Is it possible/allowed to drill a joist to run 40mm pipe through. The joists in question are about 8 inch by 2 inch. I would imagine that some form of steel plate reinforcement would be required. Any advice appreciated. Thanks.
You will probably need at least a 50mm hole to get a 40mm bore pipe in comfortably, especially if you need clearance to feed the pipe in at an angle from above the floor. If you've more than one joist to drill you may get trouble getting a fall on the waste, as holes should all be on the centre lines of the joists.
If you don't get the fall (1 in 40 recommended) the waste may be prone to blockages under the floor where you can't get at it.
I thought about doing this myself, but ended up putting the waste against the wall, and boxing round it with a piece of skirting board packed out with a 2 1/2" batten at the top.
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