Well the others have prove to me it's okay but I'm not sure because the way hardboard is made and not waterproof but I have found this from the flooring company.
Timber floors move. Whether it is individual floorboards or sheets of tongue and grooved chipboard, it moves. Movement underneath any fixed covering will cause problems as the timber will move at a different rate to the covering. This can cause gaps, tile lifting and even cracking. The existing timber floor can only be fixed where board meets joist underneath and these fixings are not generally enough to stop the floor moving. It is normally necessary therefore to fix a sheet covering of some kind, at short, regular intervals, to the existing floor.
We do not advocate the use of hardboard in these circumstances. Many people will tell you to soak hardboard overnight, lay and fix it when wet. When it dries, it shrinks back to stop it from bowing in the future....Our 30 years experience has taught us this does not work! The fixings used for hardboard are not substantial enough to keep the board tight. They either begin to pull out, allowing the board to move and bow, or, if larger headed nails are used, they work loose and can be felt, and even seen, through the covering you have installed. Our solution, as with most flooring companies these days, is the installation of 6mm plywood over the existing board. As mentioned above, this, together with the tiles, increases the depth of your floor, so it is better if the floor can run under the skirting.
