Floating floor on concrete - how much ripple?

I'll shortly be laying a floating floor of solid hardwood onto an existing concrete floor. The boards are oak, 18mm thick, 150mm wide, T&G in random lengths, and they will be laid over a combined DPM-foam underlay. The intention is to glue the boards.

By all normal standards it's a very good concrete floor - dry, and perfectly smooth and level for walking on. However, a long straight-edge reveals some very gentle ripples in height, maybe 6mm maximum variation over 1 metre.

Will this be tolerable? I don't have a feel for how well the boards will follow the contours of the floor - especially in open areas where they won't be weighted down by furniture, but will be walked on.

There may be some limited areas that would benefit from self-levelling compound, but there doesn't seem much point in covering the whole floor, since that would probably introduce more ripples than it solves.

Obviously this is all to avoid the nightmare scenario of a floor that has been glued into a solid sheet... and then sags or creaks underfoot.

Any advice based on practical experience would be very welcome.

I levelled mine first, and even so, wish I had levelled it more..

I would honestly recommend you get some levelling compound, and trowel the while thing as flat as you can..6mm is far too much - 1-2mm over a meter is better, and no more than 6mm over the whole floor.

Lay battens over the whole room and mark the high pints and draw circles round the hollows with a felt tip pen. Then mix up the compound after PVA/ing the lot, and use battens from the high points to scrape the compound level, and float smooth it.

Then when set, do it all over again.

THEN when the floor goes down - which is pretty quick - you will be rewarded by a total lack of flexure.

By the way, with actual boards - I used an engineering oak laminate from Kahrs - you will get as much as 2% expansion/contraction across the grain of the wood, and about 0.2% along the grain. This is a lot..on a 4 meter room thats 80mm overall..or 40mm per wall.

Since you are laying in winter..leave the boards inside for several weeks, and they should be at the most fully shrunk..so leave as wide a gap around the outside as you can. I am not a great fan of cork strip..better to use a deep oak skirting and 1/4 round oak if necessary, but do leave at LEAST a cm, better two, along the edges that run parallel to the boards. If its possible to remove the plaster from the walls lower down to allow expansion room, and still have a narrow skirting, this is bets of all.

On the long runs, along the grain its far less critical and 1/4" should be adequate.