(A-One) Types of Drains for Flat Roofing

One of the worst things for the health of your roof is water. Specifically, the ponding of water. Ponding is common on the flat roofing of commercial buildings. The top reason for ponding is poor drainage, as a result of incorrect installation of the roofing. Roof contractors need to account for the need to drain water. Water can pool in different areas and cause all kinds of problems. That being said, even if your roof is installed correctly, exposure to extreme weather such as hail or wind storms can damage your roof over time. This damage can affect the drainage on your flat roof causing ponding water to form. As a rule of thumb, you want 90% of water off your roof within 48 hours of a rain event.

Best Material to Use for Drainage Systems

How Ponding Water Damages A Flat Roof

Ponding water is very damaging to a flat roof. Water is heavy, and when allowed to sit upon your roof for extended periods of time, it begins to damage the materials beneath it. Some of the ways ponding water can damage a flat roof include:

  • It allows dirt and other debris to accumulate on your roof – The dirt and other debris get caught in the water and begin to rot, which in turn causes algae and other bacteria to begin growing. The strength of the roof and coating will be impacted.
  • Ponding water damages the protective coating – As the water sits on top of your roof, it is in contact with your roof coating the entire time. Contact with standing water decreases the stability of the coating and when the sun heats this water up during the day, it can destroy the roof coating. Your roof is left vulnerable to the elements.
  • Ponding water damages your roof’s membrane systems – Over time, ponding water will seep through membrane seams and as the sun continually heats the water, the roof’s membrane will become damaged. The result will be leaking into the building.

Pooled water that increases the damage and causes leaks allows water into the roofing system. During colder weather, this water can freeze, causing major damage to the interior of the roof, dramatically reducing its lifespan.

What You Can Do?

If you have ponding water on your flat, commercial roof, there are several things you can do. T best options we can advise will depend on your specific situation. As every roofing system is unique, these are the recommended steps you can take.

  • Install a tapered roof as one option – This will keep water from ponding on top of your flat roof by directing water to the building’s internal drainage system. Tapered roofs are installed at an angle that naturally leads water toward gutters or internal drains.
  • Install a better drainage system – If your current drainage is inadequate, you can correct this problem by adding a roof drain. You will need to open the existing roof down to the deck and core a hole through the deck in order to set the new drain assembly. A pipe is then used to connect the existing drain to the new drain assembly. You then tie the new drain assembly to the roof’s existing drain system.

You can also replace the roofing membrane if either of these options is too expensive. There are many PVC membranes that are designed to withstand collected water pools. In addition to this, it also helps to develop and stick to a proper maintenance schedule. Checking your roof regularly and keeping it maintained will go a long way in the longevity of the roof and the reduction of pooled water. We recommend:

  • Checking the roof regularly for ponding
  • Adding additional drains to the roof system if ponding is excessive
  • Having the drains professionally cleaned
  • Ensuring downspouts and drain pipes are not blocked
  • Scheduling regular inspections with a professional contractor

Choice of Membrane Systems

In recent times, PVC and Cast iron have become the top options for use in commercial construction. Cost and ease of use are often the justification for making the choice of PVS, but there are a few things you need to consider when making a decision about membranes.

  • Durability: Cast iron often lasts significantly longer than PVC piping but it is susceptible to rusting if not maintained properly. The lifespan of PVC is harder to predict, only because it has not been used in roofing as long as cast iron, as it’s used in drainage systems doesn’t have as long a history as does PVC. PVC has a lower temperature limit and a higher expansion rate, but PVC is not fireproof.
  • Cost: The cost for installation of PVC piping is significantly lower than that of cast iron and PVC is easy to work with. PVC does require more hangers and supports as compared to cast iron.
  • Noise: PVC piping noise is much greater as compared to cast iron because it is less dense. It also does not have the dampening effect that cast iron provides.


Whatever questions you have regarding roofing membranes, materials, and proper drainage for your roof, we are here. We will help you get your roof ready for any weather and make sure ponding is an issue you never have to worry about.

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