Install Roof Turbine w/o Cutting New Hole

I have wanted to install a roof turbine to vent the attic of my house for many years but kept deciding against it due to the possibility of water leaks. Could I instead place a roof turbine on top of an existing vent and cut a hole in the corresponding vent in the attic? There is a kitchen vent (with squarish metal cover) and a toilet vent (3" or 4" PVC pipe, open top). Also my roof has a 45 degree pitch. Would I be able to use Lowe's roof turbine which has dual (upper and lower) bearings or would I be limited to HomeDepot's which only has upper bearings in order to clear the existing protrusion from roof? Thx
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No. The plumbing vents must be unrestricted to the outside.
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You are a lot more likely to have a leak trying to hack & jimmy-rig it the way you describe. This might even make a good clip for the Red Green show.
By the time you finish the initial install and patch the inevitable leaks with roofing cement (don't care what it looks like as long as it doesn't leak any more) it will be fugly. The vibration of turning will surly create (more) leaks. They are meant to be nailed down to the sheathing, and integrated with the shingles if they are installed like regular vents. Gonna nail it to the the existing vent are ya? Ahhh, drywall screws! How about a good windstorm and that thing starts whipping around. It'll take off like a helicopter. Now you still have an open hole in the roof and some splainin' to do the the neighbor with the broken windshield.
45 degree pitch?! You planning on going up on that 12/12 roof with just sneakers? You will need roofing jacks which means you have to pop up the tabs of some existing shingles, slip the jacks under and screw them into the roof. Ahh, you're gonna try it without the jacks because you are afraid the screw holes will leak later.
As far as the different Borg turbines, you need to look at the mfgrs instructions/specs for the particular turbines they have.
And BTW, what air is it going to pull out if it's over a vent? Maybe where ever the vent is venting - the kitchen ya think? Wifey will be at the stove under the vent, turbine spinning in a wind and her hair will look like Don King. The turbine is suppose to be pulling attic air.
Based on what you are proposing and how you are considering doing it I highly suggest you abandon doing this...in any manner yourself.
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