On 18/11/2017 20:25, Tim Lamb wrote:
I have now reached to point of laying 12mm plywood over the underfloor
heating grooved chipboard.

Any suggestions for suitable screw spacing?

?None. Can you not glue the overlay onto the chipboard ?.

Why are the heating pipes in grooved chipboard ?. Don't
people normally use grooved insulation material with
metal spreaders to transfer the heat ?.

Is there a photo ?