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

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.

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

- Daniel Friedman, Publisher/Editor/Author - See WHO ARE WE?

Flat Roof Drainage Systems

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

Article Contents

[Click to enlarge any image]

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.

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

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

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

[Click to enlarge any image]

Gutter and Downspout Details (C) Carson Dunlop Associates

Carson Dunlop Associates explain in their sketch (above 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.

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, a Toronto home inspection, education & report writing tool company [ carsondunlop.com ].

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. The roof drain ketch at above left was provided courtesy of Carson Dunlop Associates, a Toronto home inspection, education & report writing tool company [ carsondunlop.com ].

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

Gutter and Downspout Details (C) Carson Dunlop Associates

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.

Clogged roof drain (C) InspectApedia 2015

In the clogged roof drain photo provided by a reader (anon. 2015) and shown at above right we see ponding at the low slope roof drain, a clear indication that even though we don't see debris around the drain screen this drain line is clogged with debris or ice.

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, a Toronto home inspection, education & report writing tool company [ carsondunlop.com ].

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, a Toronto home inspection, education & report writing tool company [ carsondunlop.com ].

Watch out: Roof drain clogs lead to flooding inside the building!

Overflowing at low slope roof drain floods apartment (C) InspectApedia BMRReader Query: I 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 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 on this page.

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 atReferences or Citations in this and each of our roofing articles.

Flat Roof Drain Installation Guides

 

Flat Roof Drainage Requirements / Specs

Question: number of roof drains needed on flat roofs

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). - 6/24/2014 David Howard, AIA

Reply:

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.

Also see the roof drain installation guides on this page as some of those have tables and guides for roof drain sizing and number and location.

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.

...


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Reader Comments, Questions & Answers About The Article Above

Below you will find questions and answers previously posted on this page at its page bottom reader comment box.

Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

Leak Problems at Canopy Marquee Roof Front - what's the drain capacity?

Please see picture below of drains on Canopy /Marquee of roof In front of building

Can you please offer me your expertise as to what is the Tar /dirt surrounding this drain

roof drain debris and broken ring (C) InspectApedia.com Anon

Sometimes random odor

I hope this sheds more light on this toxic mess And Noone else has this same problem. I just need to know who would be the best Expert to call …i e in what capacity would they be ? - On 2022-08-25 by Anonymous by private email

roof drain debris and broken ring (C) InspectApedia.com Anon

Reply by InspectApedia-911 (mod) - roofs receive various dust and debris from the surrounding area

@Anonymous by private email,

roof drain debris and broken ring (C) InspectApedia.com AnonRoofs receive various dust and debris from the surrounding area - depending on where your building is located: country, city, local climate conditions, etc. as well as wash-down from equipment or fittings on or that drain onto the roof.

Your photo is a bit blurry so I can't make out much about that dark debris around the roof drain but it looks like a combination of dirt and larger granular particles - perhaps bigger than the mineral granules on that roll roofing or modified bitumen roofing that were rolled up and over the parapet wall (likely to have a reduced life when bent like that)

- so you'd look around the roof to see what other materials are present and what particle sources (trees, roads, chimneys, equipment, etc) are nearby.

So that black muck in your photo might be simply organic dirt and debris, road dirt, or possibly even oxidized flashing cement. It looks perfectly normal, but then all we can see is a microdot of information compared with what an on-roof inspection would tell an expert.

Why are you asking? What problem are we solving? Fill us in and we may be able to offer more specific suggestions.

Help diagnose my roof drain trouble - jockey pump leaking

@InspectApedia-911,

roof leak causes bulging wall (C) InspectApedia.com AnonTks very much for your expertise

I’ve been told the jockey pump on the roof was leaking. Each time they clean the tank on the roof or a heavy rain this problem exists. Also if you noticed from my pictures …the ring around the drain is broken and I believe thats why it’s leaking and the drains will backup if it’s not replaced. It’s a Smith 1330 ring.

There is standing water under foundation of building when it rains …,,( puddles in the ground gutter ) Organic materials (algae- conducive to mosquitos)

I guess I need an on site inspection as it is waste water backing up mixed with rain water due to no separate drain ( combined)

Attached please see walls bulging from the marquee into my apt plus soapy drains /along with all sorts of bugs feeding off this standing water.

It’s hard to believe but This marquee literally abuts into my apt. It should have gone through that lobby of building. I’m on the 2nd floor. - On 2022-08-25 by Anonymous -

Reply by (mod) - debris, broken ring, and bulging wall indicate leaks or blockages

@Anonymous with roof drain trouble,

Indeed that bit of more information - leaks, broken ring (not quite so obvious in the dark area of the earlier photos) is helpful. The quantity of debris at the drain in your first photo is very small - and would not explain a roof leak,

Above on this flat roof drain repair page, we note the importance of regular on-roof inspections and clearing out around roof drains. Such drain blockages can lead to severe water leaks into the building and in extreme cases can even cause a roof to collapse.

