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shed on concrete slab

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Aug 11, 2013, 12:52 PM
shed on concrete slab
thanks in advance for your help.
I am building a 12x10 shed on a existing concrete that is 6" deep and am going to build a floor using pressure treated wood. I have a lot of concrete and the shed will be surrounded by concrete that slopes toward the shed.
Since the concrete slopes a little i was planning on shimming the bottom end to bring the floor level but i am worried about the water running towards the shed from the sloped concrete above it. How can i keep the water away. One side of the shed would be running perpendicular to the slope. I could angle it slightly but did not prefer to.
Aug 11, 2013, 01:35 PM
Why are you putting in a slab and putting in a wooden floor? That seems a bit redundant for a shed. If you are planning to use this shed for a workshop, I might look at insulating the slab before I poured it and adding some sort of radiant heat into the slab for comfort. Otherwise if this is purely a shed for storing yard equipment I'd forego the wooden flooring on top of the slab.

You don't list your location, or how high you plan on making the shed, you could get by with just a floating slab instead of footers provided it isn't attached to the house.

If it were me, I'd install a concrete curb around the slab to get my sill plate and bottom of my siding material away from the ground. I would of course use a pressure treated sill plate, and for siding I would likely use Hardi-plank or Hardi-panel type fiber cement siding.

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Any advice given here is general in nature and is not necessarily valid for your given area. If in doubt check with your local codes enforcement department for what is required when doing electrical, plumbing or structural work on your house. Permits may or may not be required in your area and home owners may not be able to DIY some tasks. I have no way of knowing if you have the skills needed to complete the tasks you are asking about, when in doubt seek professional assistance.

My advice may be worth exactly what you pay me for it. :-) For the record I did not stay in a Holiday Inn Express last night.
Aug 11, 2013, 01:53 PM
the concrete is already there. it is about 30' x 60'. it used to be an area for cars to park. I wanted to make the siding stucco, to match my house. the concrete is sloped away from my house but the shed will sit on the low side of the concrete so I am worried about water on the high side. Even if I don't put in flooring how can I keep the water from coming underneath?
Aug 11, 2013, 04:12 PM
Post a picture so we do not have to guess what it looks like please.
A slab on grade is never a good thing.
Siding needs to be at least 6" above grade.
Adding at least one row of Concrete blocks to build the walls on is the way I would go to start with.

Aug 11, 2013, 06:22 PM
Nothing short of raising the shed above the possible water level will keep the water out. There is no seal that would last very long under those conditions, nor is the shed lower end itself anywhere near waterproof enough to repel any depth of water.

The only way to keep the shed at a low level without leaking would be to cut the concrete around and slightly under the shed edge to create a moat. However, this is an expensive project and frankly, just not worth destroying the concrete pad for a shed. Also, in time it could clog and flood anyway.

You can build a pressure treated wood frame and floor that sits on the sloped concrete. You will have to custom make your joists to conform to the existing slope so that the shed floor will be level. How high you make this floor will depend on how much water you expect. While a wood floor like this will allow water to flow under the shed and on top of the slab, it will also cause a lot of moisture to get into the shed.

Aug 11, 2013, 11:49 PM
thanks to everyone. I will try and post a pic tomorrow after work. I live in San Diego so there is not a lot of water but I want to do it right.
Aug 12, 2013, 10:30 AM
I would drill holes, drive rebar (set some test blocks for measurements so you don't have to cut it all off Wink ) and epoxy it in, then set two courses of block in mortar, at the bottom, set one or two blocks on their side on each face of the block wall, and Tapcon in some cane aluminum metal sheet so you have airflow and a place for any drainaga/seepage water to exit. the cane metal will keep critters out. cement in a few J-bolts to fasten the treated plate onto. then drill your plates to fit over the bolts, shoot runs of polyurethane caulk, drop the plates, and bolt 'em. build up from there.

now you have a way for water to exit, the shed is above it, if a wind comes along and blows all the other little piggies' houses down the shed will not move, you can insulate the floor, and have a nice little shed. run some power in, it's going to be nice. if only they had built my shed/hamshack that nicely, it would have saved a lot of summer weekends rebuilding things (code doesn't allow that big a one any more where I live, but I can fix the rot.)

takes longer, you have to chalk outlines and sketch plans and all that stuff, but done right, you only have to build it once. and the bragging rights are worth at least 4 cylinders on your "cylinder rating."

This message has been edited. Last edited by: swschrad,

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