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Figure 3.14.1a. - 1

Roof construction to buildings under PG I usually use timber rafters and purlins for tiled roofs. Rooflights of combustible materials are usually used for lighting purposes. Generally, buildings under PG I are small in area and low-rise. For semi-detached and terraced houses, the separating walls would be brought right-up to the underside of or above the roof coverings to prevent spread of fire.

Figure 3.14.1a. - 2

Roofs of building under PG II may consist of dome, flat, pitched roof and other types. Roof covering may include mixture of combustible and non-combustible materials, e.g. slate, clay tiles, glass, rigid plastic. It is quite common to find hardwood timber trusses, purlins etc. being used in the roof construction. The term “roof covering” is used to describe construction which may consist of one or more layers of material, but does not refer to the roof structure as a whole. Roofs which may have different construction can be broadly grouped under the following:

a. Roofs over common corridors and means of escape. As these areas are meant for escape, it would be preferable that they be constructed totally of non-combustible materials.

b. Roof over apartment/maisonette or penthouse units.

c. Roof over linkway between buildings.

d. Roof used as terrace or floor open-to-sky. The roof covering should be of non-combustible materials to eliminate any fire risk arising from a fire involving the covering.

e. Roof over balconies

Roof can also perform two functions:

a. to contain a fire or prevent its penetration from an external source; and

b. to ensure that the external covering does not contribute to spreading of fire so that an adjoining roof might be in danger.

RC roof would be able to satisfy the above functions. However, traditional roof coverings such as tile, slates, metal roofing sheets etc., combined with timber trusses/purlins and combustible insulation materials would have a greater tendency to allow spread of flame. Rigid plastic or thermoplastic materials such as polycarbonate shall not be used. Similarly, thatched roof shall also not be used. Traditionally, tile roof construction requires timber battens and purlins for securing the tiles. However, the risk of fire spread via such timber members is taken care of by the provision of separating wall between residential units.

Figure 3.14.2b.

The provision of cavity area barrier is to prevent the spread of fire in the roof spaces. Any service penetration to the cavity barrier shall be fire-stopped.

The cavity barrier shall have minimum ½-hr fire resistance rating.

Figure 3.14.2c.

Where false ceiling is not provided, the underside of the roof would be considered as the ceiling to the space below. Hence, there is a need to control the surface flame spread of the exposed underside of the roof construction.