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Concrete deck and slab foundation built as one?

Needinglotsofhelp | Posted in Energy Efficiency and Durability on

Hello, I’m Cynthia – new member here. I’m wanting some feasibility advice.

Kansas City climate – on the line between zone 4 & 5.

Original house (1898) is built on stone foundation, with basement. This foundation is in pretty good shape, but engineer doesn’t want it to support any more weight. 

Wanting to build an addition with a deck along the side of the house that wraps to most of the back.  I don’t want a crawlspace & don’t need another basement.  Current exterior walls with little insulation, brittle vinyl siding, and windows that don’t open easily – so a well insulated addition that lets in more light and gives us some extra space makes sense compared to retrofitting existing exterior walls. There will be a deck around the addition with stairs down to grade. 

Current house sits 4′ above grade. This is what I’m thinking: 

Turndown slab foundation/(monolithic slab ?) at 2′ above grade that also extends beyond the addition to be the finish surface of a deck  (Concrete deck or raised concrete patio, whatever you want to call it). I would put the insulation above the slab for the part that is the addition. I can’t find any examples online of a deck and foundation poured together.

Here’s my concerns:
1. Moisture that migrates through the slab from the deck area to the addition area

2. Long term surface finish of concrete on the deck portion – but seems maintenance would be so much less than regular deck – seems too good to be true.

3. Finding a contractor to form/pour the concrete. It is very difficult to find somebody willing to even answer the phone, let alone do something they are unfamiliar with.
We are doing all of the other work except the structural figurings/permit drawings which are being handled by an engineer. Is it possible to do this foundation ourselves or any suggestions for a different design that we could do? I’ve read a few of the articles on here about a concrete free floor, but then we are back to a high maintenance deck. 

I’ve attached some pictures. The black outlined is the original house. The red outlined is the addition. The blue outlined is the deck. One diagram is with inset stairs, the other with typical stairs. The lot is fairly level for the first 10′ out from the house, then slopes away in both directions. Planning to regrade (?) The long portion faces west and the back faces north.

Thanks in advance,

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  1. Expert Member


    There are several of obstacles to pouring the two slabs at once.

    - Slabs with integral (as opposed to separate) footings need to be insulated and damp -proofed on the outside face. You won't be able to do that with one pour.
    - The two slabs need to be at different elevations to stop water making its way into the addition.
    - They are much more likely to crack if they are one piece.

    My suggestion would be to pour the addition either as a monolithic slab, or with stem-walls, then either pour the patio, or use pavers.

    I agree patios are a lot less work to maintain than decks, and often give you a better relationship to the surrounding landscape. Yours will be a bit more challenging as the grade at the edges of the patio will need retaining. Concrete patios aren't entirely maintenance free. They need periodic re-sealing if you want the surface to last.

    If you have good DIY skills it's possible to form up both the addition and patio, but you will need a concrete finisher to power-towel the addition, and if you want a nice surface to finish the patio too. It's not something you can pick up the necessary skills to do in a short period of time.

  2. Needinglotsofhelp | | #2

    I appreciate your reply, Malcolm. You have saved me a whole lot of time trying to figure this out. I guess that is why I could never find any examples. Interestingly, the 2 concrete contractors that did actually answer the phone never questioned my plans to pour it this way, but they also never followed up after I sent them some drawings - so maybe they didn't understand my intentions or just didn't want to mess with a screwed up design...

    I do have a question re: your statement about the grade at the edges of the patio needing retaining. Is this because I wanted it at a 2' elevation or because the ground will slope away from it? I was thinking this would be similar to a stemwall.

    Again, thanks for taking the time to respond.

    1. Expert Member
      MALCOLM TAYLOR | | #3


      Concrete contractors tend to be very good at forming up and finishing concrete - but are typically not over-interested in the wider implications of their work.

      Patios don't typically have thickened edges because there are no walls bearing on them, or much point in separating the ground outside from at underneath. I guess with your difference in height you could pour it as a mono-slab with thickened edges to retain the soil underneath, or you could retain it with a wall made of some other material (pt wood, blocks, stone, etc.), but probably the easiest solution would be to slope the grade away from the edges. This would allow you to easily add stairs and help integrate them into the landscaping.

      If you retain the soil, you need to be a bit careful as to how large a drop to the surrounding grade you end up with. I'm not that familiar with your building code,. but under ours if it exceeds 2 feet you would have to add railings to the patio - something I'm sure you'd rather avoid.

  3. Needinglotsofhelp | | #4

    Thanks again, Malcolm.
    Our codes allow 30" above grade before railings must be used. This is why I picked to place the addition and deck at 2' above grade (original house is at 4' above grade). I thought this would be a good compromise between having too many steps on the inside between original and addition and getting close enough to grade on the outside to avoid railings and also allow for some privacy. The back part of the addition will be a mudroom that will house typical stuff that would go in a garage and be used outside (like bikes) so again, closer to grade makes it easier to get stuff in/out.

    1. Expert Member
      MALCOLM TAYLOR | | #5

      Good luck with your build!

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