A common method of ventilating a roof is to make openings in the soffit and ridge to allow natural air flow. This example also has ventilated exterior walls called rainscreen construction.
Most building materials are permeable to water vapour; brick, concrete, plaster, wood and insulation all can fall victim to interstitial condensation, this is why UK Building Regulations require roofs to be ventilated, either by the use of soffit vents, ridge vents, or replacement ventilation slates or tiles.
Sheet metal in the form of copper and lead has also been used for many hundreds of years. Both are expensive but durable, the vast copper roof of Chartres Cathedral, oxidised to a pale green colour, having been in place for hundreds of years. Lead, which is sometimes used for church roofs, was most commonly used as flashing in valleys and around chimneys on domestic roofs, particularly those of slate. Copper was used for the same purpose.
In high wind areas, such as where a cyclone or hurricane may make landfall, the main engineering consideration is to hold the roof down during severe storms. Every component of the roof, as of course the rest of the structure, has to withstand the uplift forces of high wind speeds. This is accomplished by using metal ties fastened to each rafter or truss. This is not normally a problem in areas not prone to high wind or extreme weather conditions.
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