I'm almost done building a 10'x12' shed on a 20' x 20' x 6" old slab that was already in the backyard.
As I expected, I have water coming under the bottom plate on two sides. Most of it comes from the right side (as you're looking at it from the front door). And a little is coming under the front door. Note: I left the PT bottom plate across the front door because I expected some need to protect from water.

The shed has PT bottom plates and the "Smart Siding" stops just at that plate. I'm looking for recommendations to seal between the uneven concrete (probably gaps of 1/2" at the lowest part of the concrete) and the bottom plate. I have a French drain around part of the slab and just a rock-filled trench on the other three sides.

Some ideas I have:
1) Some sort of concrete caulk to fill the gaps along the seam between the board and the slab.

2) A bottom piece of trim, cut as close as possible to the contour to the slab and flashed and sealed. I could use a Trex board or a PT that would basically be a sacrificial board and replace it every few years if it rots.

3) Hydraulic cement rolled up and pushed into that seam.

4) Some sort of flexible rubber (or other synthetic) material that I can put along the side to follow the contour. My DIY version of this would start with a sand-filled rubber hose to make it heavy enough to fit in the countour.

5) Patch up the concrete and try to grade it away. (This would be the hardest for me as a DIY because I really don't have any skill with concrete.)

Combinations of all of the above? The wall on the side is much more of an issue than the front wall.

For what it's worth: yes, I'm aware that building a shed on a slab larger than the shed footprint was ill-advised. But budget, time, and future plans to make use of the rest of the slab drove me to do what I did. So here we are...

 

No problem with that!

The best solution would have been to lay a course of block around the perimeter of the walls, then apply your red head bolts through the sill plate and into the slab.

You have anchored the shed to the slab?

Honestly there is not a lot of post fixes that are going to do a lot of good if the slope of the slab is directing water to the shed.

One option would be to lift the shed, doesn't look too heavy, up and slide the solid blocks (you can get some that are 2" thick) under the wall. You could even install a composite board but if lifting Id just do the block and be done with it!

 

I would raise the shed. Anything you try to do to keep the water out will eventually fail. Best would be to pour a 4" thick concrete slab on top of the existing pad to keep the shed and its contents out of the water. Or, you can pour a new slab in another location for the shed

 

Thanks for the responses, guys. How would I go about raising the shed? This sounds really difficult. The 2" block sounds like something I could do, assuming I could figure out how to raise the shed up. It's 10' x 12' x 10' tall in the front.

 

A bottle jack or two and a couple 2x4s under the subfascia. Slip a scrap 2x4 in as soon as you have room. Then another when you have room for another. Keep stacking them until you are high enough. Block it up higher than needed temporarily then let it back down when the blocks are in place. Then go to the back side and repeat. Should take all of about 15 minutes.

If you block the sides you will be able to lay your concrete blocks in mortar on the front and back. Once they cure, move your temporary blocks to the front and back walls so you can lay your concrete blocks on the sides. Be sure you line the blocks up with your bottom plate, you don't want them sticking out. You'd want to flash over the concrete blocks after you set the shed back down and anchor it.

And you might want to put some z flashing between your upper course of siding and the lower panels.

 

When we installed, and sold, our 2200#, 8x16, swim spa all it took was a long crow bar and a bunch of 1/4" and 2x4 shims to raise it up 1/4" at a time going around the perimeter of the base. Once we had it 8" off the ground the forks for the skid loader could get under it.

 

Keep in mind that 2" block around the perimeter will raise the shed up out of the water but the floor inside the shed will still probably get wet. So, make sure to keep anything that could be damaged off of the concrete/floor.

I have lifted much bigger structures by working like an ancient Egyptian. Jack one side several inches and install blocks to hold it up. Then move to the other side and jack & add blocks. Keep repeating until you get it as high as you want.

 

Good point.

If the shed is sitting in a puddle even with mortar and solid blocks you will get water/dampness inside, that material is not fully waterproof.

My little pool/tiki shed is on blocks with a small extension on the West side for the door. If I get a driving rain I can get a damp spot inside so be aware that the block option will not be a water tight solution.

It will at least stop the bottom of the shed from rotting away which it will even with PT wood, eventually!

 

What about cutting into the concrete and around the two problem sides and putting a channel drain in? We do have a dry well about 3 feet away from the back of the slab that we could easily run this into.

Something like this: https://www.homedepot.com/p/NDS-4-in...0RTL/205938053

I think the slab is about 6 inches thick.

 

I've had pretty good success with just cutting a kerf into the concrete in problem areas like that. The kerf cut only needs to be about 3/8" deep so would accept the front lip of a 1 1/2" wide z flashing. But the cut does have to be arrow straight. Set the flashing in a heavy bead of concrete sealant then run a second bead along the front edge with a self levelling crack sealant.

I really think jacking it up then a row of 4" concrete blocks mortared down to the pad is the best solution.

 

Thanks, XSleeper. I am getting the sense that the (overwhelming) consensus is that is the best plan.

In the meantime, any recommendations to stop the bleeding until I can get organized enough to do that. Specifically, I'm looking for a recommended concrete caulk-like product that I can just put a thick bead along the seam between the bottom plate and the slab. I just want to run that along there on the next dry day.

(I know it sounds minor to a lot of you guys, but it's not to me -- and I'm in the middle of a basement bathroom build, time, money, kids' schedule, etc.... anyway I won't be able to do it right away.)

 

Any concrete sealant would be better than nothing. PL makes one that is sold in the box stores that is a concrete polyurethane.S10.

But keep in mind your siding isnt going to last long being that close to the concrete. LP Smartside recommends 6" above grade.