Why Shouldn’t Flat Roofs or Podiums have Puddles?

Flat roofs and podium decks will collect some water during rain. The good news is that pooling/ponding water is not an imminent threat to your building because rooftop waterproof membranes are designed to resist water, so leakages are not likely to occur due to a short time of water ponding on the surface. Whilst the good news is that rooftop ponding is not likely to cause a rapid reduction of the waterproof membrane life expectancy, the reality is likely to be different if there are dips/low points on the surface and/or around drains, and it takes more than two days to drain and evaporate. This can eventually cause major damage to your home or commercial property, if not rectified.

Water ponding on a flat roof is one of the main causes of waterproofing membrane deterioration because most membranes such as acrylic membranes are not designed to withstand pooling water. That being said, long-term contact with moisture is bad for any roofing material. Dust, plant matter, pollution and other items from the environment mix in with the pooling water, turning it into a muddy consistency that attacks the membrane. When the water dries out, the crust cracks and puts the membrane under stress. Over time, this often causes the membrane to blister, crack and peel – allowing water to leak through to the substrate.

This is why proper fall design is an essential consideration in the overall drainage of a flat roof. Falls create flow paths to direct the drainage of rainwater away from the roof to suitable discharge points. To be effective, it is essential to clear surface water as rapidly as possible from the flat roof to avoid ponding or stagnation of water on the roof itself.

For a flat roof to function properly and not leak, the surface water needs to be discharged quickly without ponding or stagnation. If the ponding lasts for more than a couple of days, it is time to contact Singapore Waterproofing/contact us to discuss options to mitigate any potential water ingress and damage to your property caused by a defective waterproof membrane. Our leak sealing and concrete repair specialists are qualified to diagnose the root cause(s) and tailor and deliver remedial solutions to help prolong the service life of your building.

How to fix a waterproof membrane damaged by ponding water?

How to fix ponding on a flat roof depends on the cause of the ponding. The issue may have to do with the slope of the roof or with its drainage. Sometimes air conditioning units, skylights, or vents that are installed on the flat roof may stop the intended flow of water and cause pooling or ponding in their vicinity. Whatever the cause, you cannot leave it be, or you could risk serious damage to your roof. Find out how we can reduce low points and refurbish the waterproof membrane in our case example below. (Remedial solutions may vary. Sites are assessed and a remedial solution is tailored to address the identified cause.)

This large outdoor podium on a garage roof in Brisbane just had a waterproof membrane repair carried out by our solutions leak sealing specialist team. 

The existing acrylic waterproof membrane was in overall sound condition, needing only some areas repaired and an installation of a new coat of heavy-duty modified acrylic waterproofing membrane over the existing waterproofing membrane. The damage to the membrane was observed to be caused by ponding water in low points on the surface of the rooftop podium. 

The top of the stone-clad dividing wall was exposed to the elements, allowing water to soak into the wall and track down into the building below. We recommended treating the wall with PWR (Penetrating Water Repellent) followed by installation of capping on the top of the wall.  


  • Pressure cleaned existing acrylic waterproofing membrane. 
  • Primed and installed premium joint sealant of cracks.  
  • Built up low points to reduce ponding.  
  • Installed a new maintenance coating of high-quality heavy-duty modified acrylic UV-resistant waterproofing membrane over the existing waterproofing membrane. 
    Sealed around bases of balustrade posts. 
  • Installed PWR (Penetrating Water Repellent) to the stone-clad dividing wall. (Capping will be installed on the top of the wall by other trades.)