In reply to UKC Gear:
As Sans-Plan says, if you are going to use the loft for more than storing a few boxes, then you need proper floor joists. We had to have two steel RSJs fitted either side of the house then 10 ins joists were laid across those. The original ceiling joists are still there underneath holding the ceiling up. But a separate isolated structure in engineering terms, so they don't crack. If you plan to build a gym in the loft, you also need to conform to Fire Regs.
https://www.which.co.uk/reviews/loft-conversions/article/loft-conversions/l...
Seldom quite as simple as you think.