Your additional photos show a bulging wall indoors - that could be from water in the wall from a leaking roof drain (or something else), and a photo of sudsy material around the drain -the source of that froth and its composition are unclear from our discussion so far.

But certainly by their very nature, as flat and low-slope roof drains run down through the building, a blockage or clog, especially in a freezing climate, (you've not answered our questions about country, city, building age, maintenance, leak details) can burst the roof drain and later, as ice thaws, send lots of water into the building structure, risking rot, insect damage, and costly hidden mold contamination.

Any experienced low slope roofing contractor ought to be able to inspect the drains to give you advice on what repair is needed.

But if you're stuck with people who will only go on the roof and don't consider the building interior, you might need to hire a plumber who has a sewer line camera that can inspect the roof drain throughout its route to see where there may be damage and thus where repairs may be needed and that must be made from inside the building.

If you can change to an all exterior drain or scupper drain you will eliminate the risk of clogged or leaky drain piping that bursts and leaks inside your structure. The time to do this is during new construction - which is now in your case.


My rubber roof is turning green - is that ok? What do we do?

Algae on an EPDM roof in Poughkeepsie (C) Daniel Friedman at InspectApedia.com

One story sloped rubber roof over kitchen where drain side (runs on wall) of roof appears to be turning (about 1/4 of roof that side) green.

Same side inside kitchen ceiling appears to be small damage beginning. Could this be clogged drain or what? Have tall tree over building also. Can you advise. - On 2017-10-28 by marlene gadson -

Reply by (mod) - Rubber roof turning green - watch out for slippery algae

Photo above: thick green algae on a rubber roof in Poughkeepsie, New York - the editor (DF) took a nasty fall while walking on this surface.

Watch out: green coatings on any roof surface are usually algae growth. Algae is more-likely to be found on shaded areas of roofs and in areas of standing water or ponding on a flat roof.

Algae is incredibly slippery. Even stepping carefully you can find your feet flying out from below yourself and you can take a nasty fall. Algae on roofs may be green but also often is black (a different algae species.)

Algae can be cleaned off of a rubber roof surface by gentle scrubbing with a soft bristle brush and any household cleaner, but unless the underlying cause (shade or standing water) is fixed, the algae will return.

Details are at BLACK or GREEN ALGAE STAINS on ROOFS

To cure algae on your rubber roof, also take a look at the wet areas of your roof 24 hours after a rain to see if there's a problem with ponding.

Ponding is described in our flat roof article series including

FLAT ROOF LEAK REPAIR

 

Which roof drain should I buy?

I need a new drain but I don't know which one to buy. The 4 inch drain pipe is located right next to my wall. It doesn't quite reach the 4 inch drain pipe below so it leaks

I want to buy a new drain but not sure which one to buy? Can you please advise me which drain would work. I've been considering 90Degree types. On 2016-03-04 by Anonymous

On 2016-03-05 - by (mod) -

I'm sorry Anonymous, but I can't figure out your problem from just the e text.

It sounds as if you need properly sized and properly connected roof drains to repair an improperly installed, disconnected system. When are they ask your roofer for help.

You can also use the page bottom CONTACT email link to send us photos for further comment.

 

Wasps are plugging the roof drainage canals

Have a one-story flat roof house. Noticed wasps building a nest in one of the canales and it looked like they had plugged the canale with their nest. Killed the wasps!! However, after inspecting the canale opening on the roof, I discovered that an elastomer material had been applied to the canale on the roof side, effectively disallowing any water flow off the roof through the canale.

How do I re-open the canale, either DIY or would I have to hire a roofing contractor to open the canale?

The roof repair was done 5 years years ago shortly before we bought the house. It is possible the roofer did this on purpose or simply made a giant mistake? - On 2015-09-16 by Roger -

Reply by (mod) - Wasp nests plugging "roof canal" vent openings - how to clean out wasp nests

I don't understand your reference to "canales" - or canals or channels in the flat roof on your house. So my answer has to be a bit general.

There are two types of common wasp nests found on buildings:

Paper wasp nest from Wikipedia cited & discussed at InspectApedia.com

Paper wasp nests: gray papery-material honeycomed with openings, connected to the building surface by a thin "stem".

These paper wasp nests (Polistinae vespid), provided they're not occupied by live, stinging wasps, can simply be hooked with a bent wire and pulled off for tossing into the trash.

Our paper wasp photo above is provided courtesy of Wikipedia's Paper wasp entry (https://en.wikipedia.org/wiki/Paper_wasp)

Watch out: if the gray papery wasp nest looks like the one below and you disturb live wasps therein you can be very badly stung.

