Basics of Building Construction

Building Components And Types

Lesson 19 : Roof And Roof Coverings

Types Of Roofs

Flat Roofs

Flat roof is the one which is either horizontal, or practically horizontal with slope less than 10o. Even a perfectly horizontal roof has to have some slope at top so that the rain water can be drained off easily and rapidly. Flat roof may be of reinforced cement concrete, reinforced brick work, precast concrete with flag stones supported on rolled steel joists or jack arch type. The flat roofs are properly treated to protect it from the adverse effect of rain, snow, heat, etc. Efficient water- proofing and roof drainage is an important requirement of flat roof. In addition a layer of insulating material (mud, lime, concrete, mud phuska etc.) is laid over the roof surface to provide adequate thermal insulation to the space below. This layer is known as terracing or grading. The slope of flat roof may be increased in heavy rain fall regions. The slope to the roof can be given either by varying the thickness of the terracing material or by constructing roof slab in slopes or by providing part slope in the roof slab and part in the terracing material. In situations where thermal insulation is not essential, the slope for drainage of the roof is provided in the slab roof itself and the roof surface is given adequate water proofing treatment using bitumen felts etc. to ensure water tightness.

Advantages Of Flat Roofs

• The roof can be used as terrace for playing, gardening, sleeping and for celebrating functions.
• Construction and maintenance is easier.
• Flat roofs can be easily made fire proof in comparison to pitched roof.
• They avoid the enclosure of the triangular space. Due to this, the architectural appearance of the building is very much improved.
• Flat roofs have better insulating properties.
• They require lesser area of roofing material than pitched roof.
• Flat roofs are more stable against high winds.
• They do not require false ceiling, which is essential in pitched roofs.
• Flat roofs are proved to be overall economical.
• In multi-storeyed buildings, flat roof is the only choices since over-head water tanks and other services are located on the terrace.
• The construction of upper floors can be easily done over flat roofs, if required in future.

Disadvantages of Flat Roofs

• The span of flat roof is restricted, unless intermediate columns are introduced. Pitched roofs can be used over large spans without any intermediate columns
• The self weight of flat roofs is very high. Due to this, the sizes of beams, columns, foundations, and other structural members are heavy.
• They are unsuitable at places of heavy rainfall.
• Flat roofs are highly unsuitable to hilly areas or other areas where there is heavy snow fall.
• Flat roofs are vulnerable to heavy temperature variations, especially in tropics, due to which cracks are developed on the surface. These cracks may lead to water penetration later, if not repaired in time.
• It is difficult to locate and rectify leak in flat roof.
• The speed of flat roof construction is much slower than the pitched roof.
• The initial cost of flat roof is more than pitched roof.
• The flat roof exposes the entire building to the weather agencies, while the projecting elements (such as caves etc.) of pitched roof provide some protection to the building.

Types Of Flat Roofs

The common types of flat roofs are

1. Mud-Terraced Roofing.
2. Brick-Jelly or Madras Terrace Roofing.
3. Mud-Phaska Terracing With Tile Paving.
4. Lime Concrete Terrace.
   
5. Lime Concrete Terracing With Tile Paving.
6. Bengal Terrace Roofing.
   
7. Light Weight Flat Roofing.