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Thread: Bathroom Sewer Vent

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    jtfoxman is offline Handyman
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    Bathroom Sewer Vent

    I have a sewer vent from my bathroom that goes up through the roof and it extends maybe 6" above the roof. This 2" vent freezes up on occassion in the winter and I plan on expanding the vent to 3" to help alleviate this problem. Now here's my question how far above the roof line does the vent need to extend? Does it need to be or can it be capped to help it from frosting?

    Thanks!
    Jtfoxman

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    HayZee518's Avatar
    HayZee518 is offline Deity
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    my main vent and kitchen vent extends about 2 1/2 feet above the roof. on my main vent I installed an elbow with some screening in it to prevent a bird from falling down the vent as has already happened last month.

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    jtfoxman is offline Handyman
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    Thanks!
    I was surprised when I seen mine was only 6" above the roof, that might be why it could be freezing up. I'll probably go up to 2'.
    Jtfoxman

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    Vents must terminate 6" above the roof except in regions subject to snowfall, the vent must extend 6" higher than the average snowfall. (some local codes say 1' higher than average snowfall)

    The plumbing codes also state that in regions subject to freezing all vents must be increased to a minimum of 3" at least 1 foot inside the structure. (Some local codes require 4").

    In the real world the one foot rule proves fine for older homes that have relatively poor attic insulation and lower levels of attic ventilation where heat builds up in the attic space however, in todays state of the art building practices we continually increase attic insulation & ventilation in an effort to keep the temperature in the attic space at or relatively near the exterior temperature. This works fine for HVAC energy conservation but with state of the art insulation, as the exterior temperatures drop below freezing the air in the attic space also drops to freezing.

    It must also be noted that whenever we discharge a liquid into the drain system, the liquid occupies a significant amount of space in the pipe, thus as the liquid flows the liquid displaces an equal volume of air in the pipe. It must also be noted that some of the gases that makeup sewer gas are lighter than air and will naturally rise, while others are heavier and air and will naturally settle to a lower point. There is no hard and fast rule what the percentages of those gases may be present at any given time therefore when the liquid flows in the pipe it may dispel lighter gases upwards or it may push heavier gases downwards ahead of the flow.

    At any given moment in time the only thing that remains constant is the fact that nature abhors a vacuum therefore if the air & gases in the pipe were forced upwards an equal amount of air & gases will be drawn upwards from the house sewer while on the other hand, if the gases were heavier than air they would be pushed down towards the house sewer and an equal volume of outside air is drawn into the roof vent. While most people only think of plumbing vents in terms of venting the stinky stuff to the atmosphere in reality your DWV (drain, waste & Vent) system is constantly inhaling and exhaling outside air through the vents as if it had a life of its own.

    The problem we encounter here is that when the attic space is colder and when the exterior cold air is drawn into the vents the pipe walls get cooled to a much lower point and once the flow of gases reverses the moisture laden air rising in a vent pipe will condensate on the interior walls at a much lower point. If the condensation occurs in a section of the pipe that is still above freezing temp the resultant liquid will then run down the pipe and exit out through the sewer but if the moisture laden air reaches a section of the pipe that is below freezing the liquid will condensate and immediately freeze on the interior pipe wall, thus we are seeing ice dams forming at lower and lower portions of the pipe. For this reason, even though the code only requires us to increase the line to 3" one foot inside the structure, I like to make the transition to 3" immediately upon entering the attic space, near the attic floor.

    Now in regards to your question about capping a vent. I know this answer is going to cause a myriad of feedback, but here goes anyway. NEVER CAP A VENT in a climate that is subject to freezing. First of all it must be understood that ice damning is not caused by moisture from the atmosphere. When the ambient air temperatures are below freezing all the available moisture in the atmosphere has already fallen in the form of snow and the air is dry. Any snow that actually falls into the vent pipe would be melted by warmer air rising in the pipe and it would simply run down the pipe and out the sewer. The same applies for any rain that enters the pipe. On the other hand, if you were to install a cap it would have two hindering effects. 1. The cap would retard the velocity of flow of any moisture laden air rising through the vent system and 2. The cap would be below freezing so any moisture laden air rising through the vent system would freeze to the underside of the cap on contact, thus increasing the probability of an ice damn.

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    jtfoxman is offline Handyman
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    WOW! Lot's of great information.
    So what i gathered from all this is to make the transition from the 2" vent to a 3" vent in the attic space and extend that out through the roof. I'm not to comfortable putting a larger hole in my roof is there another way of doing this without enlarge the hole in the roof? The vent is very close to the eave of the house and I can only access about a foot of the pipe in the attic before it exits out the roof. It only freezes up when the temps drop below 10F and it will unfreeze after a shower due to the hot moist air going up the vent or when temps rise a little.
    If I leave it as is and let it freeze every once in a while will this cause any harm to the system as a whole. It does it maybe 3 or 4 times over the winter and the only way you notice it is because it gurgles when you flush or run water. i have wrapped the exposed pipe in the attic with insulation which helped a little bit.
    Jtfoxman

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    Check the size of the hole through the roof from the underside. You will probably find the hole is already sufficient for a 3" pipe and if you have a flashing with a rubber grommet that may well accept a 3" pipe as well. If not, it is an easy job to install a flashing.

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    ROSALIE is offline Banned
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    hey its an interesting information.

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