convert a polycarb flat roof to a single ply roof

[dilly]

Hi my parents have a extention that has a poly carb roof at the moment and they want to put a single ply roof on it,the extention is single brick and the joist are 5x2 and they are around 600 apart.Is this going to need extra joists putting in as i want to put 18mm sheets on it and then get someone to put a covering on it,or is there some other way of strenghtening the structure if it needs or will i be ok leaving as 8s and decking
thanks

 

[JohnD]

you have told us the height and width of these timbers, and their spacing. But how long are they, unsupported span between walls?

 

[dilly]

knew i forgot something its 3m