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Piers under slab for small sunroom

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  • Piers under slab for small sunroom

    Adding a small sunroom to an existing patio slab. The owner has a really rinky dink lean to on it now, which will be taken down. We are using five pairs of 72x80 double sliding doors, one on each side, two along the long side and the fifth cannabalized for the panes (deal from someone that never used them). The weight of each pair of doors is 160lbs. We're basically just framing for the doors, adding a metal roof over the sunroom part. The Sunroom size is 10x12 ft. We're in Atlanta so not a big freeze issue (twelve inch listed, but I've never seen it in fifty years here). So, can I dig out 12x12 inch piers holes eighteen inch deep under the edges where the posts sit? The side walls carry no load, other than their own weight. The house was built in 1989, the concrete is 4 1/2" thick, no cracks or signs of movement, level grade. Concrete not my area, My work has been primarily carpentry, mostly finish.The drawing just captures a pice of the house, existing roof rafters, and the raw framing planned.
    Attached Files
    Last edited by AP Dent; 07-20-2020, 12:07 PM.

  • #2
    Obviously there are point loads at every stud pack of the structure. What you have described as "piers", 12"x12"x18" pads, are not what I call piers. In terms of load, it does appear to be minimal, although are snow loads a factor in Atlanta? If not, from a pure load standpoint it would appear to be ok, but of course for a definitive opinion, see a structural engineer.




    • #3
      I looked up the snow loads, very minimal in Atlanta 1.3-1.8 per SFT. I remodeled my mothers house and we added a beam over the rafters, dropping down the Strongties to hold the rafters for removing a wall. I had a structural engineer give me the specs on that. We had to carry the load of the roof, rafters (with 2x8 x 16ft rafters sistered on), flooring down through to the basement to the floor there. The floor in the basement was about five inches of concrete, thus the need to cut and add the footing to support the pole to the floor above. The beam was two 2x16 LVL beams bolted together holding up the ceiling made from two rooms now combined roughly thirty feet...way more weight than anything here. He spec'd two points, one at each end of the beam (23 foot) with the basement one having a sixteen inch deep by twelve by twelve for a footing, rebar at the bottom, bottom flared. That's basically what I'm doing here for way less weight and stress so, mostly overkill I think. I used his calculations from a much heavier load and that at two points, here it will be four points across the front, two on the Right side (even though it is just the weight of the studs and doors), leaving that middle post on the left wall as I'm thinking with little weight and the front carrying the full load it would be fine.
      Last edited by AP Dent; 07-21-2020, 11:24 AM.


      • #4
        I'm trying to figure out how to exactly dig under a slab at the points you need. I know you said on the edges but I cannot imagine it being a fun thing to be assigned to.

        Here we would have to go down 30 inches which may be my not quite being able to see how it can be done. I guess I can see 12 inches but still trying to wrap my head around it.

        And how do you account for the concrete shrinkage and making sure it actually is supporting the slab. Almost every pier I pour I pour to the top of my sonotube. A day or two later when it has dried it is almost always a bit lower than the top. I see a pier that my not really be supporting the slab. You going to grout the thing in there.

        Not familiar with it but what about some sort of mudjacking under the slab to stabilize it.
        Last edited by m beezo; 07-26-2020, 07:40 AM. Reason: other thoughts


        • #5
          Hey.... Thanks for this thread. I'm looking forward to customizing my sunroom as well. This helps. Cheers