Shed on Existing Concrete Slab. Waterproofing?

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Old 03-24-08, 02:14 AM
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Shed on Existing Concrete Slab. Waterproofing?



I've ordered a pre-cut 9' x 13' cedar shed that I will have to assemble. I already have a nice, flat, slightly raised (~ 4-5") concrete pad in place (it was put in by a previous owner). The pad measures 10' x 14'. I don't know how deep the concrete pad is, but it looks well done and substantial. [Why not buy a 10' x 14' shed, you may ask? Permits. Under 120sq.ft. I don't need a permit.]

I plan to use strike anchors to attach the bottom plates of the shed to the concrete pad. There is no floor for the shed; the concrete slab will be the floor. The bottom plates (sills) will be pressure-treated wood.

I plan to align two walls with the edges of the concrete pad, and set back the remaining two walls 1' from the edges of the concrete pad (see picture).

I realize that on the two walls that won't be aligned with the concrete pad that I have to be careful of water intrusion inside the shed. I was wondering what steps I need to take in order to "waterproof" the bottom plates where they attach to the pad.

Some advice I've already received from handymen is that no sill seal will prevent water intrusion. The slab is practically flat, but unfortunately it may be angled toward the shed by maybe 1-1/2mm for every 48 inches (I used a 48" level). That's not much.

So far, the feedback from others has been discouraging. Can you help me determine what is the best approach to "waterproofing" the bottom plates where they meet the cement pad? Thanks very much.
 
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Old 03-24-08, 04:50 AM
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Have you considered laying 1 coarse of block/brick?
 
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Old 03-24-08, 05:01 AM
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Originally Posted by marksr View Post
Have you considered laying 1 coarse of block/brick?
I'm not sure what you mean. Do you mean laying one course of brick along the bottom edge of the installed shed (and maybe using some flashing to deflect water), or laying one course of brick to set the shed on? Maybe something else?
 
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Old 03-24-08, 05:14 AM
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Set the brick under the perimeter of the shed [except for door] A course of brick and mortar should keep the water out.
 
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Old 03-24-08, 05:19 AM
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Interesting. Thanks for that tip. I'll check out how this might work with my shed.
 
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Old 03-24-08, 05:26 AM
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Good morning,

Could I ask a couple of questions? (Oh, you didn't answer me, so I'll ask anyway!)

If the foundation angles slightly toward the shed from the side, why wouldn't you just offset it to the opposite side?

Why do you not want to get a permit?

Raising the shed on a course of bricks is a good idea for keeping the shed walls from water standing at the bottom, but won't the water just puddle on the perimeter of the pad?

Are you going to need a ramp to move things in and out of the shed? It will not be a smooth transition from ground to pad to raised shed.
 

Last edited by connie; 03-24-08 at 05:27 AM. Reason: Poor grammer!
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Old 03-24-08, 05:31 AM
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Originally Posted by connie View Post
Good morning,

Could I ask a couple of questions? (Oh, you didn't answer me, so I'll ask anyway!)

If the foundation angles slightly toward the shed from the side, why wouldn't you just offset it to the opposite side?

Why do you not want to get a permit?

Raising the shed on a course of bricks is a good idea for keeping the shed walls from water standing at the bottom, but won't the water just puddle on the perimeter of the pad?

Are you going to need a ramp to move things in and out of the shed? It will not be a smooth transition from ground to pad to raised shed.
The slab seems to sink in the middle, of all things. Only about a mm or so, but it's going the wrong way.

Me and the inspectors don't get along.

I've scoped out a ramp. I'll probably get one after the shed is built.
 
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Old 03-24-08, 05:36 AM
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Well, if it drains to the center, Mark's idea is best. (Oh, I just had to say that yesterday, too!)

Okay, sorry I couldn't be more helpful. Good luck to you.
 
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Old 03-24-08, 09:55 AM
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Bricks or no bricks, if your slab "sinks in the middle" , water will accumulate, puddle and flow toward the path of least resistance. The door opening provides the path.

The brick idea might work, if you moved the shed forward, leaving the exposed slab toward the back instead of the front.
 
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Old 03-24-08, 09:10 PM
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Thanks to everyone for helping. I appreciate it.
 
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Old 03-25-08, 12:44 PM
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Good question

I ma struggling with the same question. I have a perfectly good 10x10 slab with a old metal shed that will be torn down. I am in the process of building the floor right now. Ive talked to many different shed sales people they all suggest putting a wood floor on top of a 2x4 frame and that on top of skids. All this keeps the wood off the ground so you do not have worry about the water proofing. But this makes you have a ramp.

I am struggling with what to do also.http://forum.doityourself.com/images...s/confused.gif

Thanks
Tim
 
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Old 03-25-08, 12:48 PM
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Welcome to the forums Tim!

Any special reason you want to use a wood floor over the concrete slab?
 
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Old 03-25-08, 02:54 PM
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I have used concrete - not bricks - in this situation. Stronger, better seal, cheaper, any dimensions you like... and easier.

With the first shed, concrete is necessary on two sides only. So, plan to build up the other two on, say, 2x4s. Therefore we need to form concrete 3-1/2" wide, and 1-1/2" thick. A few bags of redi-mix, and really simple formwork. I would clean and roughen the original concrete as much as possible.

You have an opportunity here to bolt the shed down.
 
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Old 03-25-08, 03:16 PM
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Originally Posted by Kobuchi View Post
I have used concrete - not bricks - in this situation. Stronger, better seal, cheaper, any dimensions you like... and easier.
How well does concrete adhere to concrete? The pad is old and I will be cleaning it, but it seems like any concrete form(s) I place on top would separate over time. Is there a trick to it?
 
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Old 03-25-08, 05:27 PM
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I was also concerned about water sitting on the slab. And the wood soaking up the water,
If Im not mistaken a house that sits on a slab goes all the way to the edges of the slab. Thus water would go into the ground and not on the slab. My slab is not exactly 10 x 10.

I did think about caulking the treated bottom plate if I used the concrete as a floor. Im to lazy to pour a wall to put the shed on.

Im just so confused.
Tim
 
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Old 03-25-08, 06:03 PM
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Originally Posted by Turbonium View Post
How well does concrete adhere to concrete? The pad is old and I will be cleaning it, but it seems like any concrete form(s) I place on top would separate over time.
If it gets wet then frozen a seeping crack may develop. But this is true of any material.

Besides nature, you could pop the new concrete off by hitting with a sledgehammer.


Originally Posted by Turbonium View Post
Is there a trick to it?
Yes: tooth. The old concrete should be "toothed" as much as possible. This means giving it a texture the new can bond to mechanically. Gouge it with a masonry bit or cold chisel, grinder, etc. Even chip up the old surface with a masonry nail.

If you want to make this crazy strong then drill and pin with galvanized nails, etc. + anchoring cement.

Anyway, why would it move anywhere? The weight of shed + friction between the old and new concrete will keep it fast until earthquake hits.


BTW a flashing all around that emerges from the bottom siding and overlaps the (foundation) sides will be very effective here.
 
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Old 03-25-08, 06:16 PM
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Do you think it would be better to cast my own short walls or concrete 'planks' in sections and adhere them to the pad with some kind of adhesive? I've seen references to very strong concrete adhesives on the net.

In this case, rather than setting wet concrete forms directly on the pad, I might be able to cast 8 or more sections of concrete 2" x 4"s, if you will, and then adhere them to the pad. What do you think?
 
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