How level does a sub floor need to be to have wood flooring laid on to it? I have had my concrete floor screeded with sand and cement mix covering an underfloor heating system. I had hoped for it to be completely smooth and level, smooth enough to lay engineered wood flooring directly to it without any underlay (although with a damp proof membrane) as underlay will stop the effective heat transfer. There are a few localised holes and low spots that i can deal with and a few areas where it is out of level by +/- 5mm per metre. Didnt really want to put anything else on it because height and floor thickness are critical. Was i expecting too much or should it be better and is it good enough for laying the floor?
+/- 5mm per metre actually means up to a 10mm variation over every metre. This is I am afraid much more than most wood manufacturers would allow. I seem to remember the old standard was +/- 3mm over 3 meters but every wood company seems to be different.
The problem is that the wood flooring is designed to be as flat as possible and when you have an unlevel floor all the wood will do is sit on the high spots.
I would advise using a levelling compound to level the floor (Your only filling in the low spots anyway) and then stick your wood to the levelling compound. Don't forget the DPM though, either above or below the levelling compound depending on the manufacturer.