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I have a house that has an old 8-10 inch diameter vent that used to be used as stove/oven vent. I think that newer gas ovens don't require venting like a water heater needs (ie they don't have a "smoke stack"), but this seems to be often confused with an "exhaust fan/vent" that is used to vent smoke/steam/smells from the cook top.

So that 8-10 inch vent rises up out of the roof about 18 inchs and has a cap similar to this:

roof cap

I think that I can safely terminate a 3inch double walled vent pipe into that riser (I'd prefer not to punch another hole in the roof, since this is a 4 year old "torch on" flat roof that I don't want to screw up).

My plan is to attach a firestop to the underside of the roof so that the vent pipe is snugly held in position and to limit the airflow from the top of the vent back into the attic space. A firestop lets you make a "tight-ish" seal to a hot pipe with out breaking the "1 inch to combustible material" rule for a D-Wall pipe. It looks something like:

firestop spacer

or

firestop

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1  
What is the question? –  TomG Sep 12 '13 at 0:21
    
What are you planning to use this vent for? –  Tester101 Sep 12 '13 at 14:14
    
@TomG: I guess the question is, can I skip the trouble of making a new hole in the roof by terminating a 3 inch double walled vent pipe into that 8-10inch riser? Normally risers would exit the root and have a cone shaped flashing that snugly wraps them and meets the roof. I'd like my riser to be "loose" in that large pipe, but be snugly sealed from inside the roof via a a firestop. –  mlathe Sep 12 '13 at 20:36
    
@Test101, this is for an existing gas water heater. It is a bit complicated but right now the water heater's 3 inch single wall vent is patched into (ugly, illegal, and unsafe!) the 8-10 inch "stand pipe/vent" that that is using the roof vent I'm discussing here. I've discussed it [here]( diy.stackexchange.com/questions/31222) –  mlathe Sep 12 '13 at 20:41
1  
There's no practical reason this could not work. You still may not be in full code compliance (whatever applies in your jurisdiction) because the required firestop spacers are supposed to be listed products used as part of the complete vent system, all by a single manufacturer. Field fabricated substitutes may not be allowed. It all depends on which code applies to your situation. –  bcworkz Sep 13 '13 at 1:14

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