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We have a 10x10 wooden shed sitting on a concrete slab that is under water when we get lots of rain. Is it possible to raise the entire thing, slab and all maybe 6" somehow to get it out of the water?

Edit: I'm not at the house at the moment, but here are some shots I have of it: https://i.stack.imgur.com/zgVsf.jpg

Otus
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  • Sure, but it's probably easier to re-route the rain. :P Are there any cracks or control joints in the slab? – isherwood Jul 17 '18 at 17:41
  • Or forget the slab and raise the shed... – The Evil Greebo Jul 17 '18 at 17:51
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    Remove the shed, use the slab as foundation for a raised platform, but shed back on top? – Torben Gundtofte-Bruun Jul 17 '18 at 18:16
  • https://diy.stackexchange.com/q/90906/46271 – Kris Jul 17 '18 at 18:20
  • We've tried fixing the drainage but there is no where for the water to go. At this point it's either an expensive pump + drainage system or lifing the shed out of the water. If we raise just the shed, the floor (slab) will still be under water unless we build a floor for it (prefer not to do). I don't believe there are cracks or control joints – Otus Jul 17 '18 at 18:37
  • Oh the concrete is the floor of the shed? – The Evil Greebo Jul 17 '18 at 18:57
  • I think some pics would be useful here. – The Evil Greebo Jul 17 '18 at 18:57
  • Mud Jacking is possible - but you are probably better off using the slab as footing and laying another slab on top. $$$$ vs. $ – Ken Jul 18 '18 at 04:27
  • I have looked at your picture - it looks like your entire yard there is a lake when it rains heavy. I know you don't want to hear this but I think the only reasonable solution is to lay concrete over the concrete. – Ken Jul 18 '18 at 07:01

4 Answers4

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If you “jack”...you crack.

That slab weighs a minimum of 5,000 lbs. (4” thick) and will crack when you start jacking it up a few inches.

Little known fact, concrete only has a tensile strength of 55 lbs. per square inch (as compared to compressive strength of 2500 psi ). When one edge is lifted, the top of the slab is in compression and the bottom is in tension THROUGHOUT the slab.

No amount of levers (regardless of fulcrum points) will keep the slab from cracking. In fact, where the lever is placed could accelerate the cracking process. If you’ve ever seen a contractor remove a sidewalk, you’ll know what I mean. They use an excavator to pick up an edge so it will break and they can load reasonably sized pieces in a dump truck.

Now, if the slab is reinforced and the reinforcing is properly placed, it could hold...but what are the odds that old shed slabs are reinforced.

In addition, even if you were to lift the slab, how are you going to EVENLY place gravel under the slab and then COMPACT it? Impossible. It would tip and put a stress point on the slab...causing the slab to crack.

I like the idea of trying to control the drainage around the shed and if that doesn’t work, I like the idea of removing the wood portion, using the existing slab as a footing, installing a higher slab and then reinstalling the wood shed.

Lee Sam
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  • I will give you points on this. Mud Jacking is too expensive - op is better off with the slab as footing of a new slab. Cheaper with an assured result. – Ken Jul 18 '18 at 04:24
  • @Ken I think of mudjacking as slab jacking, but I’ve only seen it move a slab 3-4” maximum. Can it raise 6-8” or is there just exposed polyurethane foam exposed? – Lee Sam Jul 18 '18 at 05:32
  • Yes Lee there are limits with Mud-Jacking - the foam or other materials that are used have limits. There are more than just height concerns as well - typically jacking will sink over time because the material will eventually fail under load, or in case of ground issues (sinking ground) it will fail again because the ground has not yet bottomed out. However the cost of doing this will easily cost the op about $3K to $4K, cheaper to pour concrete - given ops picture I assume the entire yard floods - so ops best bet seems to me is to raise it with concrete. – Ken Jul 18 '18 at 07:00
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You can do this with a little levering, in stages. My dad always said you could move the world with a long enough lever, but 3 stout 12-foot 4x4s should do.

  1. Dig three holes under one edge of the slab which are positioned at the centers of each third of the length. Each hole should allow insertion of a 4x4 to a depth of 6-8".
  2. With three sturdy friends, and using appropriate fulcrum blocks as near as possible to the slab, raise the side of the slab as far as one swing of the levers allows. Place blocking as thick as possible under the slab and lower it.
  3. Raise the fulcrums the same thickness as the blocking and lift again. Raise the blocks. Repeat until you have 10" clearance under that side of the shed.
  4. Repeat the process for the other side.
  5. Once the entire shed is raised, fill the void with inorganic gravel. Level it 3" below the slab. You could also place uniform concrete pads on a grid pattern throughout the slab area. This would ensure a level base.
  6. Place rigid blocking under the entire perimeter of the shed 1" below the gravel height (to allow for settling).
  7. Lower the shed using the levers.

Obviously the numbers are approximate here. Use your best judgement when estimating settling, etc. If you don't get close you could see the slab crack where it's under-supported.

This also depends on a strong and intact slab. It'll be spanning 10 feet while it's being lifted, so it's possible that it'll crack if it isn't already. Consider whether it has steel reinforcement and whether you're willing to take the risk.

At no time should human appendages be under the slab. This should be obvious, but it's worth mentioning.

isherwood
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  • A nice description of a process that has a fair chance of working. A quick analysis reveals a maximum tensile stress on the bottom of the slab being about 235 psi when using the technique in your answer. The tensile strength of this concrete should be about 250 psi (based on common rule of thumb of 10% of compressive strength - 2500 psi). Through the years the concrete should only be getting stronger. – Ast Pace Jul 22 '18 at 00:48
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You should make a surface swale or an underground drain to take the water away from the local depression where the shed is located. How far is it to lower ground?

You could inquire about mud jacking or professional polyurethane foam jacking.

Jim Stewart
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  • I'd be pretty surprised if you could get a company to float a building, let alone the entire thing. It would be quite a debacle keeping it level. :) – isherwood Jul 17 '18 at 19:43
  • Yeah, that's why I said, "You could inquire about . . . (it)". But just maybe it would be the solution. I wonder how much weight is in the building? – Jim Stewart Jul 17 '18 at 19:53
  • If my dad's custom fish houses are any indication, 2-3,000 lbs. – isherwood Jul 17 '18 at 19:56
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Dig around the edge of the slab and use a pancake jack to lift 1 side a little at a time. Shim each side as you go to hold them up. Once you get the slab to the right height run 6x6 or 4x4 hardwood underneath the slab then reverse the process until the slab sits on the new base of hardwood. If there are no cracks visible lifting a little at a time shouldnt crack a 4inch slab.

John
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