Flat Roofs

A roof which is approximately flat is called a flat roof. It is becoming more popular with the introduction of suitable building materials. It may be constructed in reinforced cement concrete (RCC), flag stone supported on rolled steel joist, bricks, concrete or tiled arches. This roof is provided with a slight slope in one direction to drain off the rainwater easily. The construction of flat roofs is similar to the construction of floors except that the top surface is protected against rainwater.

Advantages of flat roof

1. The roof can be used as a terrace for playing or for sleeping or for other domestic purposes.
2. The construction and maintenance of the flat roof is simpler.
3. It provides better architectural appearance to the building.
4. It is easier to make the flat roof fire resistant.
5. It possesses good insulating properties.
6. It avoids the need for a false ceiling.
7. The construction work of upper floors can be readily taken up in the case of flat roof, whereas in the case of pitched roof the entire roof has to be dismantled before construction.
8. Pitched roof needs much more area of roofing material than flat roofs.

Disadvantages of flat roof

1. A flat roof cannot be used for long spans without using columns and beams.
2. In areas of heavy rainfall, flat roofs are not suitable.
3. The initial cost is more.
4. Due to greater variations in the temperature cracks sometimes develop on the surfaces of the roof, which is difficult to repair.
5. The speed of construction is slower than that of a pitched roof.
6. If proper slope is not provided on the roof to drain off the rainwater, pockets of water are formed on the surface of the roof, which leads to leakage in the roof.

Types of flat roof

The various types of flat roof constructions include the following.

Madras terrace roof

Bricks are the major constituent and they are supported on wooden and steel joists.

1. Wooden joints are kept over rolled steel joists with a furring piece in between them.
2. A course of thoroughly burnt terrace bricks (15 × 8 × 2 cm) is laid on the edge diagonally across the joists in 1:1.5 lime mortar.
3. After the bricks are completely set, a 10 cm thick layer of brick bat concrete is laid over the course of brick. It is beaten down to 7.5 cm by wooden hand beaters. The beating is continued till the beater fails to make an impression on the roof surface.
4. Two layers of flat tiles (15 × 10 × 1.2 cm) are laid over the layer of concrete in 1:1.5 lime mortar.
5. Finally, three coats of lime plaster are applied to the surface and it is rubbed to a polished surface. Generally, a slope of 1:36 is provided to the roof by giving slope to the joists and not by increasing the thickness of terracing.

Jack arch flat roof

The brick or concrete arches are constructed with rolled steel joists, which are supported on the walls of the rooms. Some inert materials fill up the spandrels between the arches. Over it, a 10 cm thick average lime concrete (LC) terracing is provided to make it waterproof. Some waterproofing material may be used with lime concrete and the LC terracing is beaten thoroughly by wooden beaters to make it more compact.

Reinforced concrete slab roof

An average of 10–13 cm thick LC terracing with some waterproofing compound is provided over the RCC slab to make the roof leak proof. The lime concrete is thoroughly beaten by several labourers with wooden beaters for several days to make the terracing more compact and impervious. At the junctions with wall, the lime terracing is taken inside the wall for 10–15 cm depth and the corner is given a smooth and round shape such that water may not accumulate. The lime terracing is provided with a slope (1 in 60 to 1 in 100) to drain off the rainwater easily from the roof to the gutters provided for the purpose.