A roof/intermediate floors is constructed to support the dead load of the roof structure and its covering, insulation and internal finishes, snow loads and pressure or suction due to wind and live load without undue deflection or distortion. In engineering parlance it is roof/floor is termed as slab.
FUNCTIONAL REQUIREMENTS OF A SLAB
- Strength and stability
- Resistance to weather and ground moisture
- Durability and freedom from maintenance
- Fire safety
- Resistance to the passage of heat
- Resistance to the passage of sound
The strength of a roof/floor/slab depends on the characteristics of the materials used for the structure of the floor, such as timber, steel or concrete. The floor structure must be strong enough to safely support the dead load of the floor and its finishes, fixtures, partitions and services and the anticipated imposed loads. Dead loads are calculated from the unit weight of the materials set out in IS 875 : Part I and imposed loads from IS 875: Part II.
The slab should have adequate stiffness to remain reasonably stable and horizontal under the dead load of the floor structure and such partitions and other fixtures it supports and the anticipated static and live loads it is designed to support. The floor structure should also support and accommodate either in its depth, or below or above, electrical, water, heating and ventilating services without affecting its stability. For stability there should be adequate support for the floor structure and the floor should have adequate stiffness against gross deflection under load.
Resistance to weather and ground moisture
The slab should be able to protect the ingress of moisture either from the ground from the top. Obviously the degree of penetration of moisture from the ground to a floor will depend on the nature of the subsoil, the water table and whether the site is level or sloping. On a gravel or coarse grained sand base, where the water table throughout the year is well below the surface, there will be little penetration whereas on a clay base, with the water table close to the surface, there will be appreciable penetration of moisture from the ground to floors. In the former instance a concrete slab alone may be a sufficient barrier and in the latter a waterproof membrane on, in or under the concrete slab will be necessary to prevent moisture rising to the surface of the floor.
A roof excludes the rain through the material with which it is covered, varying from continuous impermeable layer of bitumen/any chemical covering or to small units of clay tiles etc.
Durability and freedom from maintenance
Ground floors on a solid base protected against rising moisture from the ground, and suspended upper floors solidly supported and adequately constructed and protected inside a sound envelope of walls and roof, should be durable for the expected life of the building and require little maintenance or repair.
Suspended upper floors should be so constructed to provide resistance to fire for a period adequate for the escape of the occupants from the building. The notional periods of resistance to fire, from 2 to 4 hours, depending on the size and use of the building, are set out in different Building Regulations.
Resistance to the passage of heat
A floor should provide resistance to transfer of heat where there is normally a significant air temperature difference on the opposite sides of the floor, as, for example, where a floor is exposed to outside air.
Obviously a ground floor should be constructed to minimise transfer of heat from the building to the ground or the ground to the building. Both hardcore and a damp-proof membrane on, under or sandwiched in the over site concrete will assist in preventing the floor being damp and feeling cold and s reduce heating required for comfort and reduce transfer of heat.
Resistance to the passage of sound
Upper floors that separate dwellings, or separate noisy from quiet activities, should act as a barrier to the transmission of airborne sound and reduce impact sound.