A roof is a covering placed on any structure that protects from weather conditions. A roof can also be used as a terrace, where it is flat and has an entryway built into it.Terraces are flat, fully supported space on upper floors that allow room for people to walk, sit and various other activities. Roof is the most exposed surface of building. It is affected directly by the vagaries of sun, wind and rain.
The roof of a building should be designed and constructed in a way to effectively drain water by means of sufficient rainwater pipes of adequate size, wherever required, so arranged, jointed and fixed as to ensure that the rainwater is carried away from the building without causing dampness in any part of the walls, roof or foundations of the building or an adjacent building. In India, terrace waterproofing has always been done with very traditional methods. Typically, systems like Brickbat Coba, China mosaic, Lime terracing or the “Mud Fuska” technique are still very much in vogue in different parts of the country. These systems have been offering waterproofing with some insulation against heat.
Some of the problems that these systems have are:
- Heavy loading on the slab
- Cracks on the surface, especially in case of Brickbat Coba
- Expertise levels of application required for lime terracing /mud fuska not as good as in the olden days
- Offer more insulation rather than a leak-proof or a watertight system
- As ageing takes place, breaking them and re-doing the same system is risky for the slab; cracks caused in the slab increase the problem of leakage
Type of Roofs
Various types of roofs used may be divided broadly into three types:
1.Flat Roofs
2.slopped / pitched Roofs
3.Shells and Folded plates
Flat Roofs
These roofs are nearly flat. However, a slight slope (not more than 10°) is given to drain out the rainwater.All types of upper store floors can serve as flat roofs. Many times top of these roofs are treated with waterproofing materials like mixing waterproofing chemicals in concrete, providing membrane, etc. With an advent of reliable waterproofing techniques such flat roofs are constructed even in areas with heavy rainfall. There should be minimum variation in pitch over the entire roof. It is preferable to have a uniform fall over the entire roof. Providing steeper falls near the perimeter of the roof should be avoided in order to prevent the collection of water at the junction between the roof slab and the parapet wall, as this junction is prone to water leakage.
The advantages of flat roofs are:
1.The roof can be used as a terrace for playing, and celebrating functions.
2.In later stages, the roof can be converted as a floor by adding another store.
3.They can suit any shape of the building
4.Overhead water tanks and other services can be located easily.
5.They can be made fireproof easily as compared to pitched roof.
The disadvantages of flat roofs are:
1.They cannot cover large column-free areas.
2.Leakage problem may occur at a later date also due to development of cracks. Once leakage problem starts, it needs proper treatments.
3.The dead weight of flat roofs is more.
4.In places of snowfall, flat roofs are to be avoided to reduce snow load.
5.The initial cost of construction is more.
6.Speed of construction of flat roofs is less.
Flat Roofs Assembly
A low slope roof or flat roof has four parts such as roof deck, vapor retarder, insulation and roof membrane. The combination of these parts is known as roof assembly.
1.Roof Deck : An entire roof system is held up by a roof deck. This is the base of roof assembly. Roof decks can be made of many different materials. Some of these materials include steel,wood, concrete, lightweight insulating concrete and cement wood fiber.
2.Vapor Retarder : Moisture vapor is created inside buildings. A vapor retarder helps keep vapor from getting into insulation. It is usually installed between the deck and insulation.Materials used for vapor retarders include felt plies, plastic sheets, aluminium foil and Kraft paper sheets. Vapor retarders are not used with every roof system. It depends on the moisture in the building and climate of the area.
3.Insulation : If there is no vapor retarder, insulation is usually the first component installed over a roof deck.The purpose of insulation is to help keep heat in a building in cold weather. It also helps keep heat out of a building in hot weather. Insulation saves money of a building in hot weather. Insulation saves money because the air-conditioning and heating systems will not be used as much. Some buildings do not need insulation. It is not used with every roof system.
4.Roof Membrane : A roof membrane makes a roof system watertight. It is installed over the insulation. In some systems, a roof membrane may be installed right over a deck.
Slopped / pitched Roofs
In the areas of heavy rainfall and snowfall, usually, sloping roofs are used. The slope of the roof should be more than 10°. They may have slopes as much as 45° to 60° also. The sloped roofs are known as pitched roofs. The sloping roofs are preferred in large spanned structures like workshops,factory buildings and warehouses. In all these roofs,covering sheets like A.C. sheet, G.I. sheets, tiles, slates, etc.are supported on suitable structures.
Advantages of slope roofs are :
1.Water drains itself quickly, giving wind and gravity little opportunity to push or pull water through the roofing material.
