Pecanfan wrote:

Mmm... but, how can I insert a temporary joist parallel to A but 6" away
from it without cutting B first (and our neighbour's ceiling falling down)?
Or have I totally missed the point? :-)

Oh hold on, do you mean a temporary joist above the floor and strap it onto
B to hold it up temporarily while I cut the end of it?

Yup - that is what I was thinking...

Wouldn't you still be relying on a strengthened Joist A to support 1/2 of
the entire floor weight?

That is not far from the situation you currently have (looking at your
original diagram) since the floor joists hang off B which in turn hangs
of A - so A is taking half the full floor load, plus possibly half the
stair load (the wall at the other end of B taking the other half of the
floor load).

If you double the strength of A, then one ought to expect it would be
able to take the additional weight of the other half of the floor. In
reality it will not be as extream as "other half", since if your drawing
is roughly to scale, thent the stair area is less than the existing
floor area, and also you would be loosing the load of the stairs.

--
Cheers,

John.

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