Downspout used for air conditionre refrigerant line (C) Daniel Friedman Guide to Flat Roof Drainage Systems for Disposing of Roof Runoff

  • 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

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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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Flat Roof Drainage Systems

Downspout used for air conditionre refrigerant line (C) Daniel Friedman

Article Series Contents

The drainage requirements for flat roofs on buildings present some special problems that we describe and illustrate here.

At page top and at left our photos show a low-slope roof surrounded by a parapet. Roofs of this design will have one or more roof drains or scuppers installed.

Watch for clogs, blockage at the roof drain or scupper.

And as our photographs below illustrate, look closely for leaks and damage around the drain opening where it penetrates the roof, especially in freezing climates.

Downspout used for air conditionre refrigerant line (C) Daniel Friedman Downspout used for air conditionre refrigerant line (C) Daniel Friedman

Gutter and Downspout Details (C) Carson Dunlop Associates

The sketch at left common leak points at flat roof drainage systems:

[Click to enlarge any image]

  • Scupper drains become blocked by leaves or other debris
  • Through building drains (upper right in the sketch) become clogged internally and burst, flooding the building, or leaks occur around the drain itself where it penetrates the roof
  • On flat or low slope roofs that drain to one end (lower right in the sketch), especially if the building rear wall does not include a projecting eave or soffit, a clogged gutter or downspout send water down the building rear wall, flooding the structure through its wall or through windows.
    Sketch courtesy of Carson Dunlop Associates.
Gutter and Downspout Details (C) Carson Dunlop Associates

Carson Dunlop explain in their sketch (left) that some flat roof designs relying on scupper drains and downspouts provide a set of secondary roof drain scuppers located 2" above the roof membrane.

These emergency drains will prevent deep flooding (and risk of collapse or catastrophic building interior flooding) should the main scupper drains become obstructed by debris.


Sketch courtesy of Carson Dunlop Associates.



Problems & Solutions for Interior Drains on Flat Building Roofs

Gutter and Downspout Details (C) Carson Dunlop Associates

Interior drains on flat roofs are commonly used in urban areas. The drain runs through the building and connects to a storm drain or in some communities where it is still allowed, to the sewer system.

(Many communities have made it illegal to connect roof drains or surface water runoff drains into the sewer system because during heavy rainfall the sewer system will be flooded, resulting in discharge of raw sewage into the environment).

Watch out: debris entering the roof drain system lead to clogging and leaks into the building interior. In freezing climates a clogged interior building drain eventually freezes and bursts near the roof surface.

When the roof has been flooded due to a clogged interior building drain and the drain subsequently bursts, a huge volume of water (from the flooded roof) enters and floods the building interior.

Inspect and clean roof drain screens and drain piping at least annually to avoid this building flood catastrophe.

Sketch courtesy of Carson Dunlop Associates.

Gutter and Downspout Details (C) Carson Dunlop Associates

Here we illustrate just how interior drains on flat roofs become clogged, causing rising ponds of water on the roof surface. Sketch courtesy of Carson Dunlop Associates.

The combination of ponding water on the roof around the roof drain and the mechanical disturbance caused by freeze-thaw cycling act to loosen the drain connections, break the roof drain seal, and cause leaks into the building below.

Clogged Roof Drains, Ponding Water, Sagging Roofs: Solutions

When the through-building roof drain system has become clogged, even if it does not burst and lead to building flooding, it causes other building problems.

The roof structure may sag due to the added weight of ponding water.

Gutter and Downspout Details (C) Carson Dunlop Associates

Watch out: in severe cases of deep ponding water (or water and ice and wet snow) on a roof with blocked drainage, the weight of the water and ice may conspire with hidden rot or insect damage (such as from prior roof leaks) to cause catastrophic roof collapse - a very unsafe condition.

Sketch courtesy of Carson Dunlop Associates.

Adding Drains for Ponding Roofs

It is possible to add new roof drains to relieve ponding on a sagged flat roof, as shown at left.

Notice that the new drain extends (through the shortest possible route) to the building exterior where it is conducted to ground or to a storm drain.

Gutter and Downspout Details (C) Carson Dunlop Associates

Using this method permits adding a drain at the roof's low point.

When the building is re-roofed, the roofer may install tapered foam insulation to direct water to roof scuppers or to a main or new-main roof drain to prevent future ponding. Sketch courtesy of Carson Dunlop Associates.

Reader Question: roof drain clogs lead to flooding in my apartment

Overflowing at low slope roof drain floods apartment (C) InspectApedia BMRI sent some picture to you and really appreciative your expertise on the water overflow in the elevators a while back.

