What is Flat Roof or Terrace Roofing ?
A roof laid at an angle of less than 10° to the horizontal is known as flat roof. Flat roof may be of reinforced cement concrete, reinforced brick work, precast concrete with channel units, waffle units etc, flag stones supported on rolled steel joists or jack arch type.
The most important requirement of flat roof is efficient water proofing and roof drainage. The roof surface has therefore to be provided with proper water proofing treatment and necessary slopes to satisfy these requirements. In addition a layer of insulating material (mud, lime, concrete) is laid over the roof surface to provide adequate thermal insulation to the space below. This space is known as terracing or grading.
A particular slope can be given to roof by adjusting the thickness of the terracing material or the roof slab can be constructed in slope. In situations where thermal insulation is not essential, the slope for drainage of the roof is provided in the roof slab itself and the roof surface is given adequate water proofing treatment by using bitumen felts etc. In places where the terraced roof is to be used for outdoor living, the top surface of the terracing should be made resistant to wear besides making provisions for efficient water proofing and drainage for the roof.
Advantages of Flat Roof
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The construction of flat roof is simplified and maintenance is easy.
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The roof can be used as terrace for playing, gardening, sleeping and for celebrating functions.
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It is easier to make the flat roof fire-proof than a sloping roof.
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They avoid the enclosure of the triangular space. This improves the architectural appearance of the building.
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Flat roofs have better insulating properties and they are more stable against high winds.
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The construction work of upper floors can be easily started. In case of a pitched roof, the entire roof is to be removed and is to be replaced by a new roof under such circumstances. It is therefore considered to be the best choice for multi-storeyed buildings.
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They do not require false ceiling, which is essential in pitched roofs.
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Flat roofs are proved to be overall economical.
Disadvantages of Flat Roof
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Intermediate columns in flat roofs give them a free hand during the construction of the span.
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Self weight of roof is more. Due to this, the sizes of beams, columns and other structural members are heavy.
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Flat roofs are exposed to sun and are subjected to violent temperature changes which may lead to cracks in the surface of the roof.
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The pockets of water are formed on the surface of the roof, if slope is not sufficient. This leads to the leakage of the roof and it sometimes proves to be difficult to exactly locate the position of the leakage on the roof.
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They are unsuitable at the places of heavy rainfall, hilly areas or areas where there is heavy snowfall.
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The dead weight of flat roof is considerable and hence it prove to be more expensive. It’s initial cost is higher than a pitched roof.
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The progress of work in a flat roof is slow as compared to that of a pitched roof.
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The initial cost of flat roof is also more than the pitched roof.
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Building with the flat roof is entirely exposed to the weathering conditions, but the projecting elements such as eaves of the pitched roof gives some protection to the building.
Types of Flat Terrace Roofing
- Mud- Terrace Roofing
- Brick- Jelly or Madras Terrace Roofing
- Bengal- Terrace Roofing
Mud-Terrace Roofing :
This type of terracing is suitable where rainfall is less. It can be provided either on tiles or on wood boards. In both the case, terracing is made with white earth mud containing large percentage of sodium salt. The mud-terracing in Punjab is provided over roof which consists of 50 mm x 50 mm x 6 mm T-sections spaced at 32 cm centre to centre over R.S.J. well-burnt tiles of size 30 cm x 30 cm x 5 cm or 30 cm x 15 cm x 5 cm are placed between the flanges of the T-sections: using lime mortar.
Over the tiles, a 15 cm thick layer of stiff mud, white in colour and containing sodium salts is spread and beaten with sticks till the surface becomes hard and the beater rebounds. The surface is then plastered with mud and cow-dung mix plaster. Finally, the surface is finished with 1 : 4 cement- cowdung plaster. In Maharashtra and Madhya Pradesh practice, mud terracing is done on teak wood boards (4 to 5 cm thick) nailed to the wooden joists. On the boards, a 2.5 cm thick layer of wood shaving is spread over which bricks are laid on edge in lime or mud mortar.
On the bricks 8 to 10 cm thick layer of mud is spread and beaten hard. Finally, a 2.5 cm thick layer of white earth containing high percentage of sodium salt is applied. This top layer has to be renewed once in a year. Such roofs do not leak, instead provide insulation against heat and thus keep the building cool and comfortable.
Brick Jelly Roofing or Madras Terrace Roofing :
Construction Procedure :
The teakwood wood joists are placed on rolled steel joists with a furring piece between the joists and rolled steel joists. The furring is placed sloping and it gives necessary slope to the flat roof. A course of specially prepared terrace bricks is laid diagonally across the joists. The size of the bricks is generally 150 mm x 75 mm x 25 mm and they are placed on edge in lime mortar. After the brick course has set, a course of brick bat concrete is laid.
The thickness of this course is about 75 mm and it consists of 3 parts of brickbats, one part of gravel and sand and 50% of lime mortar by volume. The concrete is well-rammed for a period of three days and allowed to set. The flat tiles are then laid over the layer of concrete. The tiles are laid in two courses, making the thickness of about 50 mm. Alternatively, the china mosaic pieces may be used in place of flat tiles.
Finally, the surface of the roof is finished by three coats of plaster. The surface is then rubbed and polished and given a slope of 1 in 30. As the type of flat roof construction is widely used in old madras state, it is known as madras terrace roof.
Bengal Terrace Roofing :
The roof is generally used in Bengal to cover verandah and hence it is commonly known as Bengal terrace roofing. The construction procedure of such a roof is described below :
The rafters are placed in position, sloping outward from main wall at a spacing of 30 to 50 cm centre-to-centre. One end of the rafter is inserted into the main wall for a length of 20 cm and their other end is fixed on a verandah wall-plates or bressummers. The battens or reepers are fixed to the upper surface of the rafters at right angles and are spaced at 15 cm centers. On the battens, a course of flat tiles well soaked in white wash is laid in lime or cement mortar. The roof is finally finished in following 2 methods :
a) Two more courses of flat tiles are laid on mortar and finally finished off with 2 or 3 coats of plaster. The outer top surface of the last coat is rubbed smooth and polished.
b) A layer of fine jelly concrete 4 to 5 cm thick is laid over the first course of tiles. Over this concrete layer, another course of flat tiles is laid and the surface is finally finished off with 2 or 3 coats of plaster. The outer top surface of the last coat is rubbed smooth and polished.
I like that you pointed out how the construction of flat roofing is simplified and maintenance is easy. We are currently building a small house for my grandfather and we have decided to use a flat roofing style for it. So, we should probably start calling for some experts to help us out with this project.
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Seeing how important it is to handle insulation in commercial buildings nowadays, I can see why flat roofing is so popular. If you use this model well, you can keep the air conditioning costs as low as possible and the overhead payments you make will go down. If I were to own a business, I’d definitely hire a flat roofing expert for that kind of installation.
I like that you also mentioned that good energy efficiency of flat roofs. I’m interested in looking for a good roofer soon because I want to get one of my commercial properties renovated. I plan to turn the place into a gift shop later this year.
Any book “Exploring Flat Roof Terrace Roofing” offers a thorough examination of the flat roof terrace’s construction and design factors. Commercial Roofing Whether you’re a homeowner, architect, or engineer thinking about building a rooftop terrace, this blog post has helpful advice to guarantee a fruitful and useful project.