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.

--
Ian White

Ian White wrote:
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.