Few homeowners give the roof much attention until rain begins to leak through. However, immediate action is absolutely necessary if rain or snow starts to penetrate a roof.
Periodic inspection of a roof can help you identify minor problems and correct them before they become major issues that cause serious damage.
A sound understanding of the structure of a roof is important. It’s also important to realise that the inspection and repair of a roof can be dangerous and is often best left to experts, especially as tile and slate roofs can be quite slipperyas they often have steep pitches.
Because a roof often includes a range of angles and protrusions, its surface needs to be weatherproofed and this is usually provided by the flashing. The flashing serves as a drip edge along a roof’s eaves and rakes, a collar around ventilation and plumbing pipes, a valley between two planes of the roof and steps along a chimney.
Great variation is seen in the size, shape and material of roofing. Overlapping layers of asphalt shingles, wood shingles or shakes, or tile often characterise a traditional sloping roof, although these may also be covered with galvanised steel, slate or aluminium.
Source: http://funkybunny.hubpages.com/hub/Understanding_the_roofing_structure