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Flooring over Concrete Slab

I want to install engineered flooring over a concrete slab, and understand the need to install foam underlayment before the flooring. The instructional pictures I have seen show using spacers between the first course of flooring and the existing wall to achieve the required offset spacing.

I have two issues that need to be answered: In older homes, with walls that may not be perfectly straight, how do I ensure that the first course of flooring is straight? Should I strike a chalk line? Secondly, for floating floors, how do I ensure that the first course does not move as I add additional courses? Does the underlayment prevent excessive movement between it and the flooring?

Thanks for any help!

A. Spruce
Re: Flooring over Concrete Slab

You can snap a few chalk lines to determine how square and straight your walls are. If they are severely out of whack, then you'll need to work from a chalk line and cut in your edge pieces.

Most of the time you can work directly off the wall. You use spacers along the wall to maintain the recommended gap while you build out the rest of the flooring. Once it's all down, remove the shims and install your baseboards or base shoe over the gap.

Re: Flooring over Concrete Slab

First you must determine if your slab is dry or moist. Check for cracks too. Don't lay any floor before you determine that.

If it's dry, you're good to go.

Use an underlayment recommended by the manufacturer. Floor muffler, 6mm plastic, foam or other.

Check for squareness, like recommended by Spruce. If your walls are not 90 degrees perpendicular, the first course has to be cut to match. To do that, lay the second course perpendicularly to the side walls and measure the distance between this course and the wall, then cut accordingly. Allow for shims (1/4" or so).

From this point it's smooth sailing. Finish like Spruce says.

If you do it yourself, wear knee pads. If you don't, you will very quickly find out why.

Re: Flooring over Concrete Slab

when i install floating floors i will typically put together 3 or 4 rows away from the wall then slide it all over as a unit to the spacers, this way it will stay straight when you start working against the shims straight wall or not

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