Now, IMO it's not important to have a truly flat floor, and honestly it's impossible that anyone does have a completely flat concrete or wood subfloor. The general idea I used was to take a 4' level and drag it across the floor (I also selected the straightest 10' 2x4 from Home Depot and used that too). One good idea to easily see the dips and humps in a floor is to turn off all the lights, and open a window or curtain at one end of the room. Start on that end of the room with your level, and place it on the floor, and get on your knees on the opposite side of the level from the light source. Now, slightly tip the level (or 2x4) toward you, allowing only the edge of the level to contact the floor. You will be able to see unevenness by the amount of light let through the intersection of the level and the floor. As you drag the level, you will see the light pattern change indicating humps and bumps. Mark these with a pencil. Be sure to drag it in both directions in the room, because you may have linear bumps or humps that can't be see by looking in one direction only.
How do you know which bumps to fill, and which humps to grind down, and which to ignore? That is anyone's guess, and it comes with experience. Since this is my first DIY tile job, I think I went overboard with the levelling. If you have cheap ceramic tile, you probably need more levelness than if you have more expensive, thicker tile. Also, pay attention to known high traffic areas (like common walkways, doorways between major rooms, hallways, the front entrance, etc). Those will receive the most foot traffic and need a solid install. Choose a tolerance you think your tile can handle, and fill the areas that don't meet it. If you have a huge hump that needs grinding down, you probably should. I found a couple of humps but I figured that with my smaller tiles (12" and 6") and careful thinset usage, I could get away with it. Time will tell. Grinding down concrete takes a heavy machine and I didn't want to deal with it (plus with my smaller tiles, 6" and 12", I figured I could deal with gradual humps and bumps by using more thinset and/or backbuttering the tiles as I laid them).
I did end up finding several low spots (and after removal of the old tile, I could see markings on the floor where the original installer also used SLU (Self Levelling Underlayment), so I figured I should re-fill those areas.
Self Levelling Underlayment Mess
I bought 3 50lb bags of SLU, plus enough latex primer. I used "MAPEI Novoplan2 Polymer-Modified SLU" and the associated primer (can't find the link), it's this light blue stuff you roll on like paint and let dry. So I rolled it on. Here's a tip, roll on in a much larger area than you think you need, to ensure all the SLU you put down contacts the primer. I mixed up a batch using a
paddle-style mixer attached to my 1/2" Dewalt corded drill. Then I poured a big blob on each of the areas I'd identified as needing it. Let it set for 10 minutes. Then the "fun" began. I started spreading it with a 4' section of straight 2x4 I'd cut from the 10' piece I bought. Immediately I knew I'd used WAY too much of it. The depth of the dips in my floor was no more than maybe 3/16". So the SLU was spreading out a LOT. I kept trying to work it back and forth but the puddles of SLU were getting huge (a lot bigger than I thought). It was turning into a sloppy mess, big time. This was also on New Year's Eve and I was in a hurry to finish to make a dinner with friends. I had SLU everywhere, on my hands, arms, shoes (I'd knelt down to work one area and inadvertently backed into another area with SLU). Some areas, the SLU would pool up at the edges from me wiping back and forth with the 2x4, creating a nice high bead (1") of SLU. Unacceptable. I started shovelling the SLU back into the bucket to take up excess. That made an even bigger mess. I worked the SLU to the wall in some spots, and the SLU flowed underneath the baseboards, which I knew would prevent a tile from going under there later. Also, when wet, it's really slippery and in hallways it was a pain to work a huge puddle from both sides, I almost slipped in it several times. I finally got it somewhat level, and after much cursing decided I was done for the day.The moral of the story is to start small, and add more SLU as needed, rather than trying to fill the whole thing at once. Also, 20 minutes later, you can "feather" the edges (I used a wooden paint stirrer and this worked well). Lastly, I went over all areas with a big push broom to give it a little texture in the hopes it would help the thinset bond better later.
Post-levelling
After I came back a few days later and inspected, I thought it looked like crap. There were globs of dried SLU that would have to come off. I took off the small clumps with my cold chisel, but the larger areas I wanted to sand back down, so off the rent the floor buffer again. I went back over the areas of SLU with the sander, and tried to get it more level. There were still a few spots that I didn't like, and I thought the best way would be to use MORE SLU to smooth it out. This time though I'd learned from past mistakes and just used a little at a time, and got the results I wanted. After all of the above mess, I was ready to tile. Final vacuum and wiped down the floors by soaking a few old towels in water, wringing them out, then dragging them across the floors (and rinsing after every few minutes). I also held my chisel while I was vacuuming, so that when I encountered anything (like small bits of dried SLU, or nails I'd missed from the tack strips), I took them out and vacuumed them up.
One other mistake I made was when I was dragging the level to find low spots, I only went in one direction. So I ended up missing a few humps and bumps in the other axis of the room. But after inspection, I was a) sick of dealing with SLU, and b) somewhat confident I could deal with them later during installation, so I said to hell with the SLU and decided I was ready to tile.
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