I have been planning this for some time but circumstances have changed a
bit so I need to work out a plan B.
I need to add a floor to my loft, clear of the ceiling. The ultimate
use will be storage but first it has to serve as a working platform for
sorting out some issues up there. Plan A involved fitting wall plates
(ledger boards) around the walls (there's an 18 inch wall at each eave)
then taking the joists in through the roof after stripping tiles and
before replacement. Unfortunately, the roofing job has been delayed a
bit so I want to get a working platform over part of the space ahead of
that, which will happen next summer.

There's no way I can get a 4(+)m joist into the space from indoors due
to the angles on the stairs etc. It occurs to me though that I could
get, say, a 3m length up there, so I'm wondering whether I can double up
two lengths, bolted together (ie bolts, coach screws, serrated washers
between, the works) once on the top floor. (This loft is above the first
floor but accessed from the second floor, so there's space to assemble.)
The result would be 3m then 1m in one direction and 1m then 3m in the
other. Where I'm struggling, however, is in trying to rationalise
whether this would be stronger or weaker than a single intact joist.

The interweb doesn't seem very useful as information ranges from
exhortations never to join joists, ever, to one site that said a simple
scarf joint would be fine (without doubling up or anything!). Certainly,
there are plenty of sources recommending repairs of single rotten joists
by bolting a short length alongside a butt join between old and new.
Similarly, metal splice plates are available, but I'm not sure I would
trust these without some support from whole joists flanking the join.

Just to be clear, this would be a temporary arrangement and full length
joists to be installed later when the roof job happens. I'm just
looking at making something to work from in the short term without
taking a dive through the old and fragile lath and plaster ceiling. I'm
also well aware that it's an expensive solution but sometimes it's worth
spending a bit to move things along.

Any informed (or entertaining) views on this chaps?