(Long post - sorry)

This year I had a double-storey extension done on my house. It was
started in April, finished in August.

Part of the extension includes an en-suite shower upstairs, and in
October I got round to tiling the floor and installing the shower.

However, since then, it has become obvious that the upstairs floor is
sinking. By looking at the level of the floor tiles against the
skirting boards (screwed to the wall) I can see that the floor in the
corner of the en-suite has dropped by about 6-7mm so far. While this is
not a huge problem with the floor (the grouting is still intact),
unfortunately the shower tray has also dropped by a few mm and the
waterproof seal along the edges is starting to crack and break as a result.

As far as I can tell, the settling is not happening to the same degree
across the whole of the extension's first floor. I haven't got an easy
way to tell elsewhere (no flooring yet!), but I am reasonably sure that
any other settling is only around a couple of mm, if at all.

The floor's construction is pretty standard - 2x10 (or 12? not sure)
wooden joists, laid on 40cm centres, one end bedded into the inner
blockwork leaf of the new wall, the other end suspended on joist hangers
from the existing wall. The end that's dropping is the one bedded into
the new wall. The flooring surface is sheets of T+G flooring grade
chipboard, screwed to the joists at approx 30xm intervals.

Inspecting the ceiling on the ground floor below (plasterboard screwed
to the underside of the joists) shows no signs of the ceiling dropping
or any other movement.

So as far as I can tell, this floor movement can only be explained by
some of the joists actually shrinking by about 2-3% on their vertical
axis. What do others think? Is this normal, and if so, how much
shrinkage can I expect? How long will it last - i.e. how long should I
wait before starting to repair the tiling and shower tray waterproofing?

--
Tony