Why Does the Water Pool on Your Flat Roof? – Family Roofing

Posted on by David Reid

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Why Does the Water Pool on Your Roof? 

A well-designed roofing system should prevent water from leaking into your home.  Even “flat” roofs are designed with a slight slope for proper drainage.  Ponding on your flat roof  can mean a big problem with your roof’s drainage system.  So when you see water ponding on your roof, then it should be addressed immediately.

The most common reason that flat roofs hold or pond water is because of inadequate or obstructed drainage.  The good news is that this cause of pooling water is usually the easiest to correct with scheduled roof maintenance.

 

Flat roofs drain in one of three ways:

  1. Through roof drains

  2. Sloping towards a gutter

  3. Through a scupper – collector head that leads to a downspout.

When these drains/gutters are clean and water is able to flow ponding water issues are greatly diminished or non existent.

The second cause of ponding water on a flat roof is more complicated and cannot be resolved by simple cleaning the drains.

This type of standing water is usually caused by either poor design or from deflection or sag in the buildings roof structure.

Improperly sized, not enough, or poorly placed flat roof drainage can cause mild to severe ponding water issues.

However this standing or ponding water is usually never as bad a when the roof structure has deflected or sagged over time. 

When the structure that the flat roof is installed upon has settled or sagged the amount of ponding water will be equal to the distance that the structure has deflected below the roof gutters or scuppers.

Flat roofs with internal drains are rare affected by a sagging deck as the water will still drain toward the low spot.

This is a common problem on buildings where the flat roof is installed over a concrete deck that has deflected or sagged over time.

The sag or low spot in the roof causes water to stand instead of running towards the gutter or drain.

The Damage Done By Ponding Water

When water sits in one place on your roof, it has the ability to do damage to the roofing materials underneath it.  Here are some examples of the damage that can be done by ponding water on your commercial roof:

  • The Breaking Down of Roof Coatings: As water sits on your roof, it’s in direct contact with the roof coatings.  Over time, the water itself can affect the stability of these coatings. As this water heats up in the sun, its effects on your commercial roof coatings can be even more detrimental.

  • Damage to Membrane Seams: Pooling water has the ability to slowly deteriorate and work its way through seams in your roof’s membrane.  Over time, as water continually pools and heats up in the same area, that area can be negatively affected and become vulnerable to leaking.

  • Dirt & Debris: Wherever water pools on your roof, dirt and debris will also appear.  Dirt and debris gets caught in pools of water and begins to rot.  Algae and other growths can also begin to form in these areas which can directly impact the strength of your roofing materials and its coatings.

Overall, ponding leads to the accelerated deterioration and erosion of your roofing materials, which ultimately results in water penetrating through your roof.  When water penetrates your roofing materials, it can affect the insulation and effectiveness of those materials in terms of energy efficiency.