Concrete – When pouring cement next to a home, how far below the mud line on a stucco home do you need to be to avoid water issues

Well this is a pretty big deal because we don't know the cause.

First let's go over common reasons we get cracks on new floors.

1. Soil wasn't properly compacted. Soil should be compacted with a rock bed on top.
2. Bad mixing at site. Especially in the summer contractors pump too much water in the mix. The water makes the concrete weaker and it does crack easier.
3. Bad mix at plant. Just a poor mix of materials coming in.
4. Bad environment for mix. Too hot, rain, too long to site, whatever.
5. They didn't cut the concrete or didn't cut it quick enough. The concrete needs to be cut (I suggest every 10x10 foot area) to allow for expansion. It really needs to be cut the first day.
6. Didn't use rebar. This is a common cost cutter and I have seen rebar pulled before pour.
7. House was built on unstable land. Could have been a landfill, could have springs underneath, whatever.
8. Soil type wasn't conducive to their install method. Really too many things to list here but basically different types of soil expand and contract more due to moisture levels. Also it is possible that it was pushed even further by either a lot of rain or a drought.

Those are what I got off the top of my head. I am positive there are more reasons for cracks in your basement. If I were to totally guess I would say they didn't prep right in that corner - It seems like the corner is dropping on you already and maybe it wasn't floor wasn't compacted correctly. The crack starts on the outward "L corner" which is normal but heads right across the corner - most insignificant cracks in concrete tend to be straight. This crack has its shape for a reason.

What do I think you should do? First ask questions. See if the contractor knows why it cracked. "It happens" is not an answer. My reply would be "Me not paying you happens too." If the contractor/builder doesn't want to take blame (it is their fault) then you need to get a psi reading on the floor, measure its thickness, and also prove that the floor is rebarred/reinforced correctly. You will almost certainly need an engineer. Even if the problem is obvious your builder is less likely to move until it is proved from a creditable source.

You cannot sit on this. If the mix was bad - well did they use the same mix for the walls too? If it was bad prep, what happens when you have issues in a couple years? I have seen basements that have had to be repoured and it is a mess. Also will set you back 10K in the states.

Again, no matter what anyone tells you - cracks in a fresh floor are not acceptable. This is a sizable crack. Even if an engineer checks everything out I would demand a longer warranty on the basement floor and to have them seal it. Either that or they start over.

Note (based on additional pictures: I am not there so it is a bit of conjecture. But it is not a good sign when an area is relatively dry and you have a corner that is not only wet but it seems dropped too. It is a guessing game for me too at this point, not seeing what you have going on. My top 2 guess (and they are guesses) -

#1 - They dug and compacted. While digging they left over dug out that corner and then filled it. Then they never compacted it correctly. After it rained a few times, the corner started dropping. Also when I have a slab compacted the rock is part of the equation. I don't see any rock in your pictures.

#2 - You have some sort of natural underground current or spring. I have a stream that is 3 feet under, about a foot from my house - and I can't imagine if it where a foot over and 2 more feet down.

(Do you have any pictures right before they poured?)

Typically when I pour concrete against a house foundation, (never pour concrete against any siding IMHO, it invites trouble later) I have an expansion joint set to the house. It can be cork, 1/2" thick by 4" tall set to the wall with short pins to hold it while pouring. There is a tar or asphalt impregnated expansion joint material too that may hold a straight line better with fewer fasteners.

The 1/8"/ft is a good number as mentioned by Jimmy fix-it. To set the slope, a form is set in place that sets the size (area) and the top level of the concrete surface. The form is also set at the recommended slope. 2X4s are usually used for this, then they are removed after a day or so. You will also need a concrete edger to ease the corner where the 2X forms are.

Usually pro concrete workers would not set anything within the 10 width, they don't need it, but you can set something at the middle if you choose, since an 8' screed board is a killer to pull if you don't have good help. It can either be a short piece of rebar driven in so the top is at the level for the concrete, strike your concrete in that area to the level dictated by the rebar, after the surface is set, drive the rebar further into the concrete so it is well below the surface. It works but it is easy to loose track of this pin while everything else is going on.

The other way to do the same is set a wood stake in the place you wish to use to set the top of the concrete, making sure the stake is tall enough to be well above the concrete finished surface, say anywhere between 6" to 1' above, (it can be more, but shorter would make the removal harder), set a screw or nail at the level of the concrete and when the level of concrete is set to the nail or screw, pull the stake out and plop a small amount of concrete while you are pouring to fill the void left by the removed stake. Yes you will need boots, and get in the middle of it.

When placing the concrete the edges are floated very close to the finished level, all around the edges. The pour will start from the farthest side, and the pour will proceed closer to the truck. Set the level at the house using a hand float to set the level to the expansion joint and work the far side and start going down the long side, striking the concrete off to the finished level with the screed. You will still need to float it perhaps 2 more times after that if you are going with a broom finish.

Get the rest of the concrete down, and if you are still alive, start the floating process. Use a "bull float" to get most of the surface closed up and the grout brought up, this is critical to get this right the first time!!! Setting the level properly with the screed will really make a difference too. One more time with a hand float after an hour or so should make it ready for the broom finish.

On a slab that is 50" long you will need to run control joints or cut them in later at every 10' or so. I think it is recommended to have expansion joint every 20' but it may not be required. Again, control joints would be highly recommended..