Hello:

Planning on float installing an engineered 3"or 5" hardwood strip floor (probably a glue together) over a 24yr on grade concrete slab foundation, and need expertise in dealing with inevitable cracks in the concrete subfloor. Moisture readings not an issue, but will re-do just before installation.

Carpet and pad pulled up in one bathroom and I notice hairline cracks in the concrete foundation which may or may not be caused by the rough plumbing opening. Hardwood flooring is NOT intended for this space, but it gave me a heads up of what might to expect when carpet and padding is removed in rooms in which engineered hardwood flooring is intended.

We plan to thoroughly clean and level the concrete subfloor and properly install a heavy mil vapor barrier and pad before float installation. Here's my concerns:

1. What product should be used to fill the hairline or (yikes) wider cracks?
2. What product is suitable for filling low spots?
3. Should this product be allowed to cure before installing the vapor barrier?
4. Is this process successful during cold rainy months?
5. Any other concerns when dealing with these issues?

I'm willing to go the extra mile to ensure proper concrete subfloor prep to protect the $$$ spent on the flooring.

Thanks & Regards
dg

1.) Hair line cracks can be filled, but usually not needed. It is the big cracks that are still moving to be concerned with, and if it suddenly slopes off at the crack.


2.) I like Adrex products. Their SD-F Feather Finish for screeding and their K-15 for self leveling.


3.) Yes, let it dry out, especially the thick pours that are deep.


4.) Yes, as long as your interior climate is controlled, and humidity levels are kept in check.


5.) Go for flat, the term level gets confused with this term often.

DG:

Even though this page applies to glue-down, I think you will find it helpful: Click Here

Did my first floor “flattening” pour this past week. It is truly amazing how out-of-flat a level 14 year old slab under carpet has become! Even with an inch of sub-floor plywood (1/2" + 1/2"), I am certain I’d have had “waves” or "ridges" in flooring had I skipped this step of the whole process. For the flooring method I am using see my other posts at: Click Here .

For my first couple of rooms I used regular thin set as opposed to fancy self-leveling compound due to cost ($5 per 50# bag vs $25-$30). Mixed it up good in 5 gal bucket using ½ drill and blender blade. Mix to non-runny consistency of a milkshake. Am sure fancy self-leveling polymer enhanced stuff would work wonderfully but size of my install makes this a another high dollar item with the fancy stuff. In this room, I first ground down ridges of a large crack or two that had buckled slightly causing ridges on my 14-year-old slab. Used 7” masonry wheel on grinder to feather ridges out. This makes A LOT of dust so drape painters plastic over walls and anything else you don’t want covered. Used a Star Wars mask to keep from breathing the stuff also. The cheapie rubber band paper style masks are not enough! Won't bother with this step in later rooms unless ridge is too large to be "fixed" with thinset. Scraper took off all other drywall lumps and left over carpet glue. As presented in article, used 2x4 (visually straight as possible) to screed thin set. Borrowed son’s old baseball cleats and used them when pouring/leveling. This is a great tip! As noted in article, plan on extra drying time for thin set vs fancy stuff, before you start covering it with plastic and plywood. The article recommends against concrete compounds but if you read the article on the fancy stuff's bag, it too is concrete based. My deepest thinset is less than 3/8” or so. Believe moisture will be no problem in Phoenix as weather is far warmer than most climes this time of year and I am planning on extra cure time with the windows open. The newly flat surface is not pretty but who cares? It is flat and that is what counts. Like the article says, wash everything off well before it hardens or you'll be throwing away some buckets...

Thanks for adding some more thoughts VP. I think I was trying to stear people clear of products like quik-crete--your basic masonary cement compund that takes forever to cure.

What article are you reading, that recommends against concrete compounds?

This is news to me.

Modified thinset is the same as the fancy stuff, with more sand in the mix.

I too fill deep screeds with a mix of Flexbond and Ardex SD-F. Then cap it with a skim coat of straight Ardex, because of shrinkage.

Regarding 2.), what is the difference between "screeding" and "self leveling"?