Furnace flue thru metal roof

Of course this was in a free-standing shop quite a ways from any other building - codes might not like that for something attached to a house.

Good luck,

Bill

A type B flue is only rated for "some" gas fueled appliances. Most codes were going with a thimbal thru the roof penetration when I quit working and I have no idea of the current code requirement but a triple wall thimbal was being required of most codes depending on building construction. Ask your fire or county/city building department for there current codes.

On a steel roof I personally would make a 2" high pre-curb that is seal welded to the roof decking as then you have a leak free joint for the life of the roof. The last one I made, code called for 10ga(1/8") sides and top plate but that was commerical. Residentual code might be a lighter metal, 14ga. You just need to check current codes for your area.

Vent requirements before the roof penetration is very straight forward, it's the roof penetration that will causes all the headaches with tons of codes to deal with. Not one code can be skimped on tho as this is where 99% of the vent fires that burn down homes start from.

T_Bone

There's a fitting called a storm collar which fits over the chimney like a funnel with the big end down. On insulated chimney this can be sealed with silicon caulk. A high temperature sealant would have to be used with single wall chimney. Then the boot of a regular chimney flashing is protected from rain by the storm collar. I guess that the purpose of this configuration is that the hole of the flashing is elliptical because of the slope of the roof, which makes it more difficult to achieve a close fit. The storm collar fits square on the chimeny so a close fit is easy.

All the best, Stan

99% of the time, A single wall vent roof penetration is never allowed by code and for a good reason. You can not disipate the heat quick enough to keep from the surrounding structure cool even with 6" clearence to combustables.

A single wall vent is only allowed on special appliances but the roof penetration still requires a double wall thimbal. or was required?

T_Bone

I was focused on the hardware it takes to stay waterproof and not paying enough attention to the original post to notice that Chris said it was "class B" pipe. It wouldn't have made a lot of difference in my answer, though, because I don't know how hot the outside sleeve of class B chimney gets. That would determine what kind of caulk would be required. Also, since Chris mentioned it being a steel roof on a garage I thought there was a chance that the framing pattern might allow single wall chimney with a double wall thimble or even a slight chance of steel framing. Either of those possibilities would have resulted in a single wall chimney where silicon caulk on the storm collar would be the wrong choice.

I'm not disagreeing, just explaining. You've cleared up things for me many times in the past and I have every hope that you will continue to do so in the future. When I begin to think that I know everything, that will be when it's clear that I have lost my mind.
All the best, Stan

I'm a 2nd year apprentice and I'm working in a very large SM shop, 60 men normal work load, and I built a sheep herders stove for a home project. Firebox was about 14" x 12" with a oven. I even rolled my own single wall vent pipe.

We had also just bought our first home with a car port that I turned into a garage. I get the idea to install my stove so I can have some winter heat, were in Colorado.

So I make up a flat roof jack and allow 6" clearence all around my single wall vent, cut the hole and use plenty of roofing mastic. I fire off the stove with a small fire and everything looks just fine and dandy. Well I'll go have a cup of coffee with the wife while the garage is heating up.

We got to talking and I didn't pay any attention to time then remembered the wood stove. I opened the door and the smoke was thick and smelled like roofing tar.

The stack had got so hot that is turned the roofing mastic to liquid where it ran down the vent to the top of the stove. GOD was once again watching over this fool and my garage didn't burn down.

Now had I asked a few questions I would have known to use a thimble thru the roof penetration with a minimum double wall (type "B") vent. The air space between the double wall allows the outside stack tempature to run cool thus is not a fire hazard like the signle wall vent. The thimbal keeps the surrounding combustables cool 6" below the joist to 12" above the roof on some roofs.

There are a several high efficency gas furnaces that use a PVC vent as the combustion gas is cooled before it leaves the furnace.

Some codes do allow for single wall vent pipe on wood stoves but only before the roof thimble penetrates the roof/celing.

It's the stove/furnace manufacture that determines how and what type of vent is required of the appliance. We then have several pre-approved code vent options for installation that we can choose from.

Venting combustion products is one of the most difficult problems to teach SM workers as the code is very complicated on this subject.

T_Bone

This picture is of a shingled roof, but it would be about the same principle with a steel roof.