I hope somebody can help me with this, my wife has bought a bow fronted bath and wants all the pipework hiding (i.e. under the floor). This means i will have to drill throught the joists. Can somene please tell me if there is a maximum weght a joist will take or how do i calculate it. I have been told the maximum weight of the bath with water is 275Kg and, we also have marble floor tiles which are not light so i'm worried about the joists bowing.
This isn't something you can work out yourself. It is far more complicated than you think.
Generally, you don't cut joists at all near or under a bath. Probably if the joists are 6 x 2 and close together, you can notch the tops to accept 2 x 15mm pipes. If it is the 40mm waste pipe you are considering, then that is a no no anywhere at all. All waste pipes must run above floor level or parallel to the joists.
First you don't drill through joists for pipes. You notch the joists BUT there are rules:
1. Notches should only be made in the top edge of the joist.
2. Notches should be no deeper than 0.125 times the depth of the joist.
3. Notches should be no closer to the structure supporting the joist than 0.07 times the span of the joist.
4. Notches should be and no further away from the structure supporting the joist than 0.25 (¼) times the span.
3 and 4 are to avoid the area of maximum shear force and the area of maximum tension in the bottom of the joist.
Under normal circumstances these guidelines are sufficient to ensure that you do not jeopardise the structural integrtity of the property. However, in your case the imposed load is not insignificant. Your marble floor tiles will be iro 20/m2 without the substantial weight of the bath.
Undoubtedly you'll need some additional strengthening but whether nogins will be sufficient or sistering the joist is appropriate I'm not qualified to advise you (just in case you rely on that advice!!!!).
A read of Approved Document A would seem appropriate and you could do worse than consult your local BCO to see what they say. However, it sounds as though some proper structural calculations by a professional are in order for you to be absolutely certain.
Notching joists is extremely bad practice but is obviously much cheaper than doing it properly. If you are laying pipes then try to lay them parallel to the joists and if you have to through them go through the center as this is where the stresses are least.
However, this is usually hard to do due to working space and any tradesman would notch them anyway. Just be prepared to touch up the cracking in the ceiling below.
If you were really keen you can double up the joists directly under the bath (and then notch them) and you can sleep peacefully. I have a feeling that building regulations suggest this anyway.
Do it wrong though and your wife will never let it go and will tell all of your friends!