The design of the roof and preparation required to secure it to the main structure will be detailed on your construction drawings. The roof was first considered when foundations were poured, and it was at that very early stage that the importance of wall strengths and their relation to the roof became apparent. That initial assessment should have enabled you to arrive at the current stage of construction with a suitable and solid base on which the roof can now be placed.
There are two main types of roof and the one you have chosen to build is likely to reflect how you intend using the space it will provide. Traditional roof designs offer greater flexibility because they are constructed with substantial timber beams, whilst the lightweight trussed rafter type is easier to lift into place.
The basic principle of the traditional roof construction is to bring load-bearing cross walls up into the roof space to support horizontal running purlins . These are large section timbers onto which the rafters are fixed at regular intervals, to support the roof coverings. When large spans are involved, the purlins can be replaced with lightweight steel beams, and timber is then placed on top of these, so that rafters can be fixed.
A trussed rafter roof is the lightweight alternative, and ideal if the void is only going to be used for minor storage. This type of construction relies on an array of small bracing timbers, rather than large single beams. The amount of timber involved virtually fills the void, making it fairly useless, but the frames can be lifted into position easily. Once seated on top of the walls, the frames are connected using galvanized metal plates, and diagonally braced according to the manufacturer's instructions.
Roof frames can be created in a plethora of different shapes and materials and each one will have its own set of pertinent calculations for load bearing and fixing. Your architect will have recommended a particular type of construction based on cost, style and function, in accordance with local planning restrictions and building regulations. The profile of your roof may therefore already have been determined by earlier limitations or constraints forced upon you by external authorities.