Hcfyk

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?

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

|______________||___

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.

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.