A simple rafter roof consists of rafters that the rafter foot rest on horizontal wall plates on top of each wall.The top ends of the rafters often meet at a ridge beam, but may butt directly to another rafter to form a pair of rafters called a couple. Depending on the roof covering material, either horizontal laths, battens, or purlins are fixed to the rafters; or boards, plywood, or oriented strand board form the roof deck (also called the sheeting or sheathing) to support the roof covering. Heavier under purlins or purlin plates are used to support longer rafter spans. Tie beams, which may also serve as ceiling joists, are typically connected between the lower ends of opposite rafters to prevent them from spreading and forcing the walls apart. Collar beams or collar ties may be fixed higher up between opposite rafters for extra strength. The rafters, tie beams and plates serve to transmit the weight of the roof to the walls of the building.
A roof is part of a building envelope. It is the covering on the uppermost part of a building or shelter which provides protection from animals and weather, notably rain or snow, but also heat, wind and sunlight. The word also denotes the framing or structure which supports that covering.
Condensation within the roof space is much more of a problem today due to: much less fortuitous ventilation due to tighter building envelopes with high performance windows and door and no chimneys leading. This tighter envelope means the air temperature in buildings has risen, the warmer the air in the building is, the more water vapour the air can carry.
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.
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