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I am designing a shed for my garden and I would like to build the shed inset from the concrete base by about 20cm on each side. The concrete base already exists (built before I brought my house). It has to be inset as there are fences either side and I will need a bit of a gap for construction.

My concern is that it will not be waterproof enough. As water can fall on the concrete and travel to the wooden framing.

My current idea is to lay a single course of bricks and cover then in some sort of waterproofing layer (not sure what to use for that). I am unsure if that is a good idea though.

What are your thoughts?

  • 1
    Hello, and welcome to Home Improvement. Good question; keep 'em coming! – Daniel Griscom May 28 at 10:43
  • What are the dimensions of the existing slab? How close is this slab to the obstructions on each side? What is the vertical dimension of the planned walls? – Jim Stewart May 28 at 11:24
  • It's not a direct answer here but something that is good to know: It's not just the water falling on the slab that you need to worry about, concrete can wick moisture up like a sponge so it is best to keep untreated lumber from being in direct contact with the slab. – JimmyJames May 28 at 21:16
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Michael Karas is correct in that a half-height block foundation wall is common, but that doesn't really address your concern about water intrusion. His suggestion to seal the block does, and that wouldn't require the blocks in the first place.

I would build your walls with bottom plates of treated lumber one nominal size wider than the walls themselves, and place the plates flush with the outside face of the wall sheathing, resting on foam "sill seal".

Then I'd install a metal flashing against the plate, lapping onto the concrete. This I'd caulk to the concrete with urethane or other high-performance caulk. Wrap the building with housewrap fabric lapped onto the flashing to create a full drain plane.

For extra protection you could apply a paint-on waterproofer or asphalt caulk to the area behind the flashing before it's installed.

  |          | |
  |          | | <-- sheathing
  |  stud    | |
  |          | ||
 _|__________|_||
|   plate      || <-- flashing, caulked to the slab
|______________||___
  • Will the flashing be enough or would it be worth while placing the treated timber on the concrete, fastening it down and then covering with a waterproof compound. I would then place the stud wall on top and finally the flashing. Effectively creating a wall with a double bottom plate where the last plate is pre-fastened to the slab and waterproofed? Although I have a feeling that I am going to far. – Alex_Wood May 29 at 13:26
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    That's essentially what I suggested in the last paragraph. I'd still use flashing to direct water away from the building, but sealing the lumber to the concrete is good insurance. What product you use is up to you. – isherwood May 29 at 13:29
  • Sorry i must have missed that bit. Thanks for your help. – Alex_Wood May 29 at 13:31
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    Then again, a good sill-seal tape may be adequate. Good luck. – isherwood May 29 at 13:33
4

It is fairly common on a flat slab to create a curbing using stubby height concrete blocks. These blocks are typically about 4 to 5 inches in height. When used on a garage (for example) it is typical to place the curbing adjacent to the edge of the slab/foundation.

In your case you where you want to inset from the edge of the slab you could choose to apply a tar type compound to the outside of the curbing to seal it against water intrusion. When searching for this material look for the sealers that are typically applied to the outside of basement walls on new construction before the backfill is done. Note that use of the sealer where it may have exposure to the sun and elements may require a recoat after a few years.

  • When concrete blocks are used on the perimeter of a garage is the framing sill plate put on top of these blocks? – Jim Stewart May 28 at 12:14
  • It is, Jim, with foam sill seal between. – isherwood May 28 at 13:37
1

I have seen houses constructed next to a building by the erection of fully constructed and painted walls from the inside of the house slab inches away from another wall. I did wonder how repainting would be done.

You could have the slab trimmed with a concrete saw before framing. This would not be advisable if the edges of the slab are thickened.

  • I did consider this, but it would be a lot of work and expense for a project of this size. Also the slab is bloody thick. – Alex_Wood May 29 at 13:30
  • If the slab is thick, then cutting it would be a pain. Do you think it is uniformly thick or is it thicker at the perimeter? It would be a shame to not take advantage of the full size of the slab. What is the distance from the slab to the fence? Is the fence on the property line? If so, there may be setback requirements in the building code. – Jim Stewart May 29 at 18:30

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