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Water leakage in flat roofs: What is the cause and how to prevent it with proper waterproofing?


Flat roofs are advantageous as they provide useful additional space in strata buildings. However, they are also susceptible to problems such as standing pools of water, increased building dead load and water leak, which will require costly repairs. Therefore, the process of waterproofing flat roofs requires careful consideration.

© Andriy Popov / 123RF

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Flat roofs are the most common type of roofs used in strata buildings in Malaysia as the spaces available can be used for utilities such as water tanks, pumps, antennas and lift motor rooms. Often, one can also find recreation facilities and swimming pools on the roofs of condominiums and serviced residences.

However, flat roofs cause a big problem in strata buildings: water leakage. Because of this, they are no longer allowed in new government buildings. This, to me, comes as no surprise. Years ago, I was asked by Prof Lillian Kwan of Hong Kong City University, “Flat roofs have been constructed all over the world. Why can’t they be done correctly in Malaysia?” The question has triggered me to look into this issue in greater depth.

What is a flat roof?

The most common type of flat roof in Malaysia is known as Protected Membrane Roofs. Besides rooftops, swimming pool decks and open decks also adopt such systems. The main substrate or base of flat roofs is a concrete slab. Waterproofing and heat insulation systems are installed on this concrete slab. Then, a 50mm-thick layer of cement screed is applied to the surface. This protects the waterproofing material against harsh weather. Finally, control joints on the cement screed are cut and sealed with elastic bitumen or sealant to guard against cracks.

NOTE: Cement screed is used as a top layer to finish floors. It consists of a mixture of cement, aggregates (clay and lime) and water.

What are the common problems with flat roofs?

In Malaysia, concrete structures are designed based on BS 8110: Structural use of concrete – Code of practice for design and construction. This code was developed based on the temperate climate and non-seismic zones of Britain. A study done at the University of Malaya has revealed that the average temperature on a flat roof in Malaysia is 82°C at 1.30pm and 17°C at 4.00am[1]. This huge temperature variation of 65°C is uncommon in the UK. Although the expansion and movement of concrete joints are not specified in BS 8110, they are crucial in countering such intense stress and movement induced by heat and other possible movements in the tropics. Sadly, I have yet to see stratified buildings with suitable joints in their roofing systems.

Ponding on flat roofs
© Kuan You Wai.

Flat roofs are also highly susceptible to developing standing pools of water, also known as ponding. This stagnant water causes the following:

  • Encourages the unsightly deposition of dirt and leaves, which may also obstruct outlets
  • Becomes a slip hazard
  • When there is damage, the building may suffer greater water ingress (penetration of water into the property)
  • Increases the dead load of the building and causes the concrete deck to creep (deformation of structure under sustained load), deflect (when an element of structure changes shape when a load is applied) and crack due to settlement (soils deform under a load of foundation structures).

Environmental pollution further contributes to the rapid degradation of flat roofs in Malaysia. The Meteorological Department of Malaysia has recorded a pH of 2 in some heavy industrial areas with high vehicular traffic in the Klang Valley. Harsh solar energy, ultraviolet ray and environmental pollutants are major degradation factors that influence the service lifespan of roofs in the tropical climate of Malaysia.

Therefore, crack-bridging – a system that provides protection to the upper layers of a concrete surface when the concrete and other materials move and shift – is critical to counter these behaviours in concrete components.

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Importance of durable waterproofing systems in flat roof

Sufficient gradient without standing water is the first line of defence against leaking, according to BS 6229:2018 – Flat roofs with continuously supported flexible waterproof coverings. Water management (drainage) complements a good waterproofing system to realise the intended service lifespan of the drainage system. Ironically, standing water is common on flat roofs in Malaysia. BS 6229:2018 specifies a minimum finished fall (a slope to shed/drain rainwater) of 1:80 at any point of a concrete flat roof with a liquid waterproofing system.

To ensure that the minimum finished fall is achieved, allowance should be made for deflection of structural members and decking under dead and imposed loads, and for the construction tolerance. What this means is the fall for the design should be steeper than the recommended fall, and not be limited to the surface of the cement screed but extended to the surface where the waterproofing membrane is installed. If there is insufficient fall during the casting of the concrete slab, a layer of cement screed must be applied to achieve the desired fall.

Elements of a good waterproofing system

Flexural fatigue resistance, which is the ability of a membrane to be stretched and return to its original length without permanent elongation and damage, is the key factor in a durable waterproofing system[2]. It plays a major role in catering for movements suffered on flat roofs, including fluctuation in temperature, moisture and transitional movement. On top of that, to prevent stress and breakage on waterproofing membrane on areas with anticipated cracks and dynamic cracks, angle fillets and bond breakers must be installed according to Clause 2.7 in AS 4654.2: Waterproofing membranes for external above-ground use Design and installation (40mm X 40mm fillets for sheet membranes or 15mm X 15mm bond breaker for coating system at any horizontal to vertical transition, or vertical to vertical transition).

waterproofing membrane system for flat roof
Bond breakers for cracks and dynamic joints. Source: AS 4654.1-2009 Waterproofing membrane systems for exterior use – Above ground level.