2.Steep roofs can be covered with roofing materials that are fabricated and applied in small, overlapping.
3.Top surface is in a slope and hence advantage of terrace is lost.
4.Cannot cover a building of any horizontal dimension because it becomes too tall on a broad building.
Shells and Folded plates
Shell roof may be defined as a curved surface, the thickness of which is small compared to the other dimensions. In these roofs a lot of load is transferred by membrane compression instead of by bending as in the case of conventional slab and beam constructions. The thin R.C.C. shell roofs are built to cover large column-free areas.
Advantages of shell roofs are:
1.Good from aesthetic point of view.
2.Material consumption is quite less.
3.Form work can be removed early.
4.Large column-free areas can be covered.
Disadvantages are:
1.Top surface is curved and hence advantage of terrace is lost.
2.Form work is costly.
Design Consideration for Waterproofing System
The design of the waterproofing system should include its construction detailing, specification of materials and accessories, the fixing method, the joining of waterproofing membrane with other structures and drainage. For the efficient design and laying of the waterproofing treatment, the designer shall take into consideration the following salient factors as per IS 3067:
1.Shape of the roof, such as flat, slope or curved
2.Type of roof
3.Type of finish required
4.Type of thermal insulation treatment
5.Projection through roofs
6.Drainage arrangement
7.Intensity of rainfall
8.Total weight of waterproofing treatment on the roof
Apart from the above factors, a successful waterproofing application depends on a structurally sound slab. To ensure the construction of a structurally sound slab,provisions for the following items should be included in a building’s structural design and in the design of a slab.
- Live loads
- Construction loads, such as moving installation equipment, workers and materials
- Dead loads, such as mechanical equipment, duct work,piping or conduit such as fire sprinkler and electrical lines
- Dead loads, such as a waterproofing system, soil overburden, concrete toppings
- Slab strength (gauge, density, type and thickness)
- Expected deflection
- Drainage
- Placement of expansion joints
- Placement and structural support of curb and penetration members and details
- Attachment provisions for a deck
- Suitability for adhesion / attachment of waterproofing membrane
- Suitability for water test loads
111A structurally sound slab or deck should remain so as not to deflect excessively under live loads. It should be understood that a waterproofing contractor can only inspect the surface of a slab or deck to receive the waterproofing materials and cannot assume responsibility for the slab’s slope, structural integrity, method of attachment, or any other conditions beyond his control or professional expertise.The slope of the roof should be adequate to prevent ponding of water. A sufficient slope ensures a good and economic result. The recommended slopes for the flat roofs are as follows:
With smooth finish would be 1 : 150 to 1 : 133, and With rough stone / tiles 1 : 100 and,For gravel set in cement or loosely packed concrete finish 1 : 75 to 1 : 66.
Terrace with water ponding leads to leakages and seepage.The faster the water is directed off the envelope, the less chance for leakage.The waterproofing membrane should be protected from mechanical stresses, traffic, solar radiation, air pollution and other stresses. Depending on the stress and moisture conditions of the roof structure, a suitable insulation system is selected to withstand the long-term and construction time loads.
The design should also be carried out considering climatic conditions of regions such as hot climates, cold climates In hot dry climates, monolithic concrete structural roof can provide satisfactory roofing with no membrane except at the joints. More often, the concrete is covered with a membrane or insulation and membrane system. In hot humid climates water vapor is kept from entering the underlying insulation. Insulation above the roofing is sometimes used to protect membrane from high temperature and UV exposure in hot climates.
In hot and moderate climates, a light colored surface will reduce temperatures experienced by the roof membrane. This will reduce energy required to cool the building. For this purpose, the coating should prevent UV degradation of the underlying material.In cold climates, roof eaves require special design to prevent or accommodate build-up of ice formations.Moisture within a building will move towards a cold roof. A vapor retarder may be needed to prevent condensation. An effective way to prevent condensation is to use a protected membrane roof where the insulation is placed above the membrane. This helps to isolate the membrane from temperature extremes and from freeze-thaw damage.
When temperatures are lower 4OC then require special precautions such as insulated hoses and kettles. Hot bitumen and adhesives should not be applied at temperatures below the dew point Some manufacturers provide loose-laid single ply or heat applied modified bitumen membrane systems which can be applied at low temperatures.
Apart from design considerations, there are many fixtures and parts that need to be installed properly while doing waterproofing of roof terraces.
