you wont know what will happen untill you remove the support but you know what i suspect will happen
also notice the crued filing in of the open risers[the bit between the tread backs ]
i would expect it to settle fairly level indeed i would go as far as to say without anybody on the stairs measure the drop required to make it level
if its say 18mm get yourself a bit off timber 18mm shorter than under tread to floor screw tight to the tread [18mm off the floor] as a tenporary support untill handrail and infill panel fitted
the easiest way to fill in the steps is mdf same thickness as the gap in the risers with angle holes to underneath the treads and a 2" wide strip to overlap from the riser to the mdf
from the latest pic i think the stairs themselves are the problem.
they are not what i would call traditional - they are an open stair. i don't believe this type is a strong as the traditional type ie with treads and risers (the risers here have been added after manufacture).
the side nearest the wall has received additional support by being attached to the wall. the other side has no support and is why it's bowed. i don't feel the cupboard has done anything in fact it's been fitted too late (bow had probably happened within 3 mth of being installed) to provide additional support.
in terms of what to do - for me live with it.
Okay thanks for everyones help. Filled the gaps in the treads today with the MDF and ordered the Axxys stair-parts so watch this space for the finished picys...wood can go anywhere if it's not good quality (seasoned to let the moisture dry out under weighted conditions) or assembled well (to hold it in a fixed intended position).
i'm trying to say it's not a top notch stair and i myself would move on and forget about it.
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