# Flat roof drain to municipal sewer - winter freeze?



## bob_cntrctr (Jan 30, 2008)

Gentlemen,

Maybe this belongs in roofing, we'll see - I'm looking at a 3-storey building with a flat roof and attic. The roof drains drop into the attic, into 4-inch PVC pipe. Originally I'm sure the drain was routed into the main drain stack, which is accessible in the attic. But at some point - I think not long ago, when the roof was re-done, they got re-routed out the side of the building and down to the ground - that is, the 4" pipe exits the side of the building and goes down the outside wall to ground, where it discharges into a dtich. Most likely 'cause you know municipalities are pushing getting rain water out of their drains these days.

Problem - over the winter the outside pipe fille with ice and was shredded by the expansion. Needs to be replaced.

But I'm thinking gong outside was the mistake.

So - Question - the attic is an un-heated space, with lots of air infiltration. When it's -15 outside, it's well below freezing in there too. So why don't traditional roof drains piped to the main stack freeze up like this? Or do they and I just haven't seen it yet? Or is it the warm air from the stack that keeps it clear?

In short - I'm trying to decide if I should have it switched back to draining into the stack 'cause everyone knows that's safe from this kind of freezing.

Thanks.


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## Oconomowoc (Oct 13, 2011)

Unless you have a variance you can't drain storm into sewer. Storm drains are storm drains and need to be designed as such.

There's a good chance this is happening based on undersized distribution. I'd need pictures but going to 6" would really help.


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## UALocal1Plumber (Jun 19, 2009)

There are a few issues here. The fact that you have a flat roof and an attic as well is confusing to say the least. In addition, i'm imagining the 4" making a turn and then leaving the side of the structure? How is that penetration made? Must be ugly and not water tight? Finally, when you say "shredded" does that mean that the pipe is burst? If so, that makes no sense at all - the pipe should be pitched towards the drain always, and any water that gets in there should be flowing to a point of disposal immediately. If it's not, the drain is not pitched in the right direction.

In general you'll want to avoid running storm water into the sanitary line, for a few reasons.

I would consider installing scupper drains around the perimeter of the roof. Or, if you have a parapet around three sides and the roof pitches to the rear, install a gutter to a downspout on the leading edge. If the roof pitches to the drain already, repipe it so that it is pitched at least 1/4" per foot in the direction you want to drain towards, and has a place to discharge that will not back into the line. 

Keith


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## Oconomowoc (Oct 13, 2011)

Keith, you're assuming this is a "normal" job. Lol


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## concretemasonry (Dec 1, 2006)

Going straight down through a uninsulated space should probably not be a problem because of the exposure the ability to insulate IF you can get to the exposed portion of the drain. If the original was permitted, you probably were "grandfathered" in until you changed things.

I had a one story home in MN that was built about 1915. It had stuccoed clay tile exterior walls and 9' ceilings that had some insulation. The roof top (no parapets) was about 2.5 to 3' higher and sloped to to center and about 2 feet of the 2 center cast iron drains were exposed to ventilated space between the roof and ceiling, but there was no sign of any leakage on any walls or ceiling inside (basement was finished). Apparently, the cast iron conducted enough heated and the exposure was not severe (no wind, but -20 F)), so the roof and drainage worked well and was never tampered with. Most newer homes in the area (newer than mine) also had roof drains tied in.

It drained into a combined sewer system originally and the city looked at separating the sanitary and storm sewers and requiring resident to convert, but the cost of separating was less than the cost of converting individual homes.

Once you try to change, you can lose the benefits of "grandfathering" a system that is proven to work. After that, you are on your own and it can cost you money in the end.


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## thom (Nov 3, 2006)

If the temperature in the attic space is below freezing, the temperature of the roof deck will also be below freezing so there will be no liquid water to drain into the pipe. This would keep the system from freezing. In addition, when connected to the sewer, the temperature of the air in the sewer pipe was well above freezing. The sewer pipe would probably have been part of a vent system so warmish air would constantly be rising from the city sewer through the roof. 