Hornets nest gray papery material (C) Daniel Friedman at InspectApedia.com

Mud-dauber wasp nests: yellowish-tan dirt-like material usually constructed in lumpy tubes. These wasp nests are more-difficult to remove as one entire side of the mud-tube nest is adhered to a buildng surface.

Mud dauber wasp nest on a brick wall (C) Daniel Friedman at InspectApedia.com

But mud-dauber nests are crumbly and are easily broken-up by a screwdriver, putty knife or other tool that will fit into the opening where the nest was built.

You may want to wear protective goggles and a dust mask so as not to get dirt in your eyes or face, and depending on the ease of access to these wasp nests, you might use a shop vac to help rmove the muddy dirt debris.

 


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

Reply:

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 seep into the fascia

I am having trouble with rainwater leaking around the flatroof drain, and seeping along inside the fascia board and then dripping from the corner of the wooden facai.
Is there a pipe! that can come from the roof and then fit inside the downpipe to the ground drain. If so can you inform me what it is called and where can I purchase locally to Kingston upon Thames Thank You- (Aug 29, 2012) Terry Weir MBE

Reply:

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- (Oct 11, 2012) shane

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?

Reply:

Joanne,

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, especially 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?

Reply:

Mel:

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

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: kennethdeebrown@yahoo.com Thank you. Ken (Nov 13, 2012) kenneth brown

Reply:

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 should be corrected.

Question: flat roof with internal building drain leaks

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. - (Jan 5, 2013) Molly

Reply:

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?

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 - (Feb 17, 2014) Bob

Reply:

Bob,
We need to figure out whether you are actually seeing a roof leak, a leak due to water pushed back under the roof covering or sheathing due to ice damming, or just indoor condensation. It seems to me that if there is no insulation in the roof of the room you describe, if you live in a Northern climate you must have quite a lot of heat loss.

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.

Question: how to get rid of leaky internal roof drains

(June 25, 2014) Anonymous said:
We have a flat roof on our house that's 24 years old (gravel and tar). It has never really given us a problem, however, the internal drainage system has. So far the drains have burst during the winter (due to freezing) three times over the last 15 years. Suffice it to say, the water damage inside the house was extensive!

We now need options (solutions?) to get rid of the drains. Can we go with scuppers only or do we have to reframe the roof to a slope? Please advise. Estimates for reframing are costing us upwards of $40,000. We are hoping scuppers are the cheaper route to take.

Reply:

Anon,

I suspect that the internal drain is located in a section of the roof that is NOT at a roof edge - making the scupper approach problematic unless you re-pitch the roof to drain uniformly towards one edge where scupper openings are provided.

That's an improvement I prefer to see as you're not the first to report a building that suffers from recurrent leaks in the internal drain system for the roof.

But you ought not to have to re-frame the whole roof - that sounds a bit extreme. More likely you could use a combination of shims, furring strips, plywood, tapered insulation, to add necessary pitch over the existing roof. Your local fire company may say they don't want you to build multiple layers of roofing - so you may need to

- strip off the old roof covering entirely, down to the decking (plywood or boards) that are left in place.

- check that the structure is not damaged by rot, insects, etc. and thus can take the modest added weight of shims, tapered foam boards, etc. to re-slope the surface

- install a non-structural but supported "skin" of new plywood that can pitch as needed

- install the new roof draining to scuppers.

Question: ok to use PVC for roof drains?

(Sept 21, 2014) Anonymous said:
Lesley

Need to add an additional drain pipe adjacent to the current one above the entry door. There is a gape next to the drain pipe above the entry door and the carport. Rain come down in sheets between the entry door and the carport covering.

(Jan 2, 2015) Anonymous said:
Internal roof drain pipe of a 2 family house that leads to the sewer system is split & leaks when it rains, can i use PVC pipes as a replacement?

Reply:

Yes but check with your local building department first - some communities have moved to prohibit sending roof drainage into the sewer system.

Question: prevent frozen downspouts & roof drain pipes

(May 4, 2015) Cynthia Lee said:
We have a flat roof with a new drain which extends through roof cavity to the building exterior where it is connected to a downspout on outside wall.

However, the downspout was frozen this winter and the drain couldn't work. Is there any way to prevent it?

Reply:

Cynthia

First be sure that the roof drainage system is not blocked anywhere in its route.

Second, you may be able to stop a freeze problem by using heat tapes designed for on-roof use, extending the tape far enough into the drain line that it reaches the drain line where it will thence be warmed by the building interior temperatures.

See HEAT TAPES & DE-ICING CABLES on FLAT ROOFS and also

see HEAT TAPES & CABLES for ROOF ICE DAMS for examples of the type of heating tape to use.


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