I am installing (floating) 3/8" engineered hardwood with 1/8" QuietWalk underlayment on a cement slab. I have used a grinder and leveling compound to remove the large valleys and hills. Now I have occasional small valleys I've found that are around 1/8" or less deep at their deepest points and span 1 or 2 feet in diameter. What kind of tolerance can I allow given I have a 1/8" underlayment?

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It usually says on the packet, but if there is a 1/8” dip then the floor will flex when someone like me walks across it - I am 110kg.

You may want to consider finishing the underfloor surface to a better standard, like 1/16” as the underlay will compress on the high points and still have 1/8” gap at the low points...

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