Besides, waterproofing products are known as construction chemicals which may interact with different materials on the existing subtract. Therefore, proper material selection is critical. For instance, asphalt and bitumen must not be mixed or come into contact with each other. PVC also is not compatible with bitumen and with polyester.

Workmanship usually suffers in hard-to-reach areas and areas with penetration of utilities, upturn and downturn termination. Detailing in water-tight termination and transition, which can counter movements within the design of angle fillets and bond breakers, is critical to the durability and performance of a waterproofing system. “The devil is in the details” is particularly true in the installation of waterproofing systems.

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How to diagnose a waterproofing failure?

ASTM D7052/D7052M-17: Standard guide for determining and evaluating causes of water leakage of low-slope roofs outlines a step-by-step approach for a holistic investigation of a water leak. It starts with a review of the design of the flat roof, a review of the service history, detailed visual inspections of various roofing components, interviews with the service crews to determine the extent of water damage and a review of weather records in the vicinity.

Wet insulation in roofing systems can be located using an infrared thermographic imager. Water pockets under the surface of a concrete flat roof as a result of waterproofing failure can be easily identified at night due to the relatively higher thermal capacity of water.

Diagnosing waterproofing problems on flat roof
Identifying hidden water pockets under the surface using a moisture meter. © Kuan You Wai.

Leakage from flat roofs and external walls can be easily misunderstood due to the proximity of these building components. Therefore, diagnosis of water leak from flat roofs must be extended to the external wall of the floor right below the flat roofs.

Remedial actions for flat roof water leaks are different from those for cracked walls. A drone may assist in the inspection of cracked walls due to the difficulty in accessing these walls for close inspection.

Maintenance and repair of flat roof

The service lifespan of roofing systems may be significantly extended with periodic maintenance, ideally twice a year. A maintenance manual containing basic information and guidance for inspection and maintenance should be kept, and the scope should include flashing, expansion joint covers, copings, edges, termination, membranes and choked rainwater downpipes. Other than that, soffit (horizontal, aloft underside of any construction element) under flat roofs must be checked for potential water leaks and corrosion damage.

In the event of damage before the expiry of the service lifespan, repairs should only be carried out after the type and extent of defects have been determined and their underlying cause identified. Additionally, repairs should be carried out using only materials compatible with the existing roofing system.

Once concrete spalling and rusty reinforcement steel bars have been located, diagnosis and repair according to EN1504: Products and systems for the protection and repair of concrete structures are recommended in order to mitigate and control the corrosion in the reinforcement steel bars. It is also important that the loading ability and rigidity of the affected structural components be restored in order for the remedial waterproofing system to perform optimally.

concrete spalling building defects
Spalling concrete soffit. © Kuan You Wai

Upon expiry of the service lifespan, a renewal or refurbishment exercise is needed. Design procedures for renewal should be the same as for a new roof. However, the reason why the original roof system failed should be investigated and rectified[3].

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Which is the best waterproofing system to use for flat roof?

Due to the noise and dust involved in the removal of existing cement screed and other insulation and waterproofing components, overlays with coating or preformed membranes are preferred. Meanwhile, fibre reinforcement can improve the crack bridging property of the waterproofing coating.

Modified bitumen membranes and bitumen torched membranes are widely used in overlaying refurbishment projects. Nevertheless, Zainuddin S. has observed premature failures of this system in her study of selected government buildings in Putrajaya, which revealed cracks, punctures and stretches with detached edges in such membranes[4].

Single-ply membranes include EPDM (ethylene propylene diene monomer rubber), PVC (Polyvinyl chloride), TPO (Thermal Polyolefin) and KEE (Ketone Ethylene Ester) EPDM and TPO membranes are combustible and not allowed on flat roofs under the Uniform Building By-Laws 1984. PVC membranes leach plasticiser when the temperature reaches 60°C. They also lack the elasticity to counter movements in flat roofs in the tropics.

KEE Single Ply waterproofing membrane
© Kuan You Wai.

KEE membrane is by far the most suitable waterproofing system due to its permanent elasticity, cooling ability, reparability and robustness against foot traffic.


There is no rocket science in constructing durable high-performance roofing systems for flat roofs in the tropics. Considerations must be given to the unique degradation mechanism in order to come up with a sound design. And instead of initial acquisition costs, life cycle costs should be taken as a major consideration in the selection of a waterproofing system. Quality control in detailing is crucial to overall performance, and regular inspection and maintenance are pivotal to realise the full designed service lifespan of the system.

Kuan You Wai is a member of the National Roof Contractors Association (USA), International Institute of Building Enclosure Consultants (USA) and International Concrete Repair Institute (USA), among others.

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[1] Mujaheed A., The Performance of Selected Waterproofing Membranes, University of Malaya, 2016
[2] British Standard Institute. BS 6229:2003 Flat roofs with continuously supported coverings- Code of practice. p27-32.
[3] Beech J., Durability Assessment of Roofing Membranes, International Symposium on Roofing Technology, Building Research Establishment, 1991. p 227.
[4] Zainuddin S. Flat Roof Maintenance: A Case Study of Selected Government Buildings in Putrajaya, IIUM, Aug 2009.

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