running plastic heating pipes across joists

[ajrobb]

I have the ground floor ceilings down. Should I run 15mm plastic central heating pipes just under the floorboards or should I run them through the centre of joists at 400mm centres? I found this summary elsewhere:

• Holes should be no greater than 0.25 of the depth of the joist.
  Holes should be drilled at the neutral axis.
  Holes should not be less than 3 diameters (centre to centre) apart.
  Holes should be located between 0.25 and 0.4 times the span from the support.

The longest straight run would be just over 8m. Would expansion be a problem?

 

[sooey]

Seeing as you're using plastic and the ceilings are down the easiest way will be to drill the joists.