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Thread: Pouring a slab on a (mild) grade

  1. #1
    Join Date
    May 2006
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    Pouring a slab on a (mild) grade

    I've decided to build a shed in my backyard -- nothig huge, storage as much as anything else, though it will also give me a place to store wood out of the weather. I was going to put it on skids (planned size -8x10), but I leave near a lot of critters, and I've begun to worry that I'd be creating a great place for a skunk or racoon to set up shop. So, I've decided to pour a slab (3.5" slab)

    The problem I have is that my yard slopes down about 6 inches along the 8ft dimension of my planned shed. so if a 4" slab was on-grade on the low end, the top of the slab, and thus the siding, etc., would be 2" below grade on the high end.

    Is the proper solution to regrade the yard to avoid this? Or simply use piers and use screening or PT boards to "wall off" underneath the shed from critters?

    For a variety of reasons, I'm not interested in buying a prefab shed with integral flooring. With that in mind, what would you do?

  2. #2
    a couple thoughts i had would be:

    - to thicken your slab so that top of slab on the uphill side is above finish grade.

    - Regrade the entire area as you said.
    - i also think that you would be better off building a platform out of lumber on skids. i think it will be cheaper and more easily constructed. a small amount of concrete such as what you would need might be charged a small load charge.
    and also depending on local code enforcement a wooden platform is not "permanent" where a concrete slab is.


    I went through the same thought process a few years ago and chose a wooden platform

    Cheers,
    Nick
    "there is no such thing as a mistake in woodworking, only opportunities to re-assess the design"

  3. #3
    A lot depends on your soil.

    Scratch out any vegitation.

    Form it up level and bring in fill. Typically sand.

    It wouldn't hurt to throw some #4 rod or 6x6 mesh in the mix.

  4. #4
    For our Habitat house we occasionally put sheds on concrete slabs. There have been a couple were the grade sloped away as yours does. What we did was form it up, and fill the bottom of it with rubble, old pieces of broken concrete etc, then filled it with sand and gravel before we poured. It took a bit more concrete than normal but it worked out just fine.

    Bryan

  5. #5
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    Quote Originally Posted by Geoff Barry View Post
    ...I was going to put it on skids (planned size -8x10), but I leave near a lot of critters, and I've begun to worry that I'd be creating a great place for a skunk or racoon to set up shop. So, I've decided to pour a slab (3.5" slab)...
    The slab won't necessarily keep the critters from living under the shed. I have a shed on a concrete foundation and the ground squirrels and gophers have decided that the slab makes a perfect roof for their burrows.
    Don Bullock
    Woebgon Bassets
    AKC Championss

    The man who makes no mistakes does not usually make anything.
    -- Edward John Phelps

  6. #6
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    Geoff, I'd suggest you do at least a little re-grading to both prepare the area for your slab as well as to insure that ground water on the slop has some "direction" away from your new structure. That may require a bit of a swail on the hill above the building position as well as a slight, if only short, slope back toward the hill from the edge of the slab. Do this work "now" rather than after the slab is in place as it's much easier that way. You can always rent a small machine to help you do that work if you don't already have something appropriate.
    --

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

  7. #7
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    Thanks!

    For all of the suggestions -- I believe it's time for the ole mattock and tamper combo to make an appearance again Regrading time!

  8. #8
    Join Date
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    A while back I had asked a friend of mine who lives out in the country, his opinion to the same question, concrete slab or pier and beam floor. (With the price of concrete skyrocketing ever upwards I started to debate between the two choices. Another friend just had a 30x50 building done, and the concrete alone was $9800, thus my concern.) The answer to my question kinda surprised me to say the least. I can't say he is right or wrong, because I have never lived in an all wood framed house, raised two feet off the ground. I was also concerned about critters squatting and taking up a home under the floor joists. Ok, ok, ok, here's his answer.... He said:
    "Animals will not live under the house, they may visit or go under there at times, but they will not live under the house".
    When I asked "why?"
    His answer was that it was "too cold and too close to human traffic".
    The "human traffic" thing I could understand the "too cold" part confused me.????
    For his explanation he said he's been living in a 150 year old farm house most of his life (48 years), and he's never had any wild critters make a home under the house.
    The next shop will be a pier and beam pole barn... hope this helps? If not it's an interesting story!

    Ted
    "And remember, this fix is only temporary, unless it works." - Red Green

    THIS THREAD IS USELESS WITHOUT PICTURES


  9. #9
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    For an 8x10 I'd frame it and I'd have it sitting on 4x6 runners the full length so it's easy to level, lift, shore, relocate.

    Set those runners on the little precast concrete piers (3 each runner). Even with poor soil you should have 1500psf bearing once you scruff away the organic topsoil. And if it does settle it'll be easy to bring level again by jacking up the appropriate section of 4x6 runner.

    There will be no significant code issues as with a 'permanent' structure.

    As was stated previously, set the height high enough to discourage 'live-ins', which usually means don't skirt it either.

    For a shed on up to about 12x16 I'd go w/ a frame floor.

    Chaser

  10. #10
    If I were building a small shed on concrete, I would dig down around the edges of the slab so you have a sort of barrier or footing under the slab. Wouldn't have to be real wide, just the width of a shovel, and will help with both critters and grade. Then I would run a row of 1/2" rebar around the perimeter. Depth, maybe 6" or so below the bottom of the slab. Or to the ground and a little deeper on the low side.

  11. #11
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    My 30x40 pole barn is on a slab that was poured after the walls and roof were put up. I have some signs of some critter that is tunneling under the slab. I keep filling in the holes but the critter keeps re digging. If it keeps up I will set a trap. Wish I had footings poured to keep the critters out.
    David B

  12. #12
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    Hahira, Georgia
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    Rat Guard

    On small slabs (that have no substantial load and, thus, no need of a footing) you can make a 'rat-guard' that's part of the slab.

    Just bring your form down 12" to 16" (from finish floor) so you get 10" or more below grade. Taper it back up to meet your 4" slab (like you would do with a footing, but without the flat base of a footing).

    This saves a lot of concrete and prevents burrowing. It's used a lot down in the south on grain bins and such...back when farmers still had grain bins...

    It' always recommended to put your slab edge (footing) below the frost line to prevent heaving - and then cracking.

    Chaser

  13. #13
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    Quote Originally Posted by David G Baker View Post
    My 30x40 pole barn is on a slab that was poured after the walls and roof were put up. I have some signs of some critter that is tunneling under the slab. I keep filling in the holes but the critter keeps re digging. If it keeps up I will set a trap. Wish I had footings poured to keep the critters out.
    Throw some moth balls in the hole, most critters don't like the smell.
    "And remember, this fix is only temporary, unless it works." - Red Green

    THIS THREAD IS USELESS WITHOUT PICTURES


  14. #14
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    Clarification.

    If a shed bears directly on the slab - roof load to walls to slab - it should have a footing. Even where there is no frost line codes typically require 12" below grade.

    In the case of the pole barn the slab is not taking the load of the structure and hasn't the same requirement - but I'd still consider rodents and heaving from freezing weather.

    Chaser

  15. #15
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    Ted,
    I will give moth balls a try. Thanks.
    David B

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