Pipe Penetrations
There are always different types of pipe penetrations in a roof. Their correct and watertight joining to the roofing is vital for the functioning of the entire roof. The penetration materials and systems must withstand mechanical stresses caused by changes in the roof structure resulting from ageing, temperature changes and climatic conditions.When a pipe passes through RCC slab a cement concrete fillet shall be built around the pipe and waterproofing taken over the fillet. Penetrations should be kept to a minimum as far as possible. It is preferable to have a clear and uninterrupted roof deck for continuous waterproofing. Service fixtures, planters, etc may be designed to stand over the waterproofed deck on concrete pads. A typical figure of pipe penetration through roof slab.
Roof Drains
All water that enters the roof should be channeled into roof drains with falls and into rainwater sewers via a system of down pipes. Rainwater pipes shall be constructed of cast iron, PVC, asbestos cement,galvanized sheet or other equally suitable material and shall be securely fixed. The rainwater pipes should be fixed to the outside of the external walls of the building or in recesses or chases cut or formed in such external wall or in such other manner as may be approved by the authority.
All the bends of rainwater pipes should be brought at terrace level beyond the inside face of the parapet and keep them clean.When a rainwater pipe is taken through the slab, tight fit funnel of anti-corrosive material should be provided so that water does not seep into the structure through the slab.The required number of rainwater pipes should be provided and their diameter should be selected as per average rate of rainfall as given in National Building Code.
The drain mouth with bell mouth entry shall be fixed and properly set to allow the water to flow into it. Felt shall generally be laid as on the other portion on the roof and the treatment shall be carried inside the drain pipes overlapping at least 100 mm. If possible grating cap should be provided over the drain mouth to protect chocking caused by leaves, stones, etc.
Upturns
The upturns are important features for the functioning of waterproofing. They ensure that water that ponds for whatever reason cannot penetrate underneath the roofing and into the fabric. Usually the height of an upturn is 300mm above the finished surface and on roofs at least 100 mm above the overflow level. A 100 mm upturn is acceptable at door thresholds. However, in these cases, it must be ensured that the connection with the door assembly and the wall is absolutely watertight With rubber bitumen membrane roofing, the roofing is cut along the edge of the upturn and the upturn is constructed of a separate piece of membrane. The upturn is secured with mechanical anchors and it is protected with flashing The detailing at junction of parapet and roof slab of a waterproofing system is shown in Fig below
Parapet Walls
The parapet walls should be properly leaned and waterproofing coating should be applied on internal and external faces. Provide some slope to top of parapet walls to drain off rainwater from it. Necessary grooves shall be provided in the walls to terminate the waterproofing treatment. At the junction of the wall and the floor, a round or triangular fillet of size 200 mm x 200 mm shall be provided with cement and sand mortar admixed with SBR latex waterproofing compound. The entire surface shall be cured for minimum 14 days by storing water to a depth of at least 150 mm in the entire area.
Movement Joint
Rubber bitumen membrane roofing does not require separate movement joints for the waterproofing itself. In the event of a structural movement joint under membrane roofing an approximately 500 mm unbounded strip can be used between the slab and base sheet. If necessary, a separate design detail of the movement joint should be provided.
Expansion Joint
Expansion joints and control joints are used to minimise the effects of stresses and movements of a building’s components and prevent these stresses from splitting buckling / ridging or damaging a waterproofing assembly,which includes a waterproofing membrane and wall / slab / deck, should be placed in the same location as the building’s structural expansion joints. Each of a building’s components has varying coefficients of expansion, and each is subjected to varying temperature changes and resultant thermal movement. In the design and placement of waterproofing expansion and control.
Insulation
If any protective layer is provided to prevent the waterproofing membrane from damage by construction activities, the finishes on top of this protective layer should be of sufficient weight to prevent the uplift of this protective layer under all conditions. The choice of the type and thickness of this layer depends on the overall thermal insulation required by statutory requirement
Dish Antenna
Typically, any dish antenna is installed in the parapet wall directly on the top portion with the mounting bracket. It is directly fastened in wall through bolts, and generally bolts penetrate through the roof material into a concealed wood nailer, which causes penetration of water and leakage. The remedy for the same is to install the antenna in counter flashed panel or railing. Provision of this type of panel or railing should be incorporated at the time of parapet design.
Plumbing: Service Pipes
Pipes should be 50 mm to 75 mm away from face of the wall. Fix all service pipes by clamps of one’s choice Use anti-corrosive anchor fasteners only for fixing the clamps.
Stair Cover (or MUMTY) and Canopy
These are the locations which are not cleaned for earth,debris and waste material for years. Water stagnates and slab deteriorates severely.
