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Old 07-22-2009, 02:40 PM   #1
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small shed on larger concrete slab


I'm building a 15x15' shed on an existing 15x20' concrete slab and am concerned about moisture wicking on the one side adjacent to the 5' open part of the slab. I plan to use PT sill plate with felt/membrane underneath, and to caulk the perimeter. I will install flashing on the 3 walls which run along the edge of the pad, however I can't see how to install useful flashing on the 4th wall adjacent to the open area.

The slab is level which makes a great opportunity for water to pool adjacent to the wall.

Any suggestions on what I else I should be doing for moisture proofing this?
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Old 07-22-2009, 04:12 PM   #2
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small shed on larger concrete slab


Don't use felt under the bottom plate, use foam sillseal material.

Use 6mil poly under the slab. If the slab isn't poured to shed water (and pooling occurs) there's just not too much you can do.
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Old 07-22-2009, 05:33 PM   #3
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small shed on larger concrete slab


Why not use the cement as the floor?
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Old 07-22-2009, 06:12 PM   #4
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small shed on larger concrete slab


I am using the concrete slab as the floor. The shed however will only cover a portion of the slab, meaning that a small part of the slab will be exposed to rain/snow and potential wicking under that one wall's sill plate.
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Old 07-22-2009, 06:34 PM   #5
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small shed on larger concrete slab


I think the foam sill seal KC mentioned will work
Drill & bolt the sill plate down

Any reason not to just use the whole slab & make it bigger?
I had a 10x12 & added on a 10x10 addition
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Old 07-23-2009, 06:34 AM   #6
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small shed on larger concrete slab


Sill sealer will work. I assume your door will also be along this "porch", so you'll need to use a threshold under the door and seal that to the slab. The jamb will need to be protected to prevent snow melt and water from reaching it, and wicking into it from below.
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Old 07-23-2009, 06:57 AM   #7
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Another thought
Extend the roof over the pad as a "porch"
Our friends hed is like this - to cover their pool filter from direct snow & rain

It won't stop all the rain etc, but will help
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Old 07-23-2009, 07:37 AM   #8
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Well, really all those ways aren't really the best way to deal with the situation. Eventually you are going to have problems. You'd do well to block up around the perimeter and build on top of the course of block. You will still need to lay sill sealer and pt but you are going to eliminate a ton of problems.
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Old 07-23-2009, 11:00 AM   #9
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Quote:
Originally Posted by jaros bros. View Post
Well, really all those ways aren't really the best way to deal with the situation. Eventually you are going to have problems. You'd do well to block up around the perimeter and build on top of the course of block. You will still need to lay sill sealer and pt but you are going to eliminate a ton of problems.
Agreed about the course of block, but you may get away with a half course (4" high) of either 4 or 6" block. Half courses are easier to lay IMO, especially for a DIY'er.
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Old 07-23-2009, 11:19 AM   #10
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small shed on larger concrete slab


Scuba_Dave - "the boss" won't let me make it as big as the whole pad, and although I tried the porch option out on her she shot that down too. I'd love to just build it full size but she has me out-voted, sadly.

jaros bros & jomama45 - I had considered this also but had a couple of other thoughts - what do you think of these instead of blocking:

1) Drill & pin the existing base and pour a 6"x6" concrete footing the shape of the shed

2) Saw the concrete about 2" depth where the one exposed wall of the shed will be, use sill seal and PT for the sill, but the install flashing which sits down in the new saw cut

I appreciate all your thoughts - I'm looking for a 30-40 year lifespan, not something which will give me problems in 10 years.
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Old 07-23-2009, 03:52 PM   #11
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small shed on larger concrete slab


Cut the concrete all the way through and get a slice of concrete out of there OR just cut it all the way through and let the water in there.
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