White asbestos is a compound of magnesia and silica. It is most often used form of asbestos. It is used in roofs and ceilings of industrial buildings and even residential buildings.

Flat terraced roofing is of various types. These are mud-terrace roofing, lime-concrete terracing, Bengal terrace roofing, brick-jelly roofing, lightweight flat roofing, etc.

The figure given above is of a collar beam roof. In this figure, A represents ridge piece, B represents rafter, C represents wall plate, D represents collar beam and E represents eaves board. The collar beam is provided at a raised level.

Aluminium alloy and steel decking is suitable up to a load of 0.75 kN/m². In this, width of corrugation varies from 450-900 mm, depth varies from 25-85 mm and length is up to 10 m.

There are basically two elements in a double roof. These are rafters and purlins. The purlins provide intermediate support to rafters.

Tenon and mortise joint is used to join king-post to ends of principal rafters at its head. A three-way wrought iron or a mild steel strap on each of its sides are provided to secure joint.

An overlap of at least 18 mm should be provided when country tiles are used. These tiles are liable to break easily and hence y require frequent replacement.

Steel roof truss is designed in such a way that it carries only direct stresses and not binding stresses. Direct stresses can be eir compressive or tensile. Steel roof trusses are usually made of angle sections and plates.

Wall plates actually connect wall to roof. Whereas, when two room surfaces meet toger and form an internal angle, a valley formed.

Some of disadvantages of flat roofs are y are not appropriate at places of heavy rainfall, ir self-weight is very high, and location and rectification of a leak in a flat roof is difficult. However, requirement of a lesser area of roofing material in flat roofs is an advantage.