I have a 22’ x 18’ room that is split by a crack on the long side. I have seen worse cracks, but it is bad enough to need patching to prevent moisture from coming up. The floor crowns some, but it is only visible when I lay out my straight edge. On the South side it will require some patch but is fairly flat all the way to the wall. On the north side the subfloor drops almost a full inch in the 10’ from the crack. I estimate that I will need about 5 cubic feet of Mapai to fill it. That is pricey stuff in that quantity. Is there a filler I can use, like concrete board or Portland w/sand that I can top with Mapai to cut back on the costs? I am weeks out from laying the floors as I have drywall and painting to do ahead of the floors so I have time to cure.

I could reduce the fill volume by grinding the crack down about 3/16, but I don't think that mess is worht it when I can do it with patch. Thoughts?

Thanks,

--David

It would be cheaper to grind the high place down somewhat than fill ALL the lower places. Concrete grinders can sometimes be rented. Dusty work so use one with a vacuum system.

Ya, even grinding down that hump by 25 % will greatly reduce the amount of leveler you need.

I sometimes use a top coating concrete t fill most of a large area then just skim coat it with a leveler. make sure you get something without gravel.

Thanks all.

I will look into grinding, but I think I will still have a large space to fill.

If use a top coat concrete do I have extend the cure time before I lay the floor?


Thanks,

--David

The cement does seem to take longer to cure depending on how thick is has to be. You don't have to wait a long time just until it is fully dry.

floor-flatness-t7375-15.html


I posted some pictures of a floor prepping I did years ago.

Grind and use a self leveling compound. I also have a dust free grinding system that utilizes a diamond cup wheel. Eats that concrete up. Then use the manufactures sealer, making certain all is compatable with your adhesive manufacturer. The moisture will come up through the patch out of that crack.

Hello All,

Thanks for your comments.

I made a drawing I was hopping you could look at.

Griding only seems to help if you have a 'hump' or rise. All lows need to be filled and grinding should not be used just to 'spread out' the area that is not flat. Is this correct?

Jump to the link for the drawing of my subfloor:

http://www.oneplaceleft.com/micc/Floor.jpg

Thoughts?


Thanks,

--David

Based on your drawing, it appears your slab moved and cracked. This is typically caused by poor earth compaction during construction or the earth under the slab being washed out by water. Before flattening your slab, consider looking around outside to see if you have a drainage problem. On slab construction, it is very important to have all surface water drain away from the building. This is true for raised wood foundations as well.

As I see it, you only have one option. To keep adding a cementious leveler mix until your slab is flat. You can do it in layers if you wish. It's a daunting task for the inexperienced.

Hello,

Thanks for the note. Your advice is certainly sound, but I failed to mention that my house is in Phoenix and I am miles away from natural water. The slab problems are likely due to expansive soil settlment which is pretty common out here.

I think your right about the fix. One question that comes from this are their practical or aesthetic complications that come from raising the floor on one side of the room almost one inch?

Thanks,

--David

Without looking at it it sounds like your crack is ledged quite a bit. You need to call Ken Frango in Phoenix if you want a perfect substrate. This sounds like it is not for the DIY'er. I can give you his number if you want.

In looking at your drawing, I think you COULD tackle this yourself. It will take some work and time, but does not look too complex. Build up the area at the wall first, about 1/4 inch depth and 2 feet wide, and continue this process until you have reached the point at the crack. Looks to be about four applications should do it, and many bags of compound.

I have seen Ken Frango's site and he certianly looks like he knows what he is doing. I am not prepared to call uncle yet on this as a DIY project just yet, but I will keep him in mind. I might even see if he would be interested in provideing some advice/direction based on the problem for a a comensurate fee.

Any advice on what Mapei product would be best for this? They have at least a half a dozen products that look like they fit, but not enough detail on their site to make a choice.

I had orginally planned to build up the larger void with regular quickcrete or concrete and then use Mapei to smooth out the last 1/2 inch or so. This is based on how pricey Mapei appears to be off the shelf at home improvement stores. However, I read that this can cause moisture problems if the path does not dry for some time (weeks / months) I plan to lay floor (floating) in two weeks.

Thanks,

--David

All,

Just a note to thank you for your help and advice in this project. I have managed to flatten my floor, and now move onto the rest of the house where I don’t have nearly the issues that this room had.

The problem was actually different than I originally described. The slab did not entirely ledge, but dipped and returned back up quite a ways to level toward the foundation perimeter. There also was a dip on the other side of the crack that had to be filled first. I then used my screed, screed dam, and multiple pours to extend that flatness out to the ledged part of the room.

I ground the hump with a floor grinder rental from Home Depot. What a dust machine...the neighbors thought the house was on fire.

The grinding went a long way to a flat surface but I still had huge, though not nearly as deep as I feared, imperfections to fill. On one side there was a void about 1/4" deep that covered about 180 sq feet including feathering out to flat. On the ledged side, I might have had to fill as much as 1/2"-5/8" in an area of about 100 sq feet. 13 bags of patch later the floor can be called "perfect" if you allow me a view spots that are off by about 1/8".

I put a good deal of effort to clean old paint and drywall from the surface. It almost looked like the drywall guys textured the floor then scrapped it up leaving a lot of surface covered with a think layer of that the junk. I hope that time was not wasted--cost me about 4 hours.

On a final note, I found a used 10' concrete screed on Craig's list for $60. This has been the best tool in that it allowed me to see the floor clearly and to pull the long screeds. My previous straight edge was not as straight or as rigid as I thought.

@FloormeinTucson,

You are right and this is really not a good DIY project. It took a few hail Mary's, grounded in common sense, but if I did not take the time I did at it, it could have easily meant disaster. I have now have a new found appreciation for the flooring profession and understand why I have seen sample estimates that gave a higher price on floor preparation than the actually installation.

Thanks,

--David