On 05/11/2018 12:29, GB wrote:
On 05/11/2018 12:18, Tim Watts wrote:

Mine span 3.5m and are 2x4" of 1950s grade timber. All 100+kg of me
can stand on a single one mid span with no flexing.

Whilst the timbers can take considerable loads for a while, they may
distort slowly from long-term loads (like the books you mentioned),
leading to a bowed or cracked ceiling.

I don't think the one inch chipboard increases the strength - it just
adds to the load!


15 mm OSB is far lighter and just as strong when screwed to all the
joists.

Downside is you cannot add more loft insulation.

I overlaid the trusses in my 1976 semi with 50x50mm timber 3.6
metres long at right angles to the trusses and raised up with
sections of 18mm ply about 12 inches long and 6 inches wide.
These are glued and bolted vertically and have a 2x2 slot at the
top end which carries the lateral 2x2 timbers which are about
600 mm apart carrying 15mm MDF sheets that were 600 by 600 mm
and discarded from a b&Q refit at 50p a sheet.

The 6 inch gap between the tops of the trusses and underside of the
MDF is filled with extra rockwool, running the same way as the added
50x50 timbers.