If the current drain pipe is in an area that is shaded during the winter, (where it exits the building) especially if there is an elbow or a horizontal run, it will likely freeze now. You could do a partial fix with freeze tape but that would be an energy hog. Otherwise you need to get the pipe into an area that is somewhat heated then exits in a sunny place.


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## bob_cntrctr (Jan 30, 2008)

UALocal1Plumber said:


> ...The fact that you have a flat roof and an attic as well is confusing to say the least.


Old Victorian, with steep sloped roof and partial flat roof over an attic space. Like this:

http://www.erasofelegance.com/arts/architecture/secondempire.jpg



UALocal1Plumber said:


> ...Finally, when you say "shredded" does that mean that the pipe is burst? If so, that makes no sense at all - the pipe should be pitched towards the drain always, and any water that gets in there should be flowing to a point of disposal immediately. If it's not, the drain is not pitched in the right direction.


Yes, burst pipe. It's the icicle effect. On sunny days near freezing, snow/ice thaws and flows, but then it gets to the outdoor pipe which is on the shaded side of the building where it's just below freezing, and freezes in the pipe. Eventually fills the pipe with ice, blocks, backs up, freezes, bursts.


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## bob_cntrctr (Jan 30, 2008)

concretemasonry said:


> ...I had a one story home in MN that was built about 1915. It had stuccoed clay tile exterior walls and 9' ceilings that had some insulation. The roof top (no parapets) was about 2.5 to 3' higher and sloped to to center and about 2 feet of the 2 center cast iron drains were exposed to ventilated space between the roof and ceiling, but there was no sign of any leakage on any walls or ceiling inside (basement was finished). Apparently, the cast iron conducted enough heated and the exposure was not severe (no wind, but -20 F)), so the roof and drainage worked well and was never tampered with. Most newer homes in the area (newer than mine) also had roof drains tied in.
> 
> It drained into a combined sewer system originally .....


Yes, this is about the same situation. Similar era of building (~1900). But the non-insulated ventilated space is 9 feet tall. Roof slopes to centre. The original cast iron is long gone, replaced with PVC. The main drain/vent stack is about 30 feet horizontally from the roof drain.


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## bob_cntrctr (Jan 30, 2008)

thom said:


> ...In addition, when connected to the sewer, the temperature of the air in the sewer pipe was well above freezing. The sewer pipe would probably have been part of a vent system so warmish air would constantly be rising from the city sewer through the roof.


This is what I was thinking. Yes, the main drain/vent stack comes up through the attic. So I was thinking that if the roof drain tied into that, it would have the warmish vent air in it to keep it from freezing. Just wondered if that was actually true and a well known function, or if I was just doing some wishful thinking.



thom said:


> If the current drain pipe is in an area that is shaded during the winter, (where it exits the building) especially if there is an elbow or a horizontal run, it will likely freeze now.


Ding ding ding - yes, this is exactly what happened.



thom said:


> You could do a partial fix with freeze tape but that would be an energy hog.


Thought about some kind of electrical anti-freeze line. I've seen electric lines inside outdoor drain pipes, for example. But if I can avoid recommending a solution that includes major electricity consumption, it'd be better for me.



thom said:


> Otherwise you need to get the pipe into an area that is somewhat heated then exits in a sunny place.


Wish it were possible. I suppose it is, maybe, but the run to that wall would be ridiculous.


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## KillerToiletSpider (May 27, 2007)

Oconomowoc said:


> Unless you have a variance you can't drain storm into sewer.


The entire city of Chicago disagrees with you.


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## thom (Nov 3, 2006)

KillerToiletSpider said:


> The entire city of Chicago disagrees with you.


Back in the late 1970's I was living/working in the western suburbs of Chicago. I know that back then, in Wheaton, the law was that any roof connection to the sewer system had to be eliminated. Back then they were doing smoke tests to check.


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