Monolithic slab with stepped footers?
Every picture or diagram I see of a monolithic slab foundation shows the slab as being on perfectly flat ground. I understand the concept of a monolithic slab, combining the footer / stem wall / slab into one pour and that’s easy if the ground is very flat.
If you have a “small” amount of slope on your site, can you use “stepped” footers with a monolithic slab in order to reduce the amount of concrete needed? By stepped, I mean digging the bottom of the footer horizontal for a distance, then adding a vertical step of 8″ or so when the slope of the ground allows it – like with a traditional separate footer and stem wall.
Seems like that’s a reasonable thing to do but I can’t find any examples of it actually done. Am I missing something obvious?
Thank you for any insight offered.
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Bluegoose68,
The footings on monolithic slabs are really just a thickening of the edge of the slab, and are made the width necessary to support the structure above. If you step them, or have to make them deeper to reach grade, you have to use that same thickness throughout, which means using a lot of unnecessary concrete.
You could look at a stem-wall and slab a just a way to reduce the necessary amount of concrete from footing to slab. It also simplifies the insulation, meaning you can insulate at the slab edge, rather than having to insulate the exterior and underneath the thickened footings.
Thank you for the reply.
Let me give you a few more details… The building is 38’ long x 24’ wide. Location is southern middle TN (CZ 4a, but only about 20 miles north of CZ 3). The difference in vertical height from the highest corner to the lowest corner is 18”. Since it is a one story wooden structure, I’m using a footer width of 12” and I’m going 14” deep into the ground (frost line depth is about 10”-12”).
Foundation options:
1) Monolithic slab, 12” wide “footer”, 14” deep below grade at the lowest point and no “steps” in the footer – it’s all at the same grade. Requires 25 cu yds of concrete.
2) Monolithic slab, same as option (1) but adding a “step” at the bottom of the footer when the grade allows it. Requires 22.5 cu yds of concrete (I know you don’t buy ½ yard, just for comparison’s sake.)
3) Traditional footer / stem wall / slab construction. Using a 12” x 6” footer, an 8” wide stem wall and 4” thick slab requires 21 cu yds of concrete. Slightly less concrete but more labor for the separate pours.
4) Traditional footer / CMU stem wall / slab construction. Uses 12.5 cu yds of concrete and about 375 CMU blocks for the stem wall. Significantly less concrete but now laying block is involved.
There isn’t much concrete savings going the traditional footer / stem wall / slab route. That’s why I was asking about the possibility of adding a step to the monolithic slab footer. It saves a little (between 2 and 3 cu yds.) compared to keeping the bottom of the footer all at the same grade. Even though it’s a little, why not go that route if it doesn’t cause other problems? I’ve never seen a monolithic slab drawing that included stepped footers though.
You make a good point about being able to insulate between the stem wall and the slab. To add one more wrinkle to the problem, I’m very concerned about putting rigid foam insulation in the ground. Looking at the termite probability map, in technically in the “moderate to heavy” zone but also only 20 miles away from the “very heavy” zone. I’m leaning very heavily towards putting the rigid foam insulation on top of the slab and covering that with 2 layers of plywood. It’s a design I found on the GBA website (see attached picture). Doing that takes the edge insulation advantage away from the traditional footer / stem wall / slab option.
Now that it’s all layed out, it seems that Option 1 is the safest and it doesn’t require significantly more material. But I still wonder, is there any problem with including a stepped footer with a monolithic slab?
Last question: it’s typical to put gravel under the slab to help in grading and to keep water away from the underside of the slab. Is it also beneficial to put gravel under the “footer” or turned down section of the monolithic slab? The sketch doesn’t show it.
Apologize for the lengthy post but I don’t know how to make it more concise. Thank you.
I haven't done stepped monolithic slabs but if the difference in height were significant, I would worry about differential shrinkage during drying and curing. But just a few inches shouldn't matter much.
"Footings shall be supported on undisturbed natural soils or engineered fill." https://codes.iccsafe.org/content/IRC2018/chapter-4-foundations#IRC2018_Pt03_Ch04_SecR403.1
Bluegoose,
I agree. Option #1 sounds the simplest. You will end up using a bit more concrete, but also if you add some rebar, some really strong footings.
I also agree, that insulating above the slab makes a lot of sense if you are adding a sub-floor.
As Michael said, you don't want to add fill under the footings.
I'm surprised he didn't push you towards a concrete-less slab, which he has done successfully several times.
Bluegoose68,
Something I forgot to add: When you excavate for option #1, you don't need the bottom of the footing to be level. If you keep it a consistent 14" below finished grade, it will have a slight slope, which both saves some concrete, and makes running any perimeter drains you put in easier.
That is a fantastic point. In my mind, I was assuming it had to be level (it's always shown that way.) I found this note in the building code that "The bottom surface of footings shall not have a slope exceeding one unit vertical in 10 units horizontal (10-percent slope)."
I'm well under a 10% slope if I follow the contour of the ground.
You just saved me almost 3 yds of concrete! Thank you much.
Bluegoose68,
Good luck with your build!