In message , Andrew
writes
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 ?.

No:-)

There is a 3mm foam underlay and the pipes have aluminium spreader
plates.

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

There are lots of ways to do this. I have to use an overboard set up
because the builders used random joist spacing.

Is there a photo ?

I'm taking shots as records.

--
Tim Lamb