My upstairs bathroom floor joists slope downwards about 15mm across a 6ft span. As ive taken off floor boards ready for new ply, is it worth me bothering to level the joists. I say this as i will be putting in a shower tray which i thought was supposed to be placed on a level surface for water to drain away correctly - or can i just level the shower tray instead (bath i believe can be levelled with feet)? If levelling joists is best (although alot more effort) is it possible to crank them up individually from beneath using acrow props and packing under each in turn? Am i likely to cause more problems with the adjacent rooms plastered ceiling?