Making a shower in a cupboard over trimmer joist

[rcs]

Hi
Amongst many alterations that we're planning, one that is causing us a little headache is how we can maximise the use of a huge cupboard in the bathroom. This cupboard sits on a trimmer joist and extends over the stair well of a 1930's 'U' shape stair, currently accommodating the hot water cylinder.
We figure that we can joist on top of the trimmer, 800mm over the stairwell to the supporting wall opposite and have a shower in there. We would obviously have a raised shower from the floor, the height of the new joist, minus the existing floor board, plus what ever goes on top.
Could anyone tell us what the minimum headroom is above a stair, what size joists we might need, what size stud wall we would need between the shower and stairwell and if we would need a shower tray (custom size?) or could we use an alternative. I've seen fully tiled showers on timber second floors buildings with no shower trays in NZ. Is this OK in the UK?

Many Thanks
RCS

 

[noseall]

the minimum headroom required from stair tread peak to ceiling is 2m.

a 4"x2" will comfortably span a metre. use 4"x2" timber for the stud walling and insulate the void with a quilt type insulation, affording you a little sound insulation. fix ply or tile backer board to the walls and use an armacast type shower tray bedded on 18mm minimum ply.

i would only recommend using a 'wet room' type system on the ground floor, though there are tanking systems available that will work for your scenario.

 

[rcs]

Hi Noseall, thanks for the reply. Can you get custom made shower trays or are they all standard. If I fit anything bigger than 780mm, the headroom over the stairs will be a little too close. I plan to push the wall out another 10" about 10" above the shower tray to maximise the space as this won't be a headroom restriction in the stairwell.
Also, what would be the best way to attach the joists into the supporting wall? Can you fit joist hangers in an existing wall?
Cheers