As you've had no expert replies, I thought I'd chip in with my
experience. I boarded my loft with the same sort of chipboard. It's in
a victorian terrace, and the joists are about 4" x 2". I was young and
naive and didn't think about load-bearing problems until half-way
through the job. I remember reading in the uk.d-i-y archives that my
ceiling joists were just about strong enough to hold up the ceiling and
that was it.

However in flagrant disregard of all advice I've gradually accumulated
quite a lot of weight up there, including a couple of disused kitchen
cupboards, one of which is now half-full of books. I've certainly got a
lot more up there than I theoretically should have, but the (new)
ceilings below have not even cracked.

Having said that, I have put in shelving with hefty brackets fixed to
the two party walls to take some heavy stuff to keep weight off the
joists.

Making a "room" with a loft ladder where we can put the junk and close
the hatch is one of the most useful things we've done here in terms of
keeping the habitable space habitable. Just don't don't invite your
friends up there for a party and start dancing the can-can.

Novice DIY'er wrote:
Hello all,

Continuing on from chopping up redundant water tanks, I'm now progressing
with flooring the attic area. The self doubt has crept in now with the
construction of the roof trusses and the load they could potentially take!!
Just using standard chipboard panels that span 3 'joists'/trusses with
screws to hold in position but have, so far, only done the centre (Of the
W). Now, the 'joist' dimensions are approximately 3 inches by 1.5 whereas
the area where the watertanks are is double that at least. This now leads to
the question, what could I safely store up there, weight wise, or are is the
chipboard heavy enough!!! I'm 16 stone and been crawling around up there so
I'm assuming my weight can be safely stored (If the other half had her
way!!)?

Any hints, tips or pointers greatfully received. And yes, the area where the
water tanks are/were located is probably the safest area.

Harry