to level joists or not bother

[mazears]

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?

 

[joinerjohn]

Got to be worth it levelling the joists up if your going to install a shower tray. These have to be level as you have to tile after the shower tray is put in. If it's only 15mm out I'd still fix small strips to the top of the joists to level them up.

 

[big-all]

assuming its a constant slope accross the whole width if its with the joists then use 3mm harboard in 3mm increses accross progresive joist will be within 2mm if at 400centres

if its accross the joists then 3mm strips accross the joists at 15 inch gaps
3mm at 15"- 6at 30 -9 at 45 12 at 60 and 15mm at 75"
your hardboard is still likley to be 3.175 so may finish up at 16mm

you could also add strips along the joist topped with no nails to fill the gap but no intermediate fixing untill the no nails goes off to stop the ply pulling down into the gap

 

[mazears]

Thanks big-all. nice detailed solution although i'm now doing it differently. I've pulled floor up and am in process of levelling joists individually which seems to be working. I'm loosening mortar above and around joist end and using acrow prop to lift them each then pack under.had to lift some more like 25mm!. Room will be more level one way although the joist just outside the bathroom door (landing) will end up lower now (10mm step) as i've slightly raised all joists in bathroom. Can't lift this as wall is built on it.. Maybe extra underlay at this point! More level floor but with step up or non level floor and no step - don't really know which is best???

 

[ladylola]

I've done my fair share of joist leveling in my time and it's not the most rewarding of jobs but generally the end result is worth it.

There are exceptions, however, such as a period property where a perfectly level floor will lose all of it's character.

A couple of years ago I worked at Wollaton Hall in Nottingham. At the top of the building is a room called the Prospect Room which is of a chinese lattice construction, this means that no joist goes from one wall to the other. However, it also mean't that it had sagged quite considerably something like 10 inches in the centre of the room which was about 35 feet wide if my memory serves. We levelled it using a special ply that had grain running the same way on all the layers (I forget the trade name). However, when it was all complete and level it had lost much of its charm I feel.