Flat roofs or terrace roofs

FLAT ROOFS or TERRACE ROOFS

Flat Roofs are used to buildings of any shape. They are economical also. They are suitable for buildings in plains or in hot regions, where rain fall is moderate and where there is no snow fall. Flat roofs are of two types, namely, (i) Madras Terrace Roof and (ii) R.C.C. Flat Roof.

 

1. MADRAS TERRACE ROOF

Southern States in the early 1960s were known as Combined Madras State, and hence the name Madras Terrace Roof. Madras Terrace Roof is not in use now-a-days.

 

2. R.C.C. FLAT ROOF

It is widely used. In this roof, concrete with steel reinforcement bars is used to form a Flat Roof.

1.  R.C.C. Roof Slab (Fig. 1)

R.C.C. SlabWhen the (short) span of a room is less than 4 m, it is possible to cover the area with a R.C.C. slab.

R.C.C. Roof consists of an R.C.C. Slab resting on walls. Load is. carried by the slab. Slab is directly supported by the walls. Thickness of roof slab is 100 to 150 mm. It depends on the span and loading conditions.

Weathering Course: It is provided on the top of R.C.C. roof. It prevents the entry of rain water into the roof slab. Also, it minimizes the heat radiation from the roof into the room.

Lime Concrete (lime and broken brick aggregate - no sand) is laid on the roof and compacted well. Thickness of weathering course is 75 mm. It is given with a slope of 10° (1 in 50 slope), directed towards the rain water drain pipes. Curing is done for 6 days.

Flooring: Suitable Flooring Tiles are laid in a cement mortar of 1 : 3 mixed with crude oil.

One Way and Two Way Slabs

R.C.C. Slab is of two types, based on the reinforcement provided. These are: One-way Slab and Two-way Slab. If the ratio of the length of the room to its width is more than 1.5, the slab is designed as One-way Slab. In such cases, the slab bends in shorter span direction. Hence, the main reinforcements are placed parallel to the shorter span.

. If the ratio of the length of the room to its width is less than 1.5, the slab is designed as Two-way Slab. In such cases, the slab bends in both the directions. Hence, reinforcements are provided in both the directions.

Construction of R.C.C. Roof Slab

1. Centering: Centering Sheets are arranged horizontally at the top level of walls over supports.

2. Grid of Steel Bars: Main Reinforcement Steel Bars and Stirrups are tied in the form of grid on the top of the centering sheets..

3. Cover: R.C.C. slab bends downwards, causing tension at the bottom fibres. Hence, steel bars are placed at the bottom of slab, keeping a Clear Cover of 15 mm.

4. Mixing Cement Concrete: Cement concrete of 1:2 : 4 mix is mixed thoroughly with sufficient quantity of water manually or in concrete mixer and placed on the centering sheets.

5. Compaction of the Concrete: The concrete is compacted well manually or by using mechanical vibrator to the required thickness. The top surface is levelled.

6. Slope of the Roof: Surface of the roof at the top of the building is given a gentle slope of up 1 to 10° for draining-off rain water easily and rapidly.

7. Curing: Curing is done for one or two weeks to facilitate continued hydration of cement.

2. R.C.C. T-Beam Roof Slab

See Fig. 2. When the span is long, it is divided into bays. The slab is stiffened by means of Ribs. These ribs act monolithically with the part portion of the slab and form beam in the shape of ‘T'.

Such a beam is called T-Beam. The slab portion of the beam is known as Flange of T - Beam.

T-Beam slab is most popularly used for roofs (and floors also) of residential, office and public buildings.

3. Uses

Flat roof can be used as a terrace for constructing overhead water tank, installing solar heater, for gardening, drying, recreation, celebrating domestic functions, etc.

4. Advantages of R.C.C. Flat Roof

1. Construction: Construction and maintenance are easy.

2. Concentrated Loads: R.C.C. flat roof is capable of withstanding concentrated loads.

3. Wind Load: It can withstand heavy wind load.

4. No False-ceiling: It does not require false-ceiling.

5. Roofing Material: It requires lesser area of roofing material.

6. Insulation: It has better thermal and sound insulation.

7. Fire Resistance: It is fire resistant.

8. Upper Floors: The construction of additional upper floors can be easily done over the flat roof.

9. Appearance: It presents a good architectural appearance to the building.

10. Economical: It is economical compared to Pitched Roof.

5. Disadvantages of R.C.C. Flat Roof

1. Cracks: For large span, cracks may develop due to large temperature variations.

2. Improper Drainage: Improper drainage may lead to dampness on the ceiling, causing rusting of steel bars and leakage. It is difficult to locate and rectify leakage in flat roof.

3. Span: The span of flat roof is restricted, unless intermediate T-beams are introduced.

4. Hill Regions: It is not suitable for hill regions or heavy snow-fall regions.