Flat roof ponding near drain - should roof be raised?


Yes, we all do...it was just funny how you asked. But, from me, yeah totally. If you can raise it then by all means do so. And/Or, call your roofer who did such a horrible & woefully incomplete job.

You'll have to greatly expand your puddle zone in order to keep everything not only sloped toward the scupper but also toward the center or the line of the scupper. Think of the roof as a sink, shower or tub, but the drain's not in the floor. You MUST have 2 pitches to direct any & all water to that single point.

Now, the raising issue. Of course, shim & re-shim as you raise & until your happy with everything. But, once you feel the job is complete you MUST re-secure the roof with short screws (1/2" if you don't know the thickness) into the roof decking & nails or screws into BOTH sides of the joists.

You should use galvanized bent Mending Plates or L-brackets. You want, at least, as many bracket points as there are nails that you backed out. FYI, the best support...

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For most building owners and property managers, protecting the roof system against water penetration has major ramifications for the value of the property. Ponding is the phenomenon of water pooling on flat roofs after storms, snow melts, or heavy rains. Pools should drain away without causing any damage to the structure – but owners and managers should know when to take action, how to identify the cause of the ponding, and how to resolve it.

Ponding Water 101: When Can it Damage the Roof

Ponding water isn’t an immediate threat – in fact, roof membranes are made with the same material as pond liners, so water penetration is unlikely. Over time, however, ponding water can lead to accelerated aging of the roof membrane.

Ponding water accelerates roof system aging by:

Degrading the roofing material itself: This is a particular concern for older built-up and modified roofs. Intensifying the sunlight: On modern white roofs, the water focuses the sunlight that...
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Alan Carson July 26, 2017

Roofs should not be dead flat. They should slope to the perimeter or interior drainage. Any roof that still has water on it 48 hours after a rain is defined as a ponding roof. If there has been no rain for a considerable amount of time, there may be no water, but you may be able to determine that there has been ponding. Circles that are particularly dirty on the roof, or show algae or vegetation growth, are indications that water ponds on the roof.

Ponding may be caused by:

Poor design with no slope. Sagging of the structure, disrupting the slope. No drains or an inadequate number of drains. Drains that are blocked or are at high points on the roof.

Ponding on flat roofs has a number of implications including reduced life expectancy of the roof, structural damage, leakage and possible vegetation growth.

Water on the roof is your first clue that there is a ponding problem. Dirt, algae and vegetation are other indications....

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Did You Know?

Many of us with flat roofs know that they often do not drain as well as they should.

Eventually, ponding water will deteriorate the roof coating and the roofing below, resulting in premature failure of the roof system.

Why Don't Roofs Drain Properly?

Poor roof drainage could be from settling of the building, structural changes, or construction defect.

Often roof drains are installed too high in the wall.

How We Correct Ponding Water On Roofs

We have several methods of correcting or reducing the amount of ponding water on your roof.

Enlarging the Drains

Sometimes a drain can simply be enlarged to accommodate more flow and to allow tree debris to pass and not back-up onto the roof.

Lowering the Drains

At other times the drain can be lowered in the wall, recessed in the deck, or tapered to allow for better drainage.

Raising the Roof Level

Very often, insulation can be added...

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FLAT ROOF DRAINAGE SYSTEMS - CONTENTS: Flat Roof Drainage System specifications, designs, troubleshooting problems with scuppers or interior drains on flat or low slope roofs: flat roof drains, scuppers, and screens. How to calculate the drainage requirements for roofs. POST a QUESTION or READ FAQs about keeping the flat roof drainage system working or about installing new drains in a flat roof REFERENCES

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

This article describes defects in roof leaders or downspout systems such as inadequate number of downspouts to handle the volume of roof drainage, missing, lost, or damaged downspouts. This article series discusses how to choose, install, diagnose & maintain roof gutters & downspouts, & roof drainage systems to prevent building leaks and water entry.

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By Allen Sensel

Moderate Water Ponding


There is a lot of speculation with older building designs and how their flat roofs have a tendency to collect pools of standing water referred to as ponding. The purpose of this report is to provide clarity for a commonly misunderstood notion that a flat roof, whether it’s asphaltic built-up, modified bitumen, or any kind of single-ply membrane, should not bear any kind of collection of rain water. I will provide brief examples from the IBC, U.S. Department of Housing and Urban Development, and Factory Mutual, all of whom mandate construction and renovation standards to comply with regional climates,weather patterns, and safety.


Ponding water, as described by the IBC and recognized by the U.S. Department of Housing and Urban Development, is water that remains on a roof surface longer than 48 hours after the termination of the most recent rain event.

Standing Water


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While most of the drain manufacturers have always produced plumbing products and have had very little roofing experience, it is understandable how a lot of the problems that exist with roof drains have come to be. Froet on the other hand, has experience from the roofing side, so we developed our drain from the top side down and not the bottom side up. We feel we have a better understanding of what makes a good roof drain.

The integration of the roof drain into the roofing system has also been kind of a void area due to the integration of the Architect and the Engineer. It is an issue of where one stops and the other takes over. It has been a common problem area for years. I deal with it almost every day. How is the drain being installed? How much insulation is at the drain? Is there a sump? Is the drain being installed flush with the roof surface or the roof deck? It sometimes takes a surprisingly long time to get these...

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Look how the City Hall of Chicago has embraced the unique qualities of its flat roof – due to its low slope, its inhabitants have been able to cultivate plants by containing water on its level plane. This “green roof” has been designed for this purpose, but if a regular flat roof starts containing water, serious damage is possible. Here we explore how ponding, or areas of pooling water, can damage flat roofs.

How does it work?

Gravity pulls water to the lowest point that it can go. This means that unless your roof is properly sloped and free of depressed areas, water will collect in the lowest points on your roof. Pooling water strains your roof with additional weight and accelerates deterioration. Ponding also makes your roof much more susceptible to leaks. If you notice water filled craters on your flat roof, this is indeed ponding, and you should contact a roofing professional.

Areas that are prone to ponding can often be raised so that water will drain off...

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When it comes to flat roofs, their biggest problem isn’t always water displacement – it’s where the water goes as it travels down its low slope. For that reason, the drainage you have for your flat roof can be the deciding factor between a long-lasting roof or major problems.

There are three main drainage systems that you’re likely to have for your flat roof. The type of drainage system you have will depend on a number of factors, including square footage, climate, cost, and the building’s architecture.

Internal Drains

Internal drainage systems are composed of an interior network of gutters and pipes that are placed at intervals along the roof’s field, with the main drain located at the center.


Scuppers are openings at the roof’s parapet walls or perimeter curbs that project the water off and away from the building’s foundation.


Gutters are installed at the bottom of the slope to direct water to downspouts...

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We just replaced the entire upper roof with Firestone EPDM .60 Gauge rubber about 3 weeks ago. The insulation underneath was replaced with 4 inches of insulation board. The base is corrugated steel.

A ditch had formed from ponding in the old roof along the right edge, just 10 years after the last entire rubber roof replacement. We asked him for a quote for fixing just that area. He gave us a quote for that, and a quote for the entire upper roof. We decided to do the entire upper roof. The ponding is happening in the same areas as before.

He wrote any additional work on low lying areas would be additional, and that we wouldn’t know until he tore it off. When he did tear it off, he said he felt it had good slope.

He was told that we didn’t want water going beyond the drain and ponding in the corner of the roof. During an average rain, the water is going over the edge of the roof and coming through holes in the facia. During one extremely hard storm, there was a...

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