They are referring to the floor being flat - no hills or valleys. Depending upon how your concrete floor was laid you may have to grid off high spots or fill low spots with SLC (self leveling cement.

Thanks, man. I see you caught on to the distinction between level overall (does it have a slope?), and parts where there are dips or mounds, etc. We will Google to find out the proper way to check the floor at all points (with the proper level). But the only part where there is an obvious valley is way in the corner by the boiler, where there is a drain. That part will be in our soon to be utility closet area, and we are not going to bother with installing Smart core there -- just leave as is/paint the cement.

Get an 8' straight-edge, an inexpensive plastic level. Start in a corner - and sweep it in an 180 arc. See if it's flat - or the valleys. Use chalk to mark the spots.

Thanks you two. suseyb, I will look that up. geoffrey_b, preferred length of level for this (and other) projects? Three foot? Four?

Much appreciated,

You need to look at your installation instructions and see what it requires. Usually, it is 3/16" in 10 feet. Get something straight and flat that is the length you need, and use that.

The problem with a slope is it is rarely on the same PLANE. There are several floors that I know of that can handle a slight slope....but they cannot handle humps and bumps AS WELL as a slope. It is intensely rare to find a perfectly FLAT floor with a slope that is PERFECT.

What happens? The click edges break with the stress. The little bumps and waves in a floor with a slope can and will stress the click system to the point of breaking. And it will happen over time. As it is walked on, the planks flex. As they flex, they cause stress fractures in the click system. Eventually the edges break and you get a floor that begins to gap. Once it starts, it is almost impossible to stop.

While you wait for the vinyl edges to break, you will also get this lovely 'slappy' sound under your feet. The flex in a vinyl floor can be SO big it will visibly move every time you step on it. Again, this is more pronounced on a sloped floor that isn't flat. This is where laminate and vinyl are different. Laminate is very thick and very sturdy. Vinyl is thin and often quite easy to damage without proper training/preparation. In fact, the amount of swearing going on while a vinyl click together floor is being installed is IMPRESSIVE! And I grew up with 'angry Irish'. It takes a lot for me to call swearing "impressive".

Vinyl planks are often very thin (4-5mm thick). That means the click egdes are VERY THIN (2mm deep...if you are lucky). That means there isn't much "grab". The tensile strength of the click system is very weak. A shift (when you step on it) up and down and a little side to side will rip the planks apart. Even if the edges remain intact (not broken), they simply do not have the strength to stay locked together.

So imagine purchasing 1000sf for $4/sf = $4000. And imagine you then take 3 days to install it. Now imagine it fails within 6 months to 1 year. Will you feel the money you saved on the cost of subfloor prep ($2 - $4/sf) was worth it?

Once you fix the basement subfloor (one time cost of $2-$4/sf depending on how bad it is) you can have ANY floor you want in there. You never have to fight with it again. I know that's not what you wanted to hear. Anytime there is a sloped floor (those basement drains are fun aren't they?), it is always best to remedy them before putting in flooring. The only floors that are OK with this type of floor = carpet and sheet vinyl/lino. Every other floor needs that basement slab to be fixed.

I just had LVT installed on a basement. It took $$$$ and time to level the floor to the appropriate tolerance levels so that the floor guys would install. which I probably wouldn't have done, had I realized how long it would take.

They won't install if the floor will just fail because it isn't level enough. Go sheet vinyl if you don't want to spend the big bucks.