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Building a backyard studio/shed – Slab on grade

user-4603556 | Posted in General Discussion on June 2, 2020 12:31pm

Hey all, Long time reader, first time poster! I’m building a 10×20 office in my backyard and have a concrete slab already in place. I want the height transition when walking from the hardscape around the building and into it to be as minimal as possible. Meaning I don’t want to have to step up when walking into the structure. Would it feasible to build/frame the 10×20 structure like a shed – slab on grade, with no floor framing but then put down DMX and a subfloor on top of the concrete slab on the interior. Additionally, I would be sure to seal the exterior bottom plate so no water gets underneath. The structure will have a decent overhang and should prevent water from pooling in and around the building. Thoughts or ideas on other options for a low height flooring option? Sleepers? or Battons? Or does everyone recommend a traditional 2×6 framed flooring to prevent future moisture/mold issues? Thanks! Jon

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  1. User avater
    unclemike42 | Jun 02, 2020 06:42pm | #1

    Overhang will not prevent water from flowing downhill or pooling. gravity does that.

    so it all depends on the grading of the slab vs whatever hardscape you have installed.

    you can use https://www.homedepot.com/p/DMX-1-STEP-100-sq-ft-3-67-ft-x-27-ft-6-in-Unique-Air-Gap-Underlayment-Prevents-Mold-and-Mildew-DMX-1-Step/204494289

    and a floating engineered solid floating floor of your choice. you can also just put down 6 mil poly and put a waterproof version.

    if you live where it may freeze, think about how ice would change the water flow.

    as an existing slab, was it engineered to be a building base, or to be a patio?

    and the hardscape, I ask about the engineering. is drainage properly provided for? is the base deep and compacted? will it all move?

    If you are putting a structure on a slab meant to be a patio, make it light and cheap.

  2. jlyda | Jun 03, 2020 01:13am | #2

    If slab is level with top of grade, no sealant will retain water from creeping under the plate. If the slab is above grade by at least 3” you might be ok, but not with the type of thunderstorms I get in my area. Also, if the slab is not built as a monolithic or resting on footings at frost depth it is going to move around, crack etc. If you have no freezing in your area you might be ok. If you install ground gutters around the perimeter with a wide width of permeable gravel it may keep water from getting under the plate, again this probably wouldn’t work in my area. If I were to build in my area I would have to do a bit differently. If it were me I would build the structure to float on the slab with wood floor raised a couple inches, so air can get under the joists and dry them out. Will get a ton of mold on underside of joist if they cannot dry out. I would bolt down to slab on corners and about every 4’ with a notched type angle iron that allows concrete and structure to move independently like an oversized, slotted truss clip. FYI, if you are pulling permits best to just call it a shed. If you call it an office you are going to have to do a lot more to the structure or may possibly not be allowed to build it. Again, based on my area.

  3. abdulhaih | Dec 22, 2022 02:15pm | #3

    I wouldn't risk building such low height flooring in the backyard, depending on where you live. If it's often raining or snowing, water might get inside, and if the ground freezes the floor will take damage over time. Although it also depends on how strong the foundation is. A concrete base is more reliable, but you could still add some height to the floor to assure resistance against water. In any case, you can consult a professional at https://www.gardenrooms.scot to see what's best for your backyard. They offer consultations and you can find some ideas depending on what you'd like.

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