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Thanks for your replies.
It is a truss roof. The rafters are much thicker and end on much thicker joists(13"x3"). There are some thinner vertical supports between the rafters which end on the 2x4...

Ive just seen other similar questions so Im going to measure the span of my joists and post them...

I want to strengthen the attic floor for use as another room (possibly bedroom). The joists up there now are 95mm x 42mm. I am quite confused by info found online about what I would need. To sister...