How do I ensure my concrete floor is "flat" for laminate flooring?

Question

I'm floating about 800 sq ft of bamboo over concrete. I have the entire area cleared of all previous flooring and fairly smooth. Now I'm trying to make sure it's flat,but I'm not really sure exactly how to do this.I've read the "string method" but I'm not really sure how to find the high spot across such a wide area (about 30'x25'). The room has a closet and a hallway too (i.e. it's not just square). I've also read about using a straight edge (like a 2x4), but when looking for gaps in 16ths of an inch, it seems a piece of 2x4 is not exact enough, even if I have a fairly "straight" piece of wood.
Other details: I tested for moisture using the plastic taped to the floor method and that seemed ok. The slab is about 25 years old and I live in a dry atmosphere (San Diego), so I think I'm ok with moisture and plan to use a 2in1 moisture barrier/foam padding.
Any tips would be greatly appreciated as I'm eager to start laying the flooring but don't want to skimp on the prep work. Help!

Answer 1

First: Water seeps at the same level. Use a long transparent tube (same tube used in aquarium pumps) with enough water to measure level points several inches above floor walls. You can fix one end of the tube on one area of the room, say 5 inches water level above the floor then on the opposite end of the tube, mark where the water level stops which should be the same 5 inches above the floor. Do this around your room and across areas you suspect floor is not level. Next, measure from the points you marked down to the floor as close as possible . Use the same measurement for all points. In theory, you marked points that are now level. Hammer a nail onto each of these points and string these nails. Connect these strings onto the nails across to create a network of strings. You'll see where your floor is not level.

The next question is that would you get into another trouble to level your floor before putting the laminates? It may not be necessary unless the floor is really uneven.

Answer 2

You do not really have to have perfectly level concrete to lay the floor. You need a flat surface not necessarily level floor. I have laminate over concrete in my house I did not check for level except visually for anything really off, it is fine.
If you really want to you can use a string line in several places. Along opposite walls staple a string 3 inches from the floor. measure to the floor in different places along the string if it is more or less than it is out of level.
NOBODY DOES THIS WHEN LAYING FLOOR. As long as it is close that is fine, just ask the place where you bought the flooring it comes with complete instructions.

Answer 3

You want to take out the major highs and lows so they don't telegraph throught the finish floor.
Start by finding the high spots. Get a good straight 2x4 and rub one edge with a bright color crayon, like a red lumber crayon. Drag that colored edge over the floor, and the crayon will transfer to the high spots. Use an abrasive stone to rub those down...these are grindstones with a handle on them that are made for this purpose. You can save a lot of work by finding the highest points, and put some effort into removing the spot. Don't try to level the whole floor by grinding down the top, just start with this.
After you make a couple passes with this, use a good metal straightedge (like a long level) to look for the low spots. Having a bright light behind the straightedge helps see the gaps. A nickle is 1/16th of an inch thick, so you can use that to measure the gap. Another good way is to put water on the floor and see where it pools up (but that will need to be dried out before laying the floor)
If the lows are out of spec, fill them with a leveling compound like "Henry's Featheredge". This is a very fine grained material that sets fast and will trowel on easily.

You are doing the right thing on your prep work. A good DIY person, who pays attention and takes the time to do it right will often produce a product superior to the "pros", who are driven by management to get in and get out.

Answer 4

Well you might try a 6 ft level, or at a home store purchase a heavy gague aluminum straight edged piece of corner mold.

Obviously in a large area you should know that its base is as level as possible no matter.

"High Spots" are those areas to level UP TO, not grind away. Minor depressions can easily be "floated in" with a Thin Set or similar, mortar type underlayment.

Certainly over that expanse you may have to work in smaller areas at a time, much like you will when installing any kind of flooring, and any "Underlayment" in the form of a padding material will certainly help with those depressions 1/16th inch or less.
"Perfection" might only be a process that involves "tolerance", "acceptance", some minor "compromises", and in the end enjoy the floor as the end result rather than the process to achieve the "enjoy". Without question, Perfection is a valid issue to "strive" for.

Steven Wolf

Answer 5

If your slab is clean and smooth and has no obvious peaks and you are putting down padding, you probably have to do nothing and couldn't fix it anyway. Bedding compound works but is for local dips, cracks and holes..
If you still want to check, use a marble or a steel bearing ball about the same size and use a chalk line to grid the floor. Set the marble down on the grid and see if it rolls off quickly. It will roll into depressions away from peaks.

Answer 6

To use the string method you need a length of strong string and a tool called a line level . The line level cost's under 5 bucks .
What you do is anchor one end of the string to the wall at a specific height using whole numbers- [If you use 3' you can use a 4' level to insure that you have a perpendicular measurement to the floor ] hook the line level on the string and with the help of an assistant stretch the string to the opposite wall. When the string is level ,anchor it. You now have a level line across the room. Take your tape measure and measure the distance from the line to the floor along various points. Record on a piece of graph paper , change locations and repeat as needed. You will wind up with a topographical profile of your floor.

Answer 7

There is a hanging level tool, aluminum and lightweight enough to hang parallel on a strong string or wire. It is about 5 inches long with hooks on ends. It has the same tube with water to show when it is level. So, hammer an end of the string on one side of the wall. Have someone hold the other end against the other wall. hang the hanging level tool and ask your assistant to raise or lower the string until you see the air bubble in the hanging level lies at the center.

Answer 8

I'm a guy, and personally, I don't like long nails. I know a lot of like lengthy, painted nails cause they're "so pretty", but I don't like them.

Answer 9

A little bit of nail is fine... but I agree with you. Long nails are kinda low.

Answer 10

dosnt have to be if floor isnt so sloped that it causes floor to slide against the wall or crack open at the joints thats good enough im a flooring installer ive done thousands of feet of this stuff its very forgiving of a rough floor underneath