In any building the element which is mostly susceptible to the ingress of water is the roof. So what are the roof water proofing methods? Roof slab is exposed to direct climatic variations, extremes of rainfall and structural movements caused thereby.
With the advancements in chemicals and better understanding of construction materials different methods for water proofing of roofs have been devised. There are few of methods that are more economical than other but as it is rightly said that economical methods are not always the best methods. That method for waterproofing should be chosen for roof which assures maximum protection even though it may cost little more.
All the types of roof whether flat roofs or pitched roof needs water proofing despite of the type of materials used like solar tiles, asphalt shingles, metal roofing or rubber slate. In most of the residential buildings the flat roof is built of concrete slab. Concrete slab produces hair like thin cracks and its joint with roof beam or wall would also cause seepage of water.
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.
Ideally concrete, the most widely and regularly used construction material is supposed to be water tight with co efficient of hydraulic permeability between 10-8 to 10-10 m/sec. a well-made concrete with proper mix design and with proper pouring and curing practices is regarded as a very low porous material. However it is seen in practice that concrete loses its permeability.
Concrete is known to be inherently weak in tension; cracks and voids can also form due to thermal expansion, contraction and shrinkage. As a result, water will seep through these voids. Therefore, waterproofing is required to keep the roof water-tight since they are exposed to the weather.
During construction, precautions must be taken to prevent excessive moisture from being trapped between the reinforced concrete roof structure and membrane. This is one of the common causes of flat roof waterproofing failure.
METHODS FOR WATER PROOFING FOR ROOF
In India, terrace waterproofing has always been done with very traditional methods of water proofing. 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
In order to overcome the problems arising from the progressive deterioration of traditional weathering courses, the advanced practices involve the provision of impermeable membranes are used.
One of the simplest ways of providing impermeable membranes is to use multiple coats of a waterproofing co-polymer and cementitious materials.
Traditionally, waterproofing barriers consists of multiple layers of bituminous-saturated felt or fabric bonded together with hot-applied coal tar pitch or asphalt for positive side applications, i.e. the same side as the hydrostatic pressure.
Modern Roof Water Proofing Methods
Today a number of other positive side waterproofing barriers can be selected, which include cold-applied systems.The positive as well as negative side waterproofing methods with their broad chemical compositions are given below:
- Hot applied bituminous materials: Bituminous substances with fabrics or felts
- Cold applied bituminous materials: Asphalt emulsions or asphaltic mastics, reinforced with fabric
- Liquid applied membranes: Single or multi-component products such as neoprene, neoprene-bituminous blends, polyurethane, polyurethane bituminous blends, and epoxy-bituminous blends. Epoxy emulsions allow the concrete to breathe but not allow rainwater ingress.
- Sheet-applied materials : Neoprene, Butyl, EPDM, PVC, etc. which are joined by adhesive sealing or chemical welding (solvent bonding)
- Cementitious membranes The thickness of membrane can vary depending on the type of material, its physical and environmental exposure, and loading conditions. The roof is normally waterproofed with the application of waterproofing membrane and membrane should be protected from traffic and weathering. Based on the type of material the roof waterproofing systems can be broadly classified under two categories:
- Liquid applied membrane system (LAM)
- Prefabricated or preformed membrane system (PFM)
Large amount of water vapour can evaporate from reinforced concrete or a wet screed. Once the waterproofing membrane is laid, drying out of the structural slab will mostly take place from the underside of the deck.
Cross ventilation beneath the deck is usually quite restricted due to erected partition walls. Any trapped moisture subjected to increased temperature from the sun will form vapour which will exert itself directly beneath the waterproofing membrane.
If this vapour pressure is not released or vented sufficiently, the build-up pressure will begin to form a blister on the membrane itself and residual dampness on the underside of the roof.
When the waterproofing membrane has suffered an irreversible stretch, subsequent cooling will not cause the air pocket to return to its original size. Therefore, if a dry surface cannot be achieved within a reasonable period of time, direct membrane adhesion should be postponed or an alternative method of laying should be considered.
Roof Membrane System
The waterproofing membrane is considered to be most important component of the roofing system as it serves the main function of keeping water out of the building. Below are some common roof waterproofing membranes used locally:
Liquid Applied Waterproofing Membrane method
Liquid-applied membranes Liquid-applied membranes are applied on site in a liquid form which is allowed to set and form into a water impermeable membrane.
Bituminous-based (except those containing coal tar) and polymeric-based membranes can be applied when they are cold while those containing coal tar are usually applied when heated.
Liquid-applied membranes are seamless, semi-flexible, easy to apply, detail, maintain and repair. However careful supervision and control during application is needed, particularly in ensuring proper curing of concrete, consistent thickness and uniform application.
Pre-formed sheet Waterproofing membranes
One type of pre-formed membranes is the polymer-modified bitumen membranes and they are applied by heat or attached with an adhesive.
These blended or ‘modified’ asphaltic products are bonded to a high strength fabric of polyester or fiberglass and produced into rolls.
They have elongation and recovery properties which make them suitable to protect against stresses created by wind, temperature fluctuation and normal structural expansion and contraction of the building.
Some of these products are also modified to increase their resistance to fire, thus increasing their fire-rating.
Styrene-butadiene-styrene (SBS) rolls are modified with ‘rubbers’ and compatible with petroleum products.
Atactic Polypropylene Polymer (APP) rolls are modified with ‘plasticizers’ and not compatible with all petroleum-based products, grease and oils.
Another type of pre-formed membrane is made of Polyvinyl Chloride (PVC).
Although PVC is a hard resin, it is modified with the addition of plasticisers to make it more supple and pliable for use as roofing membranes.
PVC membranes are mainly produced by either the calendering or extrusion process. In the calendering process, a reinforcement layer of glass fibre or polyester scrim is normally incorporated into the membrane to provide greater strength and dimensional stability.
Like all thermoplastic membranes, they turn soft when subjected to heat. Some proprietary membranes are formulated with heat reflective compounds capable of lowering the surface temperature of the roof membrane by as much as 15%.
Proper design of falls in reinforced concrete flat roofs is most important in creating flow paths to suitable discharge points. For a roof to be effective, surface water should be discharged quickly without ponding or stagnation. Next, it is important to select the appropriate waterproofing membrane. As the roof is constantly exposed to direct sunlight and rain, it is likely to experience tremendous thermal stresses that will affect its physical properties and performance.