Roof Vent for muliple washroom vent fans -- Ask the Roofing Sider Forum

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Date Posted: Thu, Nov 07 2002, 8:22am
Author: Paul
Subject: Roof Vent for muliple washroom vent fans

HI. I have 3 washrooms on my 2nd floor. Only one has a vent fan that is vented up through the roof with PVC piping. I was wondering whether I could also vent the other fans through the same piping by cutting the original and adding PVC t-joints. The cost of venting each fan through the roof will be through the roof! ha Any thoughts?
Thanks. I love this forum!!

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[> Re: Roof Vent for muliple washroom vent fans -- A Roofer, Thu, Nov 07 2002, 6:38pm

You kinda got me thrown for a loop with the term vent fan and PVC piping in the same use. Vent fans are usually vented using duct work and used for exhausting dryers and bathroom fans. They do require their own separate vents or else when one fan tries to exhaust it will blow into the other's vent. Anything that exhausts needs it own vent for this specific reason. If your PVC piping is for plumbing drainage you will probably be just fine tying them together. They are there to allow air into your plumbing system so it doesn't gurgle as it drains. If you have ever seen a drain that gurgles or drains slow its because they don't have an air inlet that is usually stuck through the top of the roof. Its like when you have a full glass of water in a sink then turn it upside down and lift it out of the water and the water remains in the glass until it gets high enough just out of the water so air can draw in and replace the water in the glass. Unless it is possible that all three of these drains might be in use at the same time the existing plumbing vent coming out of the roof should be sufficient enough air supply for your plumbing to drain properly.

Either of these things are simple things to do and the costs should run between $200 to $500 to add three vents out the top of the roof.

Hope this helps,
Tim Soth / A Roofer

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[> [> Re: Roof Vent for muliple washroom vent fans -- Paul, Fri, Nov 08 2002, 6:52am

Hi. Thanks so much for your info. It is not uncommon to use 3 or 4 inch PVC piping for bathroom fan applications. The flexible hosing has a greater chance of leakage and humidity and roof trusses don't mix. My hope was to use an upside down Y fitting that would prevent the exhaust from one fan to go down the pipe of the other. Plus I figure most fan must have some kind of closure to prevent outside air from coming inside. The only drawback that I could think of initially is that I might have 3 fans all trying to push air out of the same 3 or 4 inch hole. But then I figured if the the fans were small in size that maybe this was possible. My idea seemed so logical but I certainly understand and appreciate the information you gave me.

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[> Re: Roof Vent for muliple washroom vent fans -- A Roofer, Fri, Nov 08 2002, 6:41pm

You state, "It is not uncommon to use 3 or 4 inch PVC piping for bathroom fan applications."

No offense but to my ears that sounds like something coming from either another planet or another place in time. My company is from the greater Seattle area and I have been involved in construction since 1985 when I was 18 yrs old. In the over 1400 homes I have been a part of I have never seen (or have heard of in the trades) a bathroom vent's duct work made from PVC piping. For such venting there is always a "flapper vent" also known as a "fart fan" installed on the roof deck that the duct work attaches to. When the fan is off gravity closes the flapper. I have bought bathroom fans for my own place and have installed them and they never have had anything on them that closed to keep air from coming back in through them. Thats what the flapper vent does. If they all were tied together to the same flapper then they most definitely would blow into each other no matter whether you used a Y or a T to tie them together because the flapper would easily create enough resistance to the flow of air. Hang out on job sites long enough you'll find a building inspector and ask him. He will tell you the same thing. If they just stick PVC piping through a roof for bathroom fan ventilation rain water could run down it in heavy rains and with power running into it this isn't a good idea because of the fact that water and electricity don't mix.

You also state, "The flexible hosing has a greater chance of leakage and humidity and roof trusses don't mix."

More things that don't register with me and my experience.
"chance of leakage" - If they are properly installed they never leak. Its not as the duct work is a moving part that will eventually work itself loose. There are building codes in King and Snohomish County stating the requirements for attaching duct work together and the only time I saw duct work come loose is when the installers didn't follow the local building codes.
"humidity" - Are you referring to insulated duct work or the stuff that is not insulated? There are no humidity concerns with insulated duct work. The humidity concerns can't be that big of a deal because the only thing that the local building codes require insulated ducting for is for the heat ducts. But if it concerns you I suggest you should use it for these vents also.
"roof trusses" - What do the trusses have to do with this?

Thanks for your forum participation and I hope this
adds a little more info for what you were seeking,

Tim Soth / A Roofer

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[> [> Re: Roof Vent for muliple washroom vent fans -- A Roofer, Fri, Nov 08 2002, 6:53pm

I also want to add for general knowledge to locals that browse through this posting that in Washington State all PVC piping coming out of the roofs are for plumbing ventilation for draining waste water as I tried to explain in my first post in this thread.

Tim Soth / A Roofer

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[> [> Re: Roof Vent for muliple washroom vent fans -- Paul, Thu, Nov 14 2002, 7:51am

Hey thanks for the info. You're all absolutely correct. I'm a do-it-yourselfer who has a full-time job and 3 kids and just moved into a new house. I'm so exhausted that I started flapping (no pun intended) about an issue that I was obviously confused about. So sorry to take up your time. I really appreciate this great forum and the patience you show towards confused rookies like myself.

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