In any construction, the longevity of the building depends to a great extent on the waterproofing of the roof. One should bear in mind that the roof accounts for 30% of the structure’s surface, so it pays to have it correctly waterproofed to keep the rest of the structure safe from damage due to water seepage.
The roof is the most exposed surface of any building as it is directly affected 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 enough 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.
Before conducting waterproofing, 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 an efficient design and laying of the waterproofing treatment, the designer shall take into consideration the following salient factors;
- Shape of the roof, such as flat, slope or curved
- Type of roof
- Type of finish required
- Projection through roofs
- Drainage arrangement
- Intensity of rainfall
- 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, ductwork, 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
Terrace / Roof with water ponding leads to leakages and seepages. 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 the climatic conditions of the region.
The advent of the recent, new-generation chemical waterproofing coatings or preformed membranes has provided the greatest contribution to the waterproofing industry for increasing the durability of structures. These are highly elastomeric, flexible, breathable with weather ability, and have high elongation, crack-bridging abilities plus a lot of additional properties formulated to meet the specific requirements.
The liquid-applied coating of either single or multi-component can be easily applied with a hand brush or a roller. They can be acrylic, polyurethane, polymer modified bitumen, polymer-modified cementitious or epoxy-based coatings.
The preformed membranes may be of modified bituminous membranes (APP/SBS), PVC, TPO, HDPE, EPDMs, etc. having different thickness and application methodologies. They are either torch-applied, stuck with adhesive or cold-applied, fixed mechanically. But the selection of the material is more important, keeping in view the different environmental conditions and desired service life.
There are still a lot of failures of these modern waterproofing systems reasons being wrong systems of application or the behaviour and limitations of the material itself in terms of adhesion or UV stability.
In the case of application, surface preparation is very important for adhesion which can be improved by using a superior primer. The surface moisture needs to be checked before any coating application, since excess moisture will damage the coating system. In fact, it is a difficult task for a common man to choose a product or system for waterproofing from a wide range of products and systems available in the market. Therefore, the selection should be such that the material characteristics match the required physical, chemical and weather resistance properties of the coating system. In case of liquid-applied coating material, the active solid contents need to be checked, which affect the dry film thickness for effective performance. While a liquid-applied coating system can be applied easily, preformed membrane application needs skilled manpower.
Waterproofing Systems
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)
The commonly / widely used systems in Kenya are Preformed Membrane System (PFM) such as Polymer Modified Bitumen (APP / SBS) and Liquid Applied Membrane (LAM) such as fibre incorporated water-based acrylics, Polymer modified cementitious, Polyurethane, etc.
Liquid Applied Membrane (LAM)
Liquid applied membranes are well-accepted systems throughout the globe. The membranes formed are water-resistant and not affected by weather conditions like humidity, rain or extreme hot weather. Moreover, the liquid waterproofing system is easy as well as economical to repair as compared to the preformed membrane.
Preformed Membrane (PFM)
Preformed membrane can be bonded or unbonded system. Bonded system is normally preferred as it has the advantage over the unbonded system due to its leak-localizing capability.
Comparison between LAM and PFM
- LAM’s create seamless waterproofing membranes while PFMs form joints at regular intervals
- LAM’s are highly elastomeric while PFMs are rigid in nature
- LAMs show increased crack-bridging capabilities in comparison with PFMs
- On application, LAM’s are ideal for roofs with complicated shapes and details like penetrations, upstands, joints, etc. PFMs are ideal for long large roofs with minimum projected structures
- LAM’s cannot be easily guaranteed for a fixed consumption and thickness as they are dependent on the substrate porosity and applicable conditions. On the other hand, PFMs can be specified with consumption with specified lap joints
- Both systems are vulnerable to stagnated water
- Both systems normally need appropriate primers
- Both the systems are prone to blistering if the surface contains excess moisture before the application
General Requirements for Preformed and Liquid Applied Membrane
The surface to receive membrane should be cleaned, dried, free of sharp protrusions and surface defects such as cracks, voids and protrusions. It is desirable that the concrete surface to be waterproofed should be finished to provide a plain and even finish to receive the membrane. Primed surfaces to receive the preformed waterproofing membrane should not be covered until the primer is fully cured. Areas exposed to rainfall should be sufficiently air-dried before commencing or resuming installation.
The concrete or screeding should be allowed to cure for at least 7 days before the laying of the membrane. The application of the membrane to uncured concrete may result in moisture being trapped between the concrete and membrane causing blisters to be formed and affecting bonding of the membrane to the substrate. All openings, penetrations, upturns, corners, etc., should be properly detailed to ensure water tightness. Generally, more layers of waterproofing membrane are provided at these locations.
Upturns at parapet walls should be at least 150 mm high. The membrane should be tucked into the chase formed along the full length of wall. Flashing may be installed above the chase to provide additional protection. A fillet of 45°C of minimum leg size 50 mm should be formed at all intersections where the horizontal and vertical surfaces meet. Proper detailing should be provided for the bridging of the membrane over expansion joints.
Consideration should be given to the movement of the joints and where necessary, additional flashing and kerbs should be installed to ensure complete water tightness. Water tightness test should be specified for the waterproofing work. The minimum flooding period should be 24 hours. No dampness should be observed within an hour after the test duration.
Dr. Fixit has a range of waterproofing materials for one to choose from:
Fibre Reinforced Water Based Acrylic Coating
Dr. Fixit Raincoat 2 in 1 is an elastomeric waterproof coating that provides a seamless and tough waterproof barrier with an excellent crack bridging ability that can accommodate movements due to thermal variations in the waterproofing system.
Polyurethane Based Liquid Applied Coating
Dr. Fixit Flexi PU 270(I) is a liquid-applied, highly permanent elastic, cold applied and cold curing, one-component polyurethane membrane used for long-lasting waterproofing. Solvent-based with root resistance properties. Dr. Fixit Flexi PU 270(I) is based on pure elastomeric hydrophobic polyurethane resins, which results in excellent mechanical, chemical, thermal and natural element resistance properties.
Polymer Modified Cementitious Coating
Dr. Fixit Fast Flex is a High-performance polymer-modified two-component cementitious coating, composed of high-quality cement, properly selected & graded fillers, additives and acrylic polymer. It is suitable for achieving waterproofing for roofs, because it provides strong bonding, good waterproofing and excellent resistant to hydrostatic water pressure by forming highly elastic seamless coating over the applied concrete surfaces.
Preformed Membrane
Dr. Fixit Torch shield MF are polyester reinforced, APP modified bitumen membranes. The top surface is covered by a uniform layer of natural or coloured slate chippings and the underside is finished with a polyethylene film. It has a lateral selvedge for overlapping designed as a single layer for the installation of new or refurbished flat roofs (metal or prefabricated structures, walkable or non-walkable terraces, domes or shed roofs) while also ensuring exceptional, proven performance. Dr. Fixit Torch shield MF can be used for exposed application or as a cap sheet in multi-layer systems.
