New floor boards over existing ones

Hi,

After reading the relevant threads on this forum and the flooring manual from www.timber.org.au, I still have questions about laying new floor boards over the existing one. Before I ask the questions, here is the situation.

The original house was extended ages ago and as a consequence about 70% of the floor boards are 85mm wide and the rest 130mm or so. During the existing renovation few wall have been moved around, leaving openings in the floor. The openings can be patched, but the floor will look patched. One patch may be tolerated, but there would be quite a few of them, so that is not an option.

The remaining two options are to replace the existing floor boards with new ones or to lay the new floor borads over the existing ones.I prefer the second one. I'd like to lay 85mm x 19mm Tasmanian Oak over the existing hardwood floor.

The plan is to check the condition of each of the existing floor boards and put a screw or two where required to make it nice and solid.

The next step would be to rough sand the floor and when it's all clean re-check again if it is level. So far so good.

As for the following step, I am not sure which way to go.

The flooring manual basically says the boards can be laid either perpendicular to the existing ones (if they are solid and sound) or parallel to them. If they are laid parallel, the middle of the new borads should overlap the joints of the old ones. There is no mention of plywood underlays.

On the other side, there is the advice to lay playwood underlay before the new floor boards. And there is some debate whether it should be 4mm or 12mm.

I am really confused and don't know what is the right way to go. Can someone shed more light on the situation?
I really need to know if underlay is required or not and if it is required, what material to use and how to install it.

Thanks in advance,
ptt102


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