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I Forge Iron

Flashing around the chimney


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How did you guys run flashing around your chimney pipe? My shop roof has very little slope at all. Wood top with rolled roofing. I saw a flashing online that had a metal base and thermoplastic ring. Rated at 215 degrees didn't know how hot your pipes will get.

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I used a roof jack with a wide skirt because I have a steel roof and it has pretty tall ribs. This makes it a little hard to get it down tight and water proof so moving it farther from the stack allowed me to use weather seal and mastic to put a gooey gasket under it and screw it at close intervals.

Probably a better plan than asking a random bunch on the  internet is hit the library and check out a book about roofing. There is a RIGHT way to do roof penetrations for every type roof there is. What works on mine may be a disaster on yours.

Frosty The Lucky.

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  • 1 month later...

Your pipe won't get hot enough to bother the gaskets on a proper flashing.  They're rated to handle wood stoves and fireplaces that burn for hours at a time, so handling a coal forge isn't going to be any different.  The heat lost as the smoke rises up the pipe means that it's relatively cool by the time it gets up to the roof.  Then you have the roof, gasket and flashing acting as a heat sink, drawing away the heat as fast as it can.

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My walls are all cinder block.

​I can probably make the hole in the cinder block wall and flash it just as fast or faster, and better than I could a hole in the roof. Solid concrete would slow my down some, but it still wouldn't keep me from making it happen. Mark out the diameter of the hole you want, plus a little give, and drill a bunch of holes thru touching each other and knock out the center.

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I got overly cautious about the flashing this past winter when I threw a thermocouple, 6 feet up the pipe from my coal stove, in my flue and started seeing temperature peaks over 980F. Laser was still reading in the high 200's for the pipe itself. I made a custom flashing with my plasma cutter and lined it with 1" kaowool. Used high temp sealant to weatherproof it. My building is old timber and I just didn't want to take the chance. Maybe if my curiosity didn't get in the way and made me put that thermocouple in there I would have been fine without doing the extra work. Like Frosty said though, there's information out there in books about the proper way to do it.:)

J

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I believe the OP was about a coal forge. A heat stove has a much higher flue temp than a forge. A forge draws a LOT of ambient room air with the fire exhaust so it's significantly cooled or so many guys wouldn't be asking how to make their forge hoods draw. A stove is about controlling draft and keeping the flue hot enough crud doesn't condense in it.

YES, absolutely, roof penetrations and heat shielding for a heat stove is WAY more important. I over killed my wood stove's stack. It's triple wall and has 3x code for stand off distances. I also sheet rocked between the stack and the framing in the walls, floor, trusses and cupola. Yes sir I do NOT believe in taking chances with fire and code is MINIMUM acceptable.

Frosty The Lucky.

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  • 6 months later...

This is my setup. I have an old vacuum cleaner blowing through the pipe.  It forces a draft through the horizontal pipe (6"). Thus I do not need any expensive exhaust fan. These are early pics. Today I have added a piece of thin Aluminium sheet as a flashing at the exit end. Circular hole slightly less than 6" and beaten up to form a flange. The setup can be swinged away when cleaning the forge. As you can see the smoke stays in place and I have zero smoke in the shop when the vac is running. The pipe is not getting ery hot never above 200°C on the surface. I get some soot particles accumulating in the horizontal pipe so it should be swept occasionally lest it caches fire.

  trichter.jpg.715eddd7779ed12566e9702408cWand.jpg.324c67b5fc5188c2d1ca6984b97f158

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Hope you are sucessful. My object was to avoid making hole in the roof which is a bane in my climate. Draft induction is precisely the job my cleaner does. (I think we bougt it in the sixties the fan is still OK)

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