"AlexW" wrote in message
...

One more thing, if I can trouble you further...

I was planning to screw the ply down at intevals at the same width as
and centered to the existing boards across the width of the boards.
Replicating the same intervals along the length seems a bit like
overkill ... what do you think is a reasonable spacing?

I've glued 6mm ply sheets onto waterproof chipboard. Whether this is
suitable for you I don't know.