I need to cut a 40mm notch into the top of a 180mm deep 50mm joist for a wastepipe; is the "don't notch more than 1/8th depth" rule unbreakable? I can't drill a hole through the centre (I know the tolerances are greater with holes) as there are other pipes at this centre point. Thanks.
The size of your floor joists is governed by building regulations. Reducing the size of the joist by any more than the stipulated amount (also governed by regulations) is foolish in the extreme, especially in a bathroom. See our project on timber repair-beam strength and you will see an easy way of strengtheniung a beam or joist which should allow you to put pipes through. Please talk to the comapny first though as this is not just a way of cutting out large chunks of joist, calculations still need to be made.