Internal roof drains and freezing 2

[GreenSystems]

We are working stormwater projects involving the disconnection of impervious surfaces from the local CSO. Some of the buildings associated with our projects are equipped with internal roof drains directly connected to the sewer system in the building basements. We are looking to disconnect these systems and run them outside of the buildings to green infrastructure practices. A concern has been raised relative to freezing of the pipe daylighting to the outside in our cold climate and whether that could lead to backups that could compromise the roof. Given that the drains are within the heated structures and the above-freezing basements until they extend to a <6" length of pipe extending to the outdoors, we have considered freezing of the pipe to the extent that it plugs completely with ice, highly unlikely. Icicles perhaps yes, but a complete ice plug, no? Does anyone have experience or other knowledge related to this subject? Thank you.

 

[bimr]

Provide an emergency overflow drain:

http://plumbingengineer.com/sept_10/drain_feature.php

 

[Ron]

Flow through roof drains is generally too fast to freeze under the conditions you noted. If backups occur on the roof from an obstructed drain, then yes, you could get freezing of the ponded area.

Provided the drains are sized properly and adequate conveyance slopes are available, I would not consider this to be an issue. Make sure you have a good vertical drop in the roof drain outlet before turning to the horizontal. Further, prior to making a 90 deg turn, increase the pipe size by two sizes to prevent a slow-down in the flow.

 

[GreenSystems]

THank you. We have indeed incorporated such a feature into our designs that we have undertaken from scratch. Unfortunately, we were designing on the fly for a previous project. The contractor asked us where to set the outlet and we hadn't considered all of the plumbing issues at the at point. I guess we'll have to keep our eyes on that one. Thanks again!

 

[GreenSystems]

My first reply was to bimr.

Thank you to Ron for that response as well. I think the concerns have been less with fast flow associated with rain event, and more with snow melt that could refreeze at the outlet (i.e. the icycle phenomenon). I don't see that as likely to freeze up to the point that it plugs the outlet. And even then the concern would only be where that plug would remain even under a subsequent rain event wherein the rain water is backed up onto the roof causing a load exceeding the snow load typically experienced up here. Again, this seems highly unlikely to me. Thanks again!

 

[MiketheEngineer]

If water can find a way to pond - IT WILL. Mother Nature is a B&TCH and she always wins - sooner or later.

Just don't forget that.

 

[civilman72]

I think it's naive to assume that freezing will not be a problem, but it depends on how cold and how long your winter really lasts.

The one situation that I have dealt with that I can compare is with outside gutter drains. Due to the heat produced from the roof and house, the water does not freeze in the gutters (asuuming they are clean of debris), or the downspout, but there is ALWAYS freezing at the outlet. The only way I've seen to solve the freezing issue is to install heat tape or connect the downspout directly to a storm sewer system. This freezing does block the entire pipe, but rather than backing up the water, it usually just busts the pipe (which should not be an issue for you since your pipe is inside).

For reference, the average daily low temperature where I'm at is about -10 F for about four months. If anyone else deals with temps this cold for this extended period of time, and has other ways to prevent freezing at a roof drain outlet, I'd love to hear them.