Laminate flooring concrete subfloor leveling question

Old 10-14-16, 03:00 AM
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Laminate flooring concrete subfloor leveling question

I am installing laminate flooring over a concrete slab in a small house. Ripped up some old nasty carpet to do it.

One bedroom is done, however in the main living/dining room, after removing the carpet I see the slab has either a low or high (not sure how to tell) spot in an area. Such that, laying a 4 foot piece of laminate on edge on the concrete over this hump/low spot, shows about a 1/2" gap at the end (if you imagine it like a see saw)! This is way over spec.

However in googling about the issue I found a comment from a flooring installer I liked. He basically said the manufacturer recommended floor tolerances (in my case no more than 3/16" difference over 10 ft) are way too stringent and exist just to cover their butt. No floor is that flat (hell glass isn't that flat, he said), and most new construction will exhibit a 1/4" deviance over 2 feet, he said. Further, he said given that laminate is cheap, it's really not worth spending much effort or money prepping the subfloor. In that case he said you may as well do a more expensive flooring anyway.

The room I already did had a obvious low spot in the corner, and it's been walked on for a few days and seems fine. Basically, I'm leaning towards laying the floor as is, with the subfloor unlevel. If worst comes to worst I'm not out a ton of money (probably on the order of $1500 for the whole house). I dont know the first thing about concrete, and it would be a disaster to screw up something of that nature.

So given all that, I guess my question is, is there a quick, easy, way to deal with this somebody can walk me through (floor leveling compound obviously)? If it's not quick, easy, and cheap, I'm not going to bother and I will take my chances. However if I could just slop some leveling compound in the spot and have it be vastly better (I'm sure nowhere near perfect), that would be worth it. I want to hit just a area, not the whole floor.

Also, whats the difference between a hump and a low spot, they seem like two sides of the same coin. And how can you tell which one you have? A hump needs to be ground down, a low spot filled, correct? How do I know which one to do?
Old 10-14-16, 03:26 AM
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The fact that you are on a slab makes your repair much easier. you need to identify specifically where the low spot is. This would require a longer straightedge than a 4' piece of laminate. Once outlined, you would prep the area with a self leveling bonding agent/primer and then pour in some self leveling compound and smooth with a trowel. Making sure to not over fill and create a new hump.

If the low spot is up against the wall and will have heavy furniture on it all the time such as a couch or china cabinet, then you may not need to do anything. The issue with high and low spots is more of an issue around areas where the click lock mechanism will flex up and down regularly. This will cause both noise and fatigue on the joint and cause early failure. So have extreme concern in traffic areas and less concern in static areas where the floor will not move at all.
Old 10-14-16, 05:03 AM
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To find high and low areas a long straight edge and a level is required. There are very long levels but they are a rather specialized tool and can be expensive. Cheaper is to find a long straight something. It can be half of an aluminum extension ladder, a piece of steel or aluminum from a welding shop or even a good straight board. Then you can set a normal sized level on top of your straight edge. High and low areas are easily visible by looking at the gap underneath your straight edge. Best is if the level on top of your straight edge reads level but it's not uncommon for a concrete slab to be slightly out of level.

If you have low spots follow czizzi's recommendation to use a self leveling compound. If you have high spots it may not be feasible to raise the rest of the floor up to the high spot so you need to cut down the high area. Most tool rental stores rent concrete/floor grinders that you can use to grind down the high area.
Old 10-14-16, 05:44 PM
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I am not sure. I bought a 8 foot 2X4 today to try to get a better idea of the problem.

Here is one of the worst humps, there seems to be at least one other one in another area, maybe almost as bad. When the 2X4 is level on one side of the plateau, the other side is 1.5" off the ground! As demonstrated by this picture

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How should I proceed?

I'm thinking I may just have to get carpet again at this rate :/

Is there other floorings, like vinyl or tile, that deal with humps better?

edit: based on moving the 2X4 around, and keep in mind I'm coming from a place of no knowledge of this stuff, I'm starting to think i have a couple big humps/high spots/ridges. So, I'm thinking maybe a grinder?

Last edited by shark974; 10-14-16 at 06:16 PM.

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