Cutting through entire joist

[omph]

I need to run some pipework for a kitchen extractor fan. The problem is there is a joist directly above, so drilling for the pipe will require cutting through the entire joist.
The joists are 6x2 placed 400mm apart, and are supporting two ceilings (false ceiling). Roof is a flat roof. We have ripped out roof and replacing with 18mm osb + 1.3mm epdn, so now is a good time to make any adjustments to accomadate the pipework.

 

[phatboy]

If you treat it in the same way as a rooflight, and double up headers either end of the pipe and use a couple of joist hangers, you would be fine.