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I am finishing a basement roughly 900 sqft. The concrete slab for the most part is level, however, it does have some imperfections at some 18-20 high points (1/8" or less) and a couple of low areas. One low spot probably needs some self leveling concrete for a 1/4" dip. We are planning on installing laminate on this basement floor.

Options (Please see updated questions in the update section below):

  1. Install a thicker underlayment to absorb the high spots (if that's even possible)
  2. Pour self leveling concrete to correct the high/low spots?
  3. Grind the high spots down and maybe correct the bad dip?

Update:

Here's a better picture of my basement (aka the dungeon): unlevel basement

The red spots are the humps, anywhere around 1/8" - 1/4". Green are dip areas.

Thanks all for the comments. It's been a busy summer with kids activities and I found sometime and made an attempt at option #3 with a Bosch 1773AK 5-Inch Concrete Surfacing Grinder.

bosch concrete grinder

I spent 6-7 hours in the dungeon trying to grind these damn humps down and after all the dust cleared, I don't think that I really made that much of a difference and set off my smoke alarms (yes I had the vac attached with small particle filter) and massively sore forearms.

So here's my updated question:

  1. Should I attempt to rent a bigger push type concrete grinder for taking out the humps(marked red) and patch pour the low spots identified in the green above.

  2. Flood the basement with SLC (self level cement) and then rent a grinder to take out any residual high spots. (very expensive, but I'm looking for the best option)

  3. Throw in the towel and hire the pros to come in and take care of this.

Thanks so much.

  • This should be addressed in your install guide included with the laminate. In general however, the dips matter whether they are actual 'dips' (the grade changes quickly in a short distance) or normal imperfections (happens over long distances). if it's 1/8th of a inch over 10ft, it's not going to be significant. If it's over a foot, it will be significant. – Chris Aug 3 '16 at 22:12
  • Both dips and high spots can be problems. This is not an easy answer without seeing the floor. If not large cement high spots are easy to grind. Low spots can be filled with self leveling concrete. It may take a combination of both to have a long lasting floor. – Ed Beal Aug 4 '16 at 0:57
  • Make sure to fully follow the manufacturers directions for the installation in addition to the floor leveling concerns. Some types of flooring are not recommended for installation on below grade concrete floors. – Michael Karas Aug 4 '16 at 10:04
  • hey guys, never thanked you for the comments, much appreciated. Updated my question to be more clear and where I'm at with the project. – Dolph Aug 26 '16 at 5:26
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ok guys, here's what I did. Of the highs and low spots, I found that there were a couple dips that were significant, by the door and by the stairs/bedroom. I filled both of these in with SLC (self level concrete) and the results were pretty good.

Suggestions:

  • follow the directions exactly, do not over water, it will be too runny.. I used 5.5L/bag and it worked out somewhat perfect.. I had a couple of dry clumps that some how got missed. I had to scoop them up with the trowel and re-mix them with some water. I would recommend that you get a stir stick and check the mix before dumping to make sure that none was missed. I was using a paint type mixer and I think that an egg beater type would have been better.
  • get the straightest 1x4x8 and uses a screed. This is how id did SLC by the back door. The screed will slide along the high points and will push the material along, creating a perfect amount of SLC. Touch ups or moving smaller amounts of SLC with a trowel. I then feathered in the edges using a clean push brush.
  • Find some soccer cleats as it will displace the SLC way less than walking with a boot. You won't even see your foot prints after walking through.
  • don't freak out about this. If I can do it anyone can.

by the stairs back door

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