Uneven concrete floor - how to even it up?

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  #1  
Old 01-09-12, 08:48 AM
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Uneven concrete floor - how to even it up?

We recently decided to put down a wood floor in our formal lounge area, but when I pulled up the old carpet and went around the concrete I realized it was very uneven and nowhere near the tolerances we needed for the floor.

We got in a concrete guy on recommendation of a neighbor who uses contractors a lot, but the guy royally screwed up the job and made the floor worse. There's no way we would get the guy back to compound the problem, so it just comes down to trying to get our money back as far as he's concerned. Problem is we still have to fix up the floor, and we can't afford even more from our rehab fund to get someone else in.

The room is 20 x 14 and has a slight drop to one side of the room which makes it nigh on impossible to use self-leveling compound under normal circumstance to try and work on the uneven areas, as gravity will make the SLC bubble level but at odds with the sloping floor. Further, some of the affected spots are so uneven and broad that it would be a nightmare trying to screed normal cement over the troubled areas. I've ground down some of the high spots, but it still doesn't help.

The only way I can see myself getting out of this is to try and use SLC on the entire floor, but with an area of 280 square feet there's no way I think I could mix enough to do it all in one hit, because the stuff dries so fast. If I managed a layer with an average thickness of 1/4 inch I'd need 6 cubic feet of SLC - if it worked out to 1/2 an inch average thickness it would require 12 cubic feet - I think it would probably work out somewhere in between and require about 8 - 10 50lb of SLC! (Expensive I know, but I can't see how else to do this!)

I've read some real horror stories on SLC drying too fast and being unworkable over large areas, so I figured what I could do was to divide the room up into 6 quadrants of about 40 square feet, then use some timber to set up a mold to pour into. Then as each section dries and I move the frame along, I use the edge of the previous section as the 4th side of the frame/mold so that I can find the appropriate level for the new batch.

If any of this makes sense to people I'd appreciate some feedback, as I'm really stressing over hot to fix this, is we have some very heavy duty deadlines hanging over us from the bank to get our house completed.
 
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  #2  
Old 01-09-12, 10:43 AM
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You are correct. Self leveling compound almost requires a professional when doing a larger area. I've run into similar issues with epoxy floors. There is just no way to get it all mixed and down in time. For a medium sized area you could rent a mixer and hire some help but you really need to work like a well oiled machine which is rather risky unless you do it all the time. Maybe you compromise your idea of partitioning. Segment the room but do it in two or three sections instead of 6 and get some help. Worst case you could buy a cheap mixer and have one person do nothing but mix.
 
  #3  
Old 01-09-12, 11:27 AM
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You will likely need 11 bags for 1/4" or 22 bags for 1/2" thickness. If you decide to tackle this yourself, buy a couple of extra bags as the last thing you want to do is run short. You can always return them if you dont use them.

22 bags is a lot of slc. Besides the cost, slc takes some skills, a couple of helpers, and time is of the essence. This may better be a job for a professional, someone who has done this before.
 
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Old 01-09-12, 01:22 PM
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You didn't have the right contractor the first time out but this might be a good job to try another one.

I'd get a few out to bid the job so you can meet and listen to them before hiring them.
 
  #5  
Old 01-09-12, 02:16 PM
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Hi again,
Every time we get a contractor in we get badly burned - and it's not like we try and cheap out. We do our due diligence, ask around, try and see work done - bingo, we still get screwed! I really wish I could relate just how bad a job this guy did - he took a floor that had one high spot, a slight ridge, and a drop off to either end of the room that was about 1/4 inch (just outside the tolerance for gluing down a floor) and gave us a floor that's the concrete equivalent of a yo-yo!

This is why I've virtually done all the rehab work on the house myself - aside from the fact I'm handy enough to do most of it. I figure that I'm the only one I can trust to do it properly, rather that taking the lottery of whether we'll luck our or not by getting someone else to do it. That's not a knock on all the people out there who are good at their work and have the kind of integrity to do it well - it's just we seem to run into all the lemons!

The concrete floor was another matter, because I have zero experience with concrete or cement, and trying to level out a 20 x 14 floor seemed a big ask to learn on! I had visions of screwing it up and having to jackhammer (on that I'm experienced!) it all up again.

Anyway, I think I may have come up with a solution that just HAS to be foolproof - I hope!

I kept thinking that the way to make it work (and this was back before I hired the contractor) was to create some kind of mini- formwork that divided up the room. I just couldn't see how to make something that would be as shallow as 1/8" in places that would work, especially as the surface was so uneven and would contour the wood, so I gave up on the idea. But I think I just figured a different way to go about it.

