checking floor for being level

From what I read, laminate flooring manufacturers want a floor within 3/16-inch across 20'. What tool can I use across a floor to determine how level it is? A 4' level doesn't do much good.
Thanks.
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I've never used this kind of thing, but here's an idea: http://www.komar.org/projects/crawlspace/laser-level /
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J wrote:

Marble?
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A whole bag of marbles is better, strewn everywhere on the floor. If they tend to roll together, this probably indicates a low spot.
--
Don Phillipson
Carlsbad Springs
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Don Phillipson wrote:

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 to go!
- Rodger
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HeyBub wrote:

No, laminate - not tile.
- Rodger
;0)
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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.
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snipped-for-privacy@gmail.com wrote:

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.
- Rodger
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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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Exactly might thought. The simplest possible way.
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J wrote:

Clear plastic tube filled with water.
--

dadiOH
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