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Thread: adding on to garage slab for shop?

  1. #1
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    Dec 2010
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    adding on to garage slab for shop?

    I have a detached garage that I want to expand or rebuild to create a shop space. The garage was built before we bought the house, and I don't know anything about the existing concrete slab (foundation?), other than it seems to be in relatively good shape. The garage was probably built in the last 15 to 20 years and was done by one of the "garage builder" companies common in the Chicago area. Definitely not high end, but I hope it was up to code.

    Is it feasible to expand the footprint of an existing concrete slab, or do I need to remove and replace? If I were to dig down around the existing edges, what could I look for to tell if it is good enough to keep? If it is suitable to use and expand, how much more difficult would it be to add to two sides of the existing slab rather than just one? I am planning to hire someone to do the concrete work and I'm sure they will have opinions on these questions, but I want to understand what I will be facing so I can do some preliminary planning and design.

    Thanks for any help you can offer.

  2. #2
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    I doubt there is any need to tear it all out, and redo it. The tricky part will be how to move the walls, and support what's over it. They will probably saw a straight line through the concrete, do away with any irregular part that was under a wall, and pour whatever other slab adjacent to the straight line.

  3. #3
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    When we extended a slab, they drilled holes in the existing slab and epoxied in rebar to bind old and new slabs. Easy peasy.

    I think Tom's point about walls and the slab beneath the wall's plates will be your main issue.

  4. #4
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    architect and structural engineer. The first will provide ideas you may not think of and the second will make the design structurally sound. Extra expense but usually worth it. You can still do the work yourself.

  5. #5
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    Quote Originally Posted by Jon Nuckles View Post
    ... what could I look for to tell if it is good enough to keep? ...
    You can use a (1/4"?) hammer drill in 3-4 places to determine the slab thickness and if there are perimeter footings. Wayne's point about 'dowels' to bond the new to old is common, and probably a necessity (IMHO).

    Also consider the old edges - their condition and shape - and how they will blend into the overall new floor plan. Could be a problem if they are radiused? May need to grind/chip back 1" or so? Or, as Tom indicated, just saw it 'straight'. If blended properly it will prevent thin brittle edges, with resulting spalling and divots in the middle of your new floor space.

    Good luck ... and post pics.
    Last edited by Malcolm McLeod; 04-21-2019 at 10:54 AM. Reason: flood (hope not!); floor :: yes.
    Molann an obair an saor.

  6. #6
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    If you're doing a tear-down of the original structure, it may be easier to do a new slab for the overall new building foot print because of "less futzing around", but the folks who do concrete work are more than likely capable of just doing new footers and pouring new surfaces for the additional space. That will likely be clearly visible, however. But before that, you may or many not actually have to change things out if the existing footers are not up to specification for the expanded footprint of the building and it's associated weight and structure. You need to speak with an engineer about this. If the existing structure is contemporary as you state, you may be able to get information about the original setup via the building/zoning folks if it was a permitted project.
    --

    The most expensive tool is the one you buy "cheaply" and often...

  7. #7
    When I redid the garage at my old house, I added 12 feet on to the existing slab. I tied into the old slab by putting in rebar dowels at angles, so the slabs couldn't pull apart. The only wrinkle was needing to cut a hole in the existing slab for a column footing, which was just a matter of renting a concrete saw. If the existing slab is in good condition, there really is no reason to tear it out unless you really want to.

  8. #8
    Is the existing garage a 1 car or 2 car? The code where I live used to allow a 1 car garage to just be a slab with thickened edge, where a 2 car had to have a full 30" below grade by 10" wide footing. Lot of difference.

  9. #9
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    Yes, it can be done very easily. You'd be surprised. It does take some "big" equipment and the folks that know how to use it.
    If you have a "garage slab, it is probably a nominal 6" thick. It may also have a crown, or slope, for drainage. This will be taken if you want a "flat" floor, if the code allows it.
    Think about in slab heating before you start the project.
    You will be stunned at what an experienced concrete worker can do. I've seen basement walls and slabs poured with an existing house still in place. Some of those guys can do magic.
    "The first thing you need to know, will likely be the last thing you learn." (Unknown)

  10. #10
    Join Date
    Dec 2010
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    Thanks to all for the replies.

    Sounds like my first stop should be at city hall to see if there are records of how the existing garage was built. It is roughly 18' x 20'. I think I can live with 25' x 25', though it will take some downsizing in equipment and assembly surface from my rented 20' x 50' space. We don't have a large lot and, between lot line setbacks and restrictions on how much non-permeable paving and structure we are allowed to add, I don't think I can go much larger. I'll ask for clarification on that at city hall as well. My wife, though very supportive of the project, also has the crazy notion that a shop should not take up the entire back yard! Thank goodness that she does not also believe that a car needs a roof over its head in a Midwestern winter.

    I was planning to completely remove the structure above the slab and rebuild, so access to the areas where the slab would be extended won't be a problem. I might be able to keep and extend the walls that would not be moved, but they are 2x4 construction and maybe 8' high. I am guessing it would be worth going to 2x6 walls just for the ability to better insulate, and I definitely want a higher ceiling. My current space has about a 14' ceiling and I really like it, though I know I won't be able to go that high in my own shop.

  11. #11
    Quote Originally Posted by Jon Nuckles View Post
    Thanks to all for the replies.

    Sounds like my first stop should be at city hall to see if there are records of how the existing garage was built. It is roughly 18' x 20'. I think I can live with 25' x 25', though it will take some downsizing in equipment and assembly surface from my rented 20' x 50' space. We don't have a large lot and, between lot line setbacks and restrictions on how much non-permeable paving and structure we are allowed to add, I don't think I can go much larger. I'll ask for clarification on that at city hall as well. My wife, though very supportive of the project, also has the crazy notion that a shop should not take up the entire back yard! Thank goodness that she does not also believe that a car needs a roof over its head in a Midwestern winter.

    I was planning to completely remove the structure above the slab and rebuild, so access to the areas where the slab would be extended won't be a problem. I might be able to keep and extend the walls that would not be moved, but they are 2x4 construction and maybe 8' high. I am guessing it would be worth going to 2x6 walls just for the ability to better insulate, and I definitely want a higher ceiling. My current space has about a 14' ceiling and I really like it, though I know I won't be able to go that high in my own shop.
    City hall is a good place to start, the garage is likely on a floating slab, and you'll want to know how they feel about extending it. They should be good, but you never know. . . . . .

    As for height, for my current shop, I did a 6" wide cinder block with a 9' wall on top, 104 5/8 precut 2x6 studs with 9' OSB. It worked really nice, and gives me a ceiling a little over 9 1/2 feet. It was definitely easier and safer than my previous shop build, which was an 8' wall on top of 2 rows of cinder block. I also found that studs 16 on center was more handy than 24. 24 was kind a pain when trying to hang things; there never seemed to be a stud close enough.

  12. #12
    Careful about removing the entire frame. The city here will allow you to rebuild a building with addition as long as you leave part of the original structure. If you take it all down, you have to start from scratch.

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