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!