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What is the procedure for determining where to grind/fill a concrete subfloor?

What is the procedure for determining where to grind/fill a concrete subfloor?

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  #1  
Old 03-23-14, 10:26 AM
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What is the procedure for determining where to grind/fill a concrete subfloor?

I'm putting down Pergo laminate throughout my downstairs area, about 1000 sq ft. The Pergo directions say to make the room level to 1/16 within any 3' radius.

How do I actually determine what needs to be done? If I put my straight edge down on the floor, I'll find a high spot and mark it. If I rotate my straight edge 90 degrees, that same spot appears to be low. How do professionals deal with this problem?

Also, this task is hugely tedious. Moving a straight edge in every possible direction over 1000 sq ft is going to take longer than putting down the laminate. Am I making this too hard?
 
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  #2  
Old 03-23-14, 11:12 AM
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Try getting some mason string and with the help of a partner, stretch it across the floor from one end to the other in several spots. Hold one side to the ground and slowly lower the other side down and observe what you see. Mason string stretches so pull it as taught as you can without cutting into your skin. Its a good idea to wear a pair of gloves when working with mason string. Particularly if you are stretching a long run.
 
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Old 03-23-14, 11:44 AM
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A 3' radius is the same thing as a 6ft diameter. So I'd probably use a 78" level and check the floor with it.
 
  #4  
Old 03-23-14, 11:48 AM
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The mason string is an idea I hadn't thought of. This seems like a good way to look for big bumps or dips in the subfloor. Is that all I need to be concerned with? If I'm looking for bumps and dips of just 1/16 every 3', I'll be there all day. The big question I have is this: how good does it really need to be? If it really is 1/16 every 3', it seems like I need some precision measuring tools to figure that out.
 
  #5  
Old 03-23-14, 11:52 AM
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That makes sense. I have an 8' screed that I've been running across the floor. But in one direction is tells me a spot is too low, and when I rotate it 90 degrees, it tells me that same spot is too high. So what I do depends on where I place my screed, and that makes me think I've got the wrong technique.
 
  #6  
Old 03-23-14, 12:28 PM
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There probably is a technique you haven't quite hit upon. Your 8' screed should help you figure it out though. If you can identify any "humps" in the floor, you would obviously start there. You wouldn't set the end of your screed in a low spot because that will give you an inaccurate reading. You will probably find that your "humps" will be areas that haven't sagged... such an area that is directly over a beam or load bearing wall. All other areas may have dipped in relation to that area. So maybe the load bearing wall is perpendicular to your joists, and they all dip slightly on either side of that load bearing wall. Or maybe the load bearing wall is only under one joist, so it has remained straight while the rest have a dip in them. Hard to say.

If you can take a quick look under the floor and get an idea of the supporting structure, it might help you visualize where a good starting point would be. The mason's string is also a good suggestion as it might give you a good general idea of where that high spot might be. Then start from there, center your screed over the high spot and see how much it tips/rocks.

What's the makeup of your floor and subfloor?
 
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Old 03-23-14, 01:36 PM
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The subfloor is my concrete slab, and I'm installing laminate on top. I removed old carpet and ceramic tile that was cracked. Like almost all houses in the Dallas area, this house has experienced quite a bit of movement in the foundation over the last 40 years. But, the foundation has been stabilized now with perimeter piers, so hopefully there's minimal movement in the future.

Do I need to just pick a point in the room as reference point and work from there? Is that what you're suggesting with choosing a starting point? In some places, I can put down my 8' screed and observe that the floor is clearly dipping, especially near the exterior walls. But when I then put down my 3' level, I don't see any variation greater than 1/16", so I have conflicting indicators of what to do.

I can't believe that everybody who buys laminate from Home Depot goes through this process. But I guess that's why people show up here asking for advice on how to fix pops and squeaks in their floors.

Thank you very much for your advice!
 
  #8  
Old 03-23-14, 02:04 PM
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I can't believe that everybody who buys laminate from Home Depot goes through this process.
Most probably don't, but should. Any deviation from the stated subfloor requirements will void any warranty should you have issues with the floor. But on the other hand, this is a very DIY installation. The issues with unlevel floors has to do with stress on the joints from excessive movement. That along with what material the flooring is made of leads to failure. If the area in question will be underneath furniture and is not subject to movement, then I would worry less. Concentrate for your situation on your traffic areas as they will see the constant up and down movements that cause stress on the joints. If the level method or string methods fail to satisfy, you can always dry fit several rows of flooring together (say 8'x8' area) and walk on it to see if there is flex or bounce in the floor. If yes, you need to correct. Certainly not scientific, but visually, it will help paint the picture. Using just a 3' level will not provide an accurate enough reading.
 
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Old 03-23-14, 02:20 PM
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Concentrating on traffic areas sounds like good advice. I'll start there with my screed and see what it looks like. I also like your idea of dry fitting the laminate. I'll give that a shot. I'm relieved that you call this a DIY project. I've definitely been feeling in over my head with this floor leveling problem. Thanks for the advice.
 
  #10  
Old 03-23-14, 06:09 PM
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Like any project, preparation is the key to a successful install. Once you have determined that you are good with the floor level, you will need to prep each doorway and undercut the door casing and jamb to allow the floor to be tucked in underneath. If you did not take out the base board molding, then you will need to undercut the first 2" of the base board to tuck the flooring and then you will eventually cover that undercut with shoe molding. The goal, is to make the floor look like it was put in before the doors and moldings. Preparation for that look is key.
 
  #11  
Old 03-23-14, 07:35 PM
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That matches what I've been thinking. I've already undercut the baseboards in the first room with a power jamb saw, which went smoothly. Have you ever seen an installation where laminate was tucked under existing baseboards without installing quarter rounds?
 
  #12  
Old 03-23-14, 08:58 PM
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Sorry, the word "subfloor" threw me off (usually you think wood). Identifying any high spots on your slab with your screed and then hitting them with a wet/grinder will be a good place to start. Once that's done, if there are any dips that are 1/8" or more over 6 to 8 ft, a floor patch leveller (like feather patch) will take care of those areas.
 
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