There are many different roof structures to choose from when designing a building some are easier and cheaper to construct than others. In days gone by there was a standard roof construction that was used but now there are many choices.
The pitch of the roof is particularly important and varies from country-to-country dependent upon the weather and rainfall in that country for instance in the UK where we experience a lot of rain the roof needs to have a certain pitch to enable the rainwater to flow away adequately whereas in Mediterranean countries with little rainfall the pitch of the roof does not need to be as steep. Trussed roofs are designed with an appropriate pitch but although a common roof structure does not give much internal loft space.
Flat roofs have traditionally been used for extensions mainly because they are cheaper and easier to install however the life of a traditional felted flat roof is only about ten to fifteen years so it will need regularly replacing.
A relatively new roof design that is proving popular in modern builds is the curved roof. These are constructed from metal such as copper and are mainly used to give a striking appearance to a property.