Use a bag of marbles (or bearings) that are 3/16 of an inch in diameter -
you can then go to the edge of the room and place your head on the floor
(facing the conflagration of marbles) and if you see any, then you're good
I had this same question several weeks back. I found two things
useful - 1) check the straightness of several 8-10' studs (12 or 16
would be even better) at the hardware store with a string. Hold the
string to one end/corner and taughtly pull it to the other end (having
a friend helps). Find the straightest one you can, and then take it
home and use it all over to find high or low spots. Sure you look
weird at the store, but it's the cheapest level you can buy for this
purpose. 2) fill a 5-gal bucket of water and pour it out slowly all
over the floor. It will pool in certain places. These are the areas
you'll need to fill with floor leveling compound. Just measure the
depth of the water. If it's less than 3/16, you're fine. If you
notice areas where the water goes away from, you may need to use more
leveling compound to "build up" the floor to keep the high points from
sticking more than 3/16th up.
Incidentally the second method wouldn't work for me, since my entire
basement floor slopes toward one corner where the sump pit is. Even
though the entire floor is fairly "flat" it is not really "level".
Therefore all my water ran down toward the sump pump and i just had to
clean it up without finding anything out. But, considering flatness
is what I really care about (for flooring also), my flat, un-level
floor is actually fine.
Uh, in all seriousness, why not use a taught snap line (or 20' piece of
string) and stretch it across the sub floor in various areas? Hold it up
3/16ths at either end and if it touches anywhere... If it doesn't touch
anywhere, drop it to the floor and if the floor drops below the line,
measure the drop.
I used an aluminums straight edge that was made for cutting plywood. Laid on
the floor it is easy to find high spots that may need belt sanding down and
low spots that need filler to get it level. I also used the same straight
edge to scrape the filler flat with the surrounding areas. Despite problems
with a join between the original house and an addition, I managed to get the
floor quite flat. I had a ridge over the original foundation wall with a
1/2" deep low spot next to it. Used some plywood filler strips to fill the
deepest to avoid using solid floor leveller compound.
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