I located where the high spot was on the floor, and found that it was about a third of the way across the room. So I divided the room lengthways into thirds with lines on the floor, then, starting with the line on the high spot, used my level, an 8' piece of dead straight timber, and a multitude of 3" long wood strips I cut to different thicknesses, to set them as shims along the length of the line, with the high spot as the reference point. The wood strips now mark the desired floor level for the entire line, and this line was then used as the reference point for the entire floor. Using my level and the long piece of timber, I then worked across to the adjoining lines and set strips in place across the entire room to mark out the level.

Next is the point where I got the brainwave - instead of worrying over trying to make some kind of miniature formwork out of wood, all I have to do is lay down a 2" - 3" wide strip of cement (not SLC) along the lines between each wood strip (which will be glued down) then simply use the wood strips as guides/rests to help me screed the cement to the correct level, repeating the process between each set of wood strips, and making sure to clean up any excess so i don't have any unwanted high spots sticking up. When it's all dry (and barring shrinkage) I should have five dead level lines of cement, 3" wide, going the length of the rooms to act as mini-molds or guides to pour the main body of cement into, one section at a time - the lines of hardened cement should be perfect as guides to screed the rest of the floor.

So what do you think? The theory seems sound and it would take all the guesswork out for my inexperienced hands, but I'd like to run it by you guys before I give it a whirl. If I can't do this then we'll probably have to bite the bullet and get in another contractor.

In case anyone mentions it, I could probably take out the glued down wood strips afterwards, but it doesn't seem all that necessary. I'm covering the slab with ply anyway once it's nice and flat, over which I'm going to nail down the wood flooring.

A last point to Here's Johnny - that much cement mix? The contractor who screwed up the floor only went through 8 bags of Sandmix and seemed to lay down a fairly decent thickness in places of 3/8ths or more, down to a mere 1/8 in others. I was thinking of using Patcher with the idea I had in mind.

Thanks for reading this far.
 

Last edited by timbo59; 01-09-12 at 02:33 PM.
  #6  
Old 01-10-12, 05:03 AM
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Your idea is similar how they do finish concrete veneer (almost stucco like) work in Central America. They tack up a straight board at the elevation they want. Then they sling mortar between the board and wall, sometimes pushing to pack it under the board to fill the space. Remove the board and you have a mortar ridge with a nice flat top at the correct elevation.

They go another step further and do these mortared lines about every 4-6 feet. Something they can span with a screed board. Then they sling mortar onto the wall between their guide strips and work a board back and forth screeding off the area. Slinging in more into low spots as needed. You could do the same thing on your floor using regular mortar or something similar that has a slower cure time than SLC to give you plenty of time to work.

If you look closely at this picture you can see the guide strips under the corners of the window opening. I've only got a photo of it done on a wall but since many poured floors down there are quite crappy they often have to do similar to smooth out the floors before applying tile.

 
  #7  
Old 01-10-12, 01:03 PM
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Thanks for that - nice to know I'm on the right track. My only concern now is how much, if any, shrinkage I might get along the cement lines. If so I'll just go over them a second time.

I'll take pictures, and if it works out fine I'll post them online.
 
  #8  
Old 01-10-12, 02:01 PM
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A last point to Here's Johnny - that much cement mix? The contractor who screwed up the floor only went through 8 bags of Sandmix and seemed to lay down a fairly decent thickness in places of 3/8ths or more, down to a mere 1/8 in others. I was thinking of using Patcher with the idea I had in mind.
One bag of slc will give you about 25 sq ft at 1/4" in thickness. So you can do the math. All slc's are a little different but for the slc I use thats about it.

As to sandmix at 3/8" to 1/8" thickness, sandmix should never be used for that application.

If there are just a few low spots, they do make portland cement based patching compounds to fill in low spots to a feather edge thickness.

I'm not sure how you intend to attach the plywood to the concrete slab. I'm a tile guy so this piece is new to me. I can tell you that if you try to nail thru slc you will make a mess out of it.
 
  #9  
Old 01-10-12, 06:17 PM
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Funny you should say that - I read the notes on the Sandmix bags they left behind and thought the same thing.

I was going to use Patcher or something similar that can be used thinly, but are there going to be any issues laying it over the layer of Sandmix that's already been put down?

As for how I'm going to put the ply down, I've been on some speciality wood flooring forums and found that there's two methods, the older version being to Tek-screw the ply to the concrete, while the more modern option favoured by the pros is to 'float' the floor, using two or even three layers of thinner ply layered over each other and screwed together. Both are fine, though the latter has become more popular because it's a little easier for the pros and it's better as far as moisture is concerned due to the moisture barrier that can be placed underneath. Plastic can of course be laid below the ply being screwed down, but it gets compromised somewhat by the holes made by the screws - at least that's what the pros tell me!

