Why aren't even flat roofs made with a little slope? I have a bitumen roof and, although it is not leaking, it does have a bubble/lump. It is only about 4 years old. Should I be worried?

all the flat roofs i do have a gradient on the top ,this is achieved by applying furring pieces say 4-1 on top of joists this way the roof has a slope on the top and is level underside where the internal cieling is,But your average cowboy either wont know or wont bother installing these to save money ,like i say pay cheap pay twice no bargains in building ,just professionals who quote to do the job properly and most of the time seem like the most expensive where in actual fact the cheaper quote is cheaper because of something like this he had not priced for furrings which can be up to 300 pounds for an average flat roof

Substructure is correct with both the technical stuff and the comment on the type of tradesmen out there.If your flat roof is literally flat,then it has not been done properly.With regards to the other bit,when are people going to learn.I know for a fact that there is someone on here that hasnt completed ANY of his last 7 jobs and has quoted way under the price for the job to be done correctly on every one.Why some people spend 40K on a car but go for the cheap and get stung on their homes is something i have never quite understood.

very good answer agree with substucture