On Wednesday, 26 June 2019 18:17:47 UTC+1, DerbyBorn wrote:
I need to bore a hole in a joist to take a shower waste. Reading the regs
tells me that in a 20cm Joist the max hole can be 32mm

I need to pass a pipe of 42mm diameter!

The joist is right next to a wall at the outer edge of the house - load on
it will never be high.

Pragmatically - would you go ahead and bore the hole?

Glue and screw an extra bit of wood on to the joist spanning the hole. PVA would do the jpb

Drill the hole through both joist and wood.