Basics of Building Construction

Building Components And Types

Lesson 19 : Roof And Roof Coverings

Types Of Flat Roofs

Curved Roofs

These are just the modifications of pitched roofs and are frequently employed in modern age to cover large areas. Shell roofs and domes are the varieties of curved roofs. They are useful for big structures such as factories, monumental works, etc. Curved roofs may be constructed of timber or R.C.C. There are two common-forms of a shell roof.

1. A north light shell roof
2. A barrel vault shell roof

A dome is a rounded vault forming a roof. It is useful when roof is to be provided on circular brick work or regular polygon shaped walls. Curved roofs afford pleasing appearance and in most cases, the construction of curved roofs results in the economy of steel and concrete. Due to arch action, the stresses are considerably reduced which result in thin section for the curved roofs. Also they provide space in the roof which can be utilized for some purpose.

A recent development, known as doubly curved tile roof has combined the advantages of flat roofs and curved roofs. Following is the procedure of making such a roof.

1. Pre-cost units of cement concrete tiles are prepared at site. The size of tile is kept at about 70cm x 70cm with uniform thickness of at least 20mm. the tiles are in the form of domes with a size of about 50mm. Cement concrete of proportion 1:2:4 is adopted and the units are properly curved.
2. Pre-cost units of R.C.C beams are prepared and reinforcement is provided as pre design. The dimensions of beams are usually 90mm deep and 130mm wide. The stirrups are kept projecting about 50mm above the precast portion so as to have bond with cast in-site-concrete. Cement concrete of proportion 1:2:4 is adopted and the units are properly curved.
3. The beams are suitably laid across the supporting walls.
4. The tiles are placed in position after spreading some mortar on the edges of beams. The minimum bearing of this on beams should be 25mm and that on walls should be 50mm to 70mm.
5. The haunches between the humps of tiles are filled up with cement concrete of proportion 1:2:4.
6. Suitable water – proofing treatment to the roof is given at the top. The roof thus exhibits a flat surface at the top and curved surfaces at the bottom

The Advantages of doubly curved tile roof are

1. It can be constructed in short time as it affords advantages of prefabrication.
2. It does not require constant skilled supervision and can easily be adopted for villages.
3. It is cheap in construction.
4. It requires less frame work.