I'm looking to create a flat area inside a sloped, concrete basement. I'm thinking of using wood to create concrete forms 16" apart. One end of each concrete form would be at the wall (depth near 0) and the other would be several feet out (depth ~6 inches). The idea is that gravity would level the concrete. I could then put 2x4s perpendicular to the concrete forms and put plywood on top of that for a level surface.

I would like to keep the weight of concrete, cost, setting time and mixing procedure reasonable. Any recommendations on what kind of concrete to use? Self-leveling? Lightweight? Some combination? I'm also wondering if 16" seems correct.

 

Welcome to the forums! Why use concrete if you are flooring it? Just use framing lumber cut to fit and make it level, frame it like a floor and install Advantech subflooring. How much out of level is this floor?

 

Any wood touching concrete should be pressure treated to help avoid rot.

 

Yikes! It's hard to imagine a basement with a 6 INCH difference from the wall to an area into the room. I HOPE the bad area is relatively small? If that's true, you can thoroughly clean it (vacuum then mop and let thoroughly dry out), then put down a coating of something like redgard. After that cures, pour SLC (self leveling cement) and you'll end up with a perfectly flat and level floor area. The membrane is necessary to ensure adhesion. SLC won't stick to concrete (just like concrete doesn't stick to concrete).

Using SLC over a large area could get pretty expensive. Concrete isn't perhaps not an ideal application, as it usually can't be feathered-out to a very thin edge and needs (if I recall) at least 1.5" or more of depth.

If you have decent carpentry skills, you can skip the SLC etc steps. A laser level is pretty essential and will cost perhaps $80-100. Then use pressure treated 2x4s - ripped down to create your flat level surface. Then install Advantech (spaceage plywood that is impervious to moisture - won't warp or rot) on top of that.

It is the SIMPLEST solution - IMO. The downside is one must manually rip a lot of lumber to create the flat/level structure. If you aren't great with carpentry, it could be well worth hiring a good carpenter for one day (figure $250-350) to help make this happen.

 

The basement gets wet from time to time. My current solution involves wood directly on the sloped concrete, but I know that the wood will eventually end up in standing water, and will have problems even if it is pressure treated.

Hence the concrete.

 

I do think that using SLC for this large an area would be too expensive. Hence the idea of concrete forms a couple of inches wide and 16" apart that the "floor" can sit on top of.

 

It would have been useful if you told a more complete story from the start. The "basement wet" thing is a different issue and should be fixed. Often, the culprit is one (or both) of these:

1. Downspouts come down directly to too close to the foundation walls (solution - extend the bottom pieces far enough to carry the water to where the ground will continue the job on its own. We had one house with this exact issue and I fixed it with some 4" outside PVC, downspout-to-pvc connectors (from HD), a shovel and some elbow grease. Figure $100 or less and your time/effort.
2. Soil near foundation walls slopes toward the house, not away. Usually (not always) there is room to add topsoil to fix this.

IMO, until this is addressed, I wouldn't bother creating a floor down there. You'll just end up with other issues and problems.

The only other thought is you could create a small well (lowest point of basement) and put a sump pump into it - where all the water gets caught and pumped out. But that isn't really fixing the root cause.

 

I agree. "wet" is a condition that precludes the use of wood, even pressure treated.

 

I appreciate the detailed analysis, but I already know why the basement occasionally becomes wet and that fixing the root cause is not feasible at this juncture. I don't live in the world where you always get to do the best possible thing.

I'm only hoping for an understanding of what type of concrete might work for my plan.