The original intention had been to glue down the flooring, which we specially bought for the purpose (3/8", which is about as thick as is normally recommended for glue-down) but what with the botched job, the fact that they used Sandmix, and the matter that I'm correcting the flaws with something else, has left me doubtful regarding the adhesive qualities of the floor. That's why I decided to go for the ply, as the room in question is a sunken living room, so the additional height won't compromise anything. Once the ply is down it then becomes a question of whether to nail down the flooring or glue it in place. Nailing's undoubtedly cheaper, so I may go that route.
 
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Old 01-11-12, 05:16 AM
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If you are going over the entire area with ply and then your flooring I would not worry about shrinkage of your leveling material, SLC or mortar. It's going to be covered. Minor shrinkage cracking should not be a problem as long as you are not going overboard and make a really thin mix that becomes weak and cracks excessively. You just need something to provide continuous support to whatever you put on top.
 
  #11  
Old 01-11-12, 06:15 AM
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as you are not going overboard and make a really thin mix

If you are going to use slc, dont play with the formula use exactly the amount of water they tell you. No more, no less. Since you are inexperienced with slc, do not get the rapid set.
 
  #12  
Old 01-11-12, 06:25 AM
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Important. Do not skip the step of using the latex primer that the manufacturer of the slc requires. Do not use any substitutes. Yeah, I know, expensive stuff but absolutely necessary.
 
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Old 01-11-12, 09:59 AM
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@Here's Johnny. No, I've decided to skip the SLC for two reasons. Firstly, I got scared off by all the stuff I've been reading on how difficult it is to work quickly over large areas when you're on your own. Secondly, the entire floor has a slight drop to one side of the room, so I think the SLC, if it lives up to its name, will want to try and pool towards that area - or at the least sag towards it! A final point on the matter - the only stuff available locally is the rapid set stuff, which is what got me nervous!

I've gone with the stuff the contractor should have used in the first place, Sakrete, at least so far as I've been told by another contractor contact. It's supposed to be used to thicknesses UP to 1/2 an inch (unlike Sandmix) which is the exact job I have at hand.

Two general questions, from a concrete newbie.
Firstly, Is Sakrete stiff enough at the 6/1 water mix suggested to 'stand up' in the narrow lines to be leveled out that I mentioned in an earlier post, or will it try and slurry/sag away? I'm only going to mix small quantities to start with while making the lines, as the work may be tedious. Can I just put in less water to make it stiffer, or leave it a little while before applying?

Secondly, what with the crap job already done, the fact I'm adding another layer of cement, plus the ply and wood flooring, I'm going to be raising the level of the floor by about 1 ½”, which is going to be compromising the two separate steps leading down into the area - one step by the foyer, another by the family room. Before, the total drop into the room was 7", now it's going to be around 5 ½. That meant a 3 ½” drop to the step from the main floor of the house, then 3 ½” down to the sunken lounge. Now the latter is going to be more like a 2" high step! Should I build the step up so that everything gets back to being equal (2 Ύ” height) or would that be too shallow and I should simply build up the step so that it’s at the same height as the main floor – it would mean a single step up of 5 ½”.

Lastly, how do I attach a picture in the same manner that another was inserted into this thread by Dane? I was going to place a picture of the step in question here, but couldn't figure out how to do it.
 
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Old 01-11-12, 10:23 AM
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Old 01-11-12, 10:37 AM
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Hi Mitch.....I did see that before and noted that it asked for a URL, on top of which I do have a photobucket account. But unless I'm mistaken, all that will do is essentially add a hyperlink on my post to access the photo at Photobucket. No big deal, I can do that, I was just curious how Dane actually inserted a picture into the body of his post in this thread, which would be the simpler and easier option. Otherwise I can happily put the picture up on Photobucket.
 
  #16  
Old 01-11-12, 04:52 PM
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Okay, my first attempt at using the cement this afternoon was a bust! I mixed it as described, only to find that it was a semi-dry mess that didn't bond into any kind of slurry at all. I finally added a bit more water to get it into a pasty mix, then tried laying it down along one of the lines. It just came out of the bucket very clumpy, so I had to really pat it down with a trowel. Then I tried to screed it flat, only to have it either come up in clumps on the timber I was using if I tried tamping it down between the wooden high points, or break away from the concrete I'd laid it on if I tried pulling the screed across. Very frustrating! Should I have mixed it differently, am I using the wrong cement, or should I be looking at something else entirely to try and build up the lines? I'm even toying with the idea of using thinset for the lines, as I'm much more familiar with its properties than cement mix.
 
  #17  
Old 01-12-12, 11:43 AM
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Timbo

Thinset would not be any good. Thinset is for bonding. It cant be applied in thickness of more than 1/4" as it shrinks. Additionally, it doesnt screed well. If you are just filling in certain areas try a portland cement based patching compound. It might be helpful if you posted some pictures so we can see what you are up against.
 
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