Flat roofs of buildings are exposed, for the maximum duration of the day, to the scorching heat of the summer sun. It is but natural that the roof gets badly heated. The rooms below them also become unbearably hot as the heat gets transferred below. How can we avoid or reduce this heat?Here is the logic of the cause, and the remedy. Concrete is a very dense material. When concrete gets heated, it retains the heat for a very long time. This heat is radiated into the rooms at night when the surroundings are getting cool. Due to the stillness of air below the roof, the air in the room also becomes hot. The ceiling fan revolving below the roof pushes the warm air down. Needless to describe the discomfort.
The remedies are:
Avoid the roof getting heated
Insulate the roof from above
Insulate the roof from below
Avoid the heat transfer from the roof to the room
Cool the roof with some means
Extract the hot air collected below the roof
Extract the heat from the roof before it gets transferred
Shade the roof
If the sun’s heat is unbearable, we stand under a tree or open an umbrella to shade ourselves. We have also experienced that the temperature inside houses shaded by trees are very low and pleasant, in summer also. We park the car under a tree to avoid the car getting heated. Similarly, if the sunlight is heating the roof, shade it with a lightweight roof made of GI Sheet, Mangalore Tiles or canvas. The shadow prevents the roof slab from heating. The shelter can also be used for gatherings.The roof can also be sheltered by having shading features like pergolas. Creepers can be grown over the pergolas to increase the shade.High walls or trellises along the perimeter of the roof can also shade the roof. However, it will be a costly affair.
Plants are the best material to shade a roof. Unlike a tin roof, the plants do not get heated. They absorb heat and light. Place potted plants on the roof, shelter the roof and enjoy the vegetables they yield. Make a terrace garden.The topmost floor of an apartment is the service floor which contains the overhead tank, pressure pumps, lift machine, electric panels, CCTV camera recorders, access monitors, fire safety equipment, water tanks etc. It does not matter if this floor gets heated. It definitely prevents the last habited floor from getting heated.
Terrace gardens will prevent heat absorption. The plants and grass shade the roof. The earth will insulate the roof. Wetness in the mud will certainly cool the roof. However, consult your structural engineer if the roof can carry the garden load and a landscape architect to design a maintenance-easy garden.
Traditionally, “Surkhi”, a mix of brick bat, lime, maravajra (natural adhesive) and Antvalakai (gum fruit) was used as weather proof layer to protect the roof from getting heated. It is not popular nowadays because of lack of skill and know-how. Weather proof clay panels are commonly used for insulation. Each clay panel comes with three tube-like holes.This void helps to avoid heat transfer. Besides, clay does not absorb and retain heat too much. It is laid over the concrete roof with cement mortar.Foam concrete panels can be used instead of clay panels mentioned above, in a similar manner. The sponge-like perforation in the lightweight concrete block prevents heat transfer to the roof slab.
A layer of 40 mm gravel (railway jelly) laid on the roof can effectively prevent the roof from getting heated. The gravel shades the roof by day. The polygonal surface of the gravel makes minimal contact with the roof. This prevents heat transfer. At night, the gravel gives out the heat to the cool air. The disadvantage of this remedy is that we cannot walk comfortably on the roof to use the terrace.
In Jodhpur, Rajasthan, roofs are painted white with lime. White lime reflects a great deal of heat that will otherwise be absorbed by the roof. However, the top will have to be painted almost every year.Many manufacturers use the same technology and offer roof coatings that will last for many years. While some coats are white the others are silver in colour to enhance reflectance. The vendors claim 5 degree reduction of heat.
Bituminous sheets with silver coating on one side are available for top insulation of the roof. They not only reflect heat and insulate but also protect the roof from water leakage.They are about 8 mm thick and have a silver coating on the exposed side. They are glued to the roof with bitumen liquid. Silver coating reflects the heat and the bitumen insulates.
In buildings under construction, insulation can be built into the roof. Hollow clay panels and light weight foam concrete panels are available to be used as infills in the roof. Such roofs are called ‘filler slab’ roofs.
The technology has 3 benefits. The concrete consumption is decreased, the load on the structure is reduced, and the heat gain in the roof is controlled.