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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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.
The sketch at left common leak points at flat roof drainage systems:
[Click to enlarge any image]
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.
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.
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.
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.
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.
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.
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
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 located...in 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:
In addition to those general concerns I see the following concerns in your photo:
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:
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.
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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Frequently Asked Questions (FAQs)
Question: converting a flat roof garage, what do we do with its drain system?
(Aug 19, 2012) Hatty said:
Have a flat roof on garage with shingle and interior roof drain fitted to it . The downspout pipe runs into garage and there is a rain gutter in garage. Soon we wish to convert garage to a room but stuck on wot and how to replace and removing this down spout system?
We are on a budget so firstly if we keep this system can i plumb and connect the rain water downspout pipe straight into a internal toilet sewage pipe ? (we have a guest w.C next to garage so toilet sewage pipe runs through garage room) if we connect the rainwater pipe from garage roof into the sewage toilet pipe we block the rainwater drain on floor to prepare flooring to convert garage to room easily. Is this legal? It is only 16ftx 8ft garage roof size so i am sure the toilet waste pipe could take it. If not i dont know wot else can be done do we have to re sort garage roof and gradient and put a new slope and new guttering or could we create a new chanel or summat to divert water to new gttering is their a solution? Plz reply to my email
You'd need to either run the drain through the occupied space (a common but worrying design in my opinion) or you'd need to add tapered roof insulation to re-shape the roof surface to drain to a scupper at a low side or end of the roof. Unfortunately adding tapered insulation to re-pitch a roof deck costs about as much as the rest of a re-roof job. It might be easier to re-frame a sloped surface to drain to a suitable roof edge.
Question: leaks around the flat roof drain seerp into the fascia
(Aug 29, 2012) Terry Weir MBE said:
I am having trouble with rainwater leaking around the flatroof drain, and seeping along inside the facia board and then dripping from the corner of the wooden facia.
(Oct 11, 2012) shane said:
terry you need to lift the flat roof felt then make a lead flashing around the drain going into your down pipe make sure the lead comes up that it also sits on the plywood and then torch the felt back down over the lead this should work
Question: recurrent drain clogs and drain connected to sewer
(Nov 4, 2012) Joanne said:
We have a flat roof that was installed a year ago. Everytime it rains the drain gets clogged and the roof is like a swimming pool and is leaking in the bedroom. When the roofer came out he took out the cage that was around the drain on the roof because it was getting clogged with leaves and stopping it from draining. But the roof is still leaking everytime it rains. The drain is on the front of the house and the down spout goes in to a drain which goes into the sewer. Is there a drain or something we can do that will stop this from happening?
While I might temporarily remove a roof drain strainer as a final step in drain cleaning, completely removing and leaving-off the roof drain strainer to fix a clog is a very bad idea, epecially if the roof drain runs through the building. The strainer's job is to prevent debris from clogging the drain piping system. If that clog occurs, and if the drains run through the building, you're not only going to have a roof that no longer drains (leading to ponding, leaks, damage) but there is risk of that the drain bursts, flooding the building.
If there are overhanging tree branches that can be trimmed or similar steps to reduce roof debris that may help; so can a much larger on-roof strainer that will block debris while allowing drainage for a longer interval. But ultimately you need a regular roof inspection program to clean the drain and inspect for problems.
Watch out: connecting roof drainage or surface runoff drains to a sewer system overloads the sewer in wet weather, often causing discharge of raw sewage and often illegal in many jurisdictions.
Question: leak on commercial flat roof, ponding problems
(Nov 12, 2012) mel said:
We have a commercial building with a flat roof leaking. This is a one story building with three units dental offices. There is one unit where all the water goes down because it the lowest point of the roof. I did not notice any roof drains except scuppers around parapet walls which are higher than the roof membrane so it create a ponding. The roof seems sagging. The original roof was made of asphalt with gravels on top, one roofer cleaned the gravels and coat it with asphalt but it still leaking every time rains. To fix it they installed a TPO and it still leaks. The roofer did not have a permit when they installed the membrane. They should isn't it? The City asked us to provide a drawing plan to eliminate the sagging point of the roof. What is the best way for us to fix this problem?
From your note it sounds as if you need a competent roof inspection. If there has been leakage that led to sagging, there could be, in fact there is likely to be a significant risk of structural damage, even risk of a dangerous roof collapse. Before adding roof support to "eliminate the sagging" a more competent inspection of the condition of the roof structure is needed. In my opinion it makes no sense to proceed to a solution before we understand the problem.
Question: standard protocols for testing flat roof ponding
(Nov 13, 2012) kenneth brown said:
Are there standard "protocols" for testing of flat roofs for ponding to identify non-conformity with standards for roofing roof installations? If so, what are the protocols for such studies and/or how can I access them? I can be reached at 858-794-4684 or by email: firstname.lastname@example.org Thank you. Ken
Most experts agree that if there is standing water on a flat/low slope roof 24 hours after the end of rainfall then that constitutes ponding that shoudl be corrected.
Question: flat roof with internal building drain leaks
(Jan 5, 2013) Molly said:
I have a flat with flat roof it has a internal building drain [ secret drain] it has been there since construction. The roof needs recovering now. However I am getting a lot of mould in the rooms nearest to the secret drain. As this drain has been there for 38years. Is it possible that the drain is leaking and causing internal problems.
Certainly yes. You can track down the roof drain as an in-building leak source by following the water or water stains in the building.
Question: condensation vs roof leaks below a poorly drained flat roof?
(Feb 17, 2014) Bob said:
I have a flat roof and my back bed room roof joists are exposed and the roof boards are 1x10 and when I wipe my finger between the gap of the boards myfinger is wet with condensation and the roof leaks between the other gaps I was just up on the roof and where it is leaking there in no water or puddling only over by the down spot which is about 8to 10 feet away there is standing snow that turned to ice any ideas where the water is coming from
About the puddling 10 feet away, on a flat roof water can travel horizontally quite a distance from the leak point in the roof covering to an entry point in the roof decking where it shows up inside. I'd say if it's not raining and you squeegee off the ponded water and the leak inside stops that'd be a good diagnostic.
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