I want to run two pipes along under our landing floor (which
means going through all the joists).

Joists are 180×50mm Unfortunately there are
already notches and holes, with the result that the calculator
he

http://www.diydoctor.org.uk/projects...ist-calculator

which mostly agrees with the wiki says there is only room for
one more hole. But it also says that the maximum hole size is
45mm, which means that I could put the two 22mm pipes through
one big hole. I dont want to do that because one is hot and the
other cold. So my question is, can I put two 22mm+minimal
clearance holes say 66mm apart (3 diameters as per the wiki) and
still meet regulations?

--
Jón Fairbairn