The way they build things around here (Albuquerque) is they just put the slab on the soil. Of course the soil is very sandy. Homes are almost always slab-on-grade, but sometimes it looks like they use a stem wall.

Maybe, the need for frost protection is based on the drainage characteristics of the soil, the amount of rainfall, and the temperatures. We certainly get a lot of freezing temps here, but our annual rainfall is only about 8 to 9 inches, the frost line is 16", and by the time the low temps hit, the soil is pretty dry. I noticed that my driveway is also just a 4" slab, with no edge of anything, just sitting in the soil.

This is what the city has in their Patio Cover handout:

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Old 06-15-16, 10:41 AM
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Concrete porches, patios, walways and driveways around here are just a 4" thick slab with no perimeter footing. A footing is only done when some structure above like a wall or column demands it.
 
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Old 06-15-16, 04:14 PM
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When the soil is not "undisturbed", do they compact it or just use a footer to get down to undisturbed ground?
 
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Old 06-15-16, 04:50 PM
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Footers get dug until firm enough soil is found. In most cases virgin, undisturbed ground is firm enough. If there is soft soil the area still needs to be excavated until firm enough soil is reached. Then the footer can be poured all the way from the bottom up to the needed height. Or, the excavation can be filled with clean stone like #57 which naturally assumes a high compaction ratio just by being poured into the hole. Then the footer can be poured on top of the crushed stone. The key being there is a load path all the way down to good bearing soil.
 
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Old 06-16-16, 12:06 AM
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That sounds about right. Now I'm looking at the ground there and wondering how much loose soil and gravel is on top. I think I should probably scrape it off and see what I've got. I'm guessing that firm soil is about 6 inches down. If it is, do you think it's better to just go with a 6" slab, or put in a couple inches of the stone? I don't mind the extra cost of the concrete.
 
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Old 06-16-16, 09:12 AM
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I assume your going to meet the meet the minimum for a ready mix delivery. In that case whether or not to go 4" with stone or 6" thick concrete depends on logistics. It may be easier and cheaper to go with the thicker slab and not deal with the stone. If it's a big pour generally it's cheaper to bring in the stone and keep the slab to a more standard 4" thickness.
 
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Old 06-16-16, 09:29 AM
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Yes, that makes sense. It's a small slab approx. 12'x15' so I'll have so see how the numbers work out.
 
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Old 06-19-16, 04:40 PM
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OK, so I leveled off the soil to see if a 4" slab would work, and it looks fine. I sloped the soil away from the house at 1/8" per foot. Now, I realized I have a small challenge here. As you can see in the pic, the ground slopes left to right. I need to allow for drainage in that direction, to the right side of the patio. I am doing the remainder of the yard with pavers, and the pavers off the end of the slab would have to be sloped. I'd have this weird step off the concrete to the pavers, that would start at 0 on the left corner and be almost 2 inches on the right. A tripping hazard for sure. So now I'm thinking of running the concrete out further, to within a foot so of the concrete blocks that form the base of the shed (where the shovels are). Then I could just put gravel in that remaining space between the end of the concrete and the shed base for drainage. The other option would be to slope the concrete in two directions, from the near left corner to the far right corner (away from the house and left to right). That seems kinda funky to me but maybe it's done that way sometimes.

What do you think?

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Old 06-19-16, 04:49 PM
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Just to clarify - if I did the "double slope" thing, the slab would be level at the near edge, where it meets the house porch slab. The left side would slope away from the house at 1/8 per foot. The far edge would slope from left to right at 1/8" per foot. That means the right side would slope away from the house at 1/4" per foot. Is this a workable idea?
 
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