I've got a flat on the 3rd floor of a Victorian building. It's great but the floors are sagging and creaking. The rooms are large generally 5.5m x 5m and 8m x 5.5m so I reckon the joists are under quite some strain. They bounce quite a bit. I really want to get it sorted but I've not got access from below and cannot damage the ceiling below as it belongs to someone else.
If anyone has any advice or can point me in the right direction I'd be really grateful.