Keeping a Shed on Slab Foundation Dry?

Old 09-03-19, 06:14 AM
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Keeping a Shed on Slab Foundation Dry?

Hi Everybody,

I'm building a 12x20 shed on slab foundation. I live in a cold/snowy climate and I plan on using this shed a lot, so this means there will be a lot of heat/cold exchange inside the shed this winter.

So one of my primary concerns as I do research is the base of the shed leaking where it meets the slab foundation. I imagine if I just have wood floor/framing on slab it's bound to melt and ooze all inside making all my materials (including floor insulation to get all wet/moldy).

So I'm trying to take a number of things into consideration like sheathing hang, bushing materials and whatever is required to shield water from intruding at the base.

Is there a video on this or does anybody know what the de-facto standard for this sort of thing might be?

Thanks for any suggestions.
Old 09-03-19, 06:37 AM
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You said you will have a slab and then also mentioned a wood floor. Are you gong to have both? It is more common to simply have the concrete slab as the floor in a shed.

To prevent water entering onto your shed floor the floor should be above grade as much as possible. It is also a good idea for the side sheeting of the shed to hang down over the slab so water sheds away. If your slab protrudes beyond the walls and exterior sheeting it is very difficult to prevent water from eventually seeping in.

Since you are somewhere you get snow and ice elevation is your friend. Getting the slab as high off the ground as possible will help a lot to keep water out. Especially during spring thaw when there might be snow and ice piled up against the building.
Old 09-03-19, 07:15 AM
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There are many ways to do this and this is my approach. Install slab 6 inches above grade. Make sure ground slopes away from slab on all 4 sides. Use pressure treated wood for sole plates. Apply large amounts of non-hardening caulk to bottom of sole plates when installing walls frames After wall frames are up, install 6 inches minimum aluminum coil stock to walls around perimeter (3 inches below top of slab). Install wall sheathing and finish (2 inches below top of slab). Bend exposed 1 inch aluminum out to 45 degrees so rain running down the wall finish is directed away from wall. Build roof with at 6 inches minimum of overhang on all 4 sides. Install gutters.

Last edited by beelzebob; 09-03-19 at 07:17 AM. Reason: deleted
Old 09-03-19, 07:25 AM
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If your slab is on grade, put a row of concrete blocks around the perimeter so that your bottom plate will be 8" above grade. Anchor the blocks with rebar 4' on center (and at each corner) and slug with concrete. Put anchor bolts in the wet concrete to anchor your bottom plate. When you put on sheathing, let the sheathing hang over the blocks 1". This ensures your siding will be 6" above grade as it should be.
Old 09-03-19, 07:46 AM
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So I'm in the process of building a shed on an existing slab that was used for a swim spa.

What I did was similar to what XS stated but I used 4x6x18 solid and hollow blocks to get the sill plate above the slab but I had to use anchor bolts since slab was existing.

The other advantage is that if you have grass around the shed then you wont be beating your siding with the string trimmer!
Old 09-03-19, 01:22 PM
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If your slab is on grade, put a row of concrete blocks around the perimeter so that your bottom plate will be 8" above grade.
I'm going to get A LOT of snow with runoff, so this sounds like the idea I'm hunting for. Kind of like having concrete waders for the legs of the house, then lay the siding over that.

This shed does not have to look nice. It can look like a man cave, ice fishing hut etc but just has to hold up through the winter.

After talking a good deal about this we're even considering building the entire thing (except roofing) out of cinder/concrete, but I'm still learning more about that process and the pros/cons (so don't be surprised if you see questions about that)

Thanks everybody!

Last edited by soupking; 09-03-19 at 02:03 PM.

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