This is the latest picture I have of what took place yesterday while the roof tank was supposedly being cleaned. This particular picture is directly outside of widow where this canopy drain is other words believe it or not it can overflow into apt. Its very close to my windows

How the building did that I will never know. Terrible disarray

However can you please offer me again your expertise on this issue as it is very scary for me to live with this condition that is totally ignored by management

This is all in addition to other picture I have sent you i. e. weep hole in brick hot spots on floors along with vibrations I inadvertently tapped 2014 as opposed to 2013 on my camera.

Don't know where to go with this or who to trust weather it be a plumber /electrician /or home inspector. - BMR 10/09/2013

Reply: recommendations to reduce the risk of building leaks due to roof drain SNAFUs

BM: There is additional important information that an on-site inspector would have to observe and then consider before offering a confident diagnosis about the roof drain overflow problem that you and your photo describe in your email. But I can offer a few comments, and also I refer you and your building superintendent to the low slope roof drain design discussion and suggestions found at FLAT ROOF DRAINAGE SYSTEMS.

Low slope roof drains are indeed subject to clogging that in turn can lead to serious, even dangerous conditions such as:

  • Roof flooding, weight and structural failure worries, an occasional roof collapse, particularly if the flooding and weight on a roof are combined with structural damage from rot or insects as may have occurred on an older building or one whose roof had a history of leaks
  • Roof overflow sending water into nearby windows, doors, entering the structure - such as the case you describe.
  • Roof leaks - leaving standing water on a roof not designed for that condition invites roof leaks and failure both from the continued presence of water and in freezing climates, from further damage to the roof covering from freeze-thaw cycles.
  • Building interior flooding also is more likely when a roof drain pipe passes through the building interior rather than outside the structure; a freeze-up or clog followed by freeze-up of the roof drain segment that is at or close to the roof top leads to leaky or burst drains and building damage.

In addition to those general concerns I see the following concerns in your photo:

  • What looks like algae formation around the roof drain, suggesting a history of clogging, or imperfect drainage and standing water;
  • The location of the roof drain in a corner invites accumulation of on-roof debris and thus clogging - worse if there are nearby trees or other sources of on-roof debris;
  • The location of the roof drain inches from a chimney invites leaks into the building around the chimney, particularly if the roof floods in this low area, bringing water levels above the chimney flashing. These conditions also invite freeze-thaw damage and thus leaks at the chimney if your building is located in a freezing climate. See FLASHING, CHIMNEY Mistakes & Leaks

While an on-site expert would undoubtedly have more suggestions and could perform a more reliable inspection than anyone can do based on just a photograph, it seems to me that at a minimum your building maintenance people will want to:

  • Inspect the current roof drain(s) and the drain line through the building for leaks and assure that the materials are in good condition;
  • Inspect the roof surface and roof drains on a regular schedule to find and clear away any debris clogging the drains; just how often this is needed depends on local condition such as the rate at which debris accumulates on the roof.

On some buildings it is possible to adjust the roof slope and amend the roof drainage to convert to roof scuppers that drain out through a parapet wall to the building exterior - eliminating a good part of these clog and leak difficulties.

Whom do you trust to fix these problems? It is the building owner's responsibility to find and employ knowledgeable, reliable people; you may just aggravate the owner or maintenance staff if you are too directive, and worse, if you try to specify the exact repair then you will own the problem rather than leaving it in the hands where it belongs.

But certainly the solution to roof drainage troubles is not something to leave to an electrician and usually not a typical home inspector. A roofing contractor who has experience with low slope roofing installation and repair is where management should turn; I'd expect such a person to be in general agreement with the information given here as our views are based on guidance from roofing industry expert sources cited at REFERENCES in this and each of our roofing articles.

Specifying the Drainage Requirements for Flat Roofs

Reader Question: number of roof drains needed on flat roofs

6/24/2014 David Howard, AIA said: What is the general area of a flat (1/4"/1'-0" slope) single-ply roof for each roof drain (ratio for the number of roof drains / roof area).


To design a sloped or flat roof drainage system we need to know the anticipated maximum rainfall rate and the flat or projected roof area. I examined several roof drainage calculators and tables to see what they tell us.

Please see ROOF DRAINAGE CALCULATIONS for a detailed answer to your question.

Watch out: keep in mind that a design without adequate safety margins risks dangerous structural collapse of the roof or other catastrophes such as a building flood. Just consider what happens in "real life" when roof drains become clogged because no one remembered to inspect and clear the drains.


Continue reading at ROOF DRAINAGE CALCULATIONS or select a topic from the More Reading links shown below.

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