I was just going to say something similar, even a second piece of timber
suitable attached across the weakened point might be enough. It very much
depends on the place and design of the building I guess.
Brian

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DerbyBorn wrote:

the regs tells me that in a 20cm Joist the max hole can be 32mm I
need to pass a pipe of 42mm diameter!

Reinforce the joist by bolting metal plates to both sides?