Pipes have been in there for some time. There is evidence of a leak but I think this was from the shower. Mains pressure cold and low pressure hot, everything seems water tight, allbeit not that well put together. Also seems to be gas running accross too.
To be honest Id be inclined to glue and screw 2x2 to the top of the exiting joists and re-deck in 18mm wbp but I havent go the hieght to play with, door also opens into the room making this a no go.
So I suggest to the customer that boards need to come up, add in 4x2 where I can and then re-deck with 18mm wbp. However what if after all this there is still deflection?