Loo cistern in attic /loft

[Dee]

@nickfromwales.....how much can I notch out the top of 2x4 joists? 

These joists only support a plasterboard ceiling and the notches are needed to provide fall for a poo pipe 

[PeterW]

Not much ... although you could hang the lower joists from the upper ones to stop any movement. 

[Nickfromwales]

49 minutes ago, Dee said:

@nickfromwales.....how much can I notch out the top of 2x4 joists? 

These joists only support a plasterboard ceiling and the notches are needed to provide fall for a poo pipe 

You can take more out if you sister each joist with plywood, one each side of each joist. You'd need to fit these the same height as the joists and about 1200mm long. Glued with proper resin based waterproof wood glue, clamped and screwed every 100mm an inch down and inch up ( so two screws at each 100mm interval ) which will replace almost all of the strength in the section  you have cut away. 

Pre cut the plywood according to the notches you've made in the joists . 

Hanging them off 'binders' as @PeterW suggests will also help with the worst affected ones but not needed all the way if only taking half inch or so out of start / end of the run. 

 

[Nickfromwales]

Inch ( 25mm ) plywood would be best. Local yard may have a saw and be able to rip it down for you. 

[Dee]

Oh poo!

 could I double up 12mm ply?

It is only taking the weight of existing pb and the new poo pipe! 

So,  is the strength of a joist compromised that much by notching? I've seen some massive chunks taken out and holes drilled! 

[Nickfromwales]

@Dee

If it genuinely is only performing the duty of ceiling rafter, not floor joist, then whack away ma'am. A couple of binders ( timbers to connect upper joists to lower rafters ) will be a belt for the braces, and if the above is correct then the ply is unnecessary. 

[Dee]

Definitely only supporting the ceiling below so I feel confident it won't collapse! 

[Nickfromwales]

I think a few binders for the worst affected rafters would be a good idea, and no more.. Try and lay the soil pipe on some 50mm fluffy rockwool to offer some sound deadening to the room underneath. It'll compress down to 5-10mm.