Low Pitch Tie In To Existing Roof

 
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Old 08-27-2009, 09:29 PM   #1
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Low Pitch Tie In To Existing Roof


Hello. I currently have a home which I built a couple years ago myself - which is a 32 by 20 rectangular home. It has a hip roof, metal roofing, 3/12 pitch plus 2 foot overhang all the way around. I am going to be adding on an addition the whole entire front south facing side (32 feet long). It is going to be a 14 by 32 foot addition (14 feet out and running the whole length of the house). I am going to be taking off the gutter and facia and installing a beam (properly spec'd to handle roof load where it attaches to the overhang). The new roof will be a 1/12 pitch and tie in under the existing metal (also I put water and ice shield on the entire roof deck under the metal when I built it a few years ago) - the new roof will be rubber roof and will run at least 2 feet up underneath the metal.

The new rubber 1/12 pitch roof will be a cold roof - 2.5" of polyiso on the roof deck and rubber roof over that. (2by 10 rafters to get filled with batt insulation: this is the proper foam-to-batt insulation as per the requirements in my location in ohio per the building science corporation guide).

My question is this... The continuous soffit vent under the whole front side will now not be venting the attic. There is going to be a place under the (current 2 foot overhang) that is not insulated and I will be insulating that part of the new ceiling, but the quesiton is how to get proper venting to existing roof.

My thought was to run a 4" pipe up underneath the truss overhang (holes in the pipe) of which the pipe will run the whole distance across and vent to either one side or both sidea of the 32 foot lonf run, and I consdiered installing a small dc inline fan (solar powered) to provide airflow through that area. In other words, this pipe would run between the insulaiton that is above the ceiling at the current two foot overhang and the metal roof above and be getting air from the ends of house where the pipe will stick out and then bend down with a screen on it.

I do not know if this is making sense. Is there any other way to provide venting? I am attaching a photo of the front of house where the 1/12 pitch roof will tie into the existing roof.

Thanks,


Scott
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Last edited by Tickridgescott; 08-27-2009 at 09:38 PM.
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Old 08-28-2009, 12:16 AM   #2
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Re: Low Pitch Tie In To Existing Roof


Why are you mixing cold roof assemblies and warm roof assemblies? This will certainly trap moisture at the roof deck. You will not be able to ventilate this roof. You would not need to if you go with a warm assembly.

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Old 08-28-2009, 06:19 AM   #3
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Re: Low Pitch Tie In To Existing Roof


The reason is that because of needing the extra headroom I need to do a cold roof. The other option was to extend the ridge up and come out with a longer roof and make everything vented. However, this is a situation in which we are cramped for space (500 sq ft house for 4 of us) and having some financial hardships due to economic conditions right now, and we desperately need space.

I had another thought that this thing could be vented by installing hat vents along the lower edge of the front roof.
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Old 08-28-2009, 11:58 AM   #4
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Re: Low Pitch Tie In To Existing Roof


Quote:
Originally Posted by Tickridgescott View Post
The reason is that because of needing the extra headroom I need to do a cold roof. The other option was to extend the ridge up and come out with a longer roof and make everything vented. However, this is a situation in which we are cramped for space (500 sq ft house for 4 of us) and having some financial hardships due to economic conditions right now, and we desperately need space.

I had another thought that this thing could be vented by installing hat vents along the lower edge of the front roof.
I don't know much about metal roofs, but have you considered edge vent?
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Old 08-28-2009, 09:25 PM   #5
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Re: Low Pitch Tie In To Existing Roof


You do understand that a cold roof means insulation under the roof... and a warm roof assembly means that the insulation is on top of and in contact with the roof deck, right?

If youre going to have open ceilings whereas the ceiling is also the underside of the roof deck, then use a warm roof assembly. Warm roof assemblies require no vetilation, since there is no attic, and no attic space to ventilate.
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Old 08-28-2009, 11:33 PM   #6
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Re: Low Pitch Tie In To Existing Roof


I would start the flat roof under your existing overhang so as to have a damn of sorts at least 2 inches tall just be sure to flash it well. If you come out with flat from the facia you can run into problems in ice and snow melts......I see alot of porches built flush on facia with ice damns cuasing the melting water from above to seap through the closures and back up the roof and into the old soffit.

You could rafter up 6 foot and put some pitch in and stay metal. That would be my choice

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