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Discussion Starter · #1 ·
Hello community,

I been building a small studio/man cave for last year or so. The roof sheathing and asphalt felt is now on the roof. Next up is the actual roof. I have these stone coated steel roof sheets I am going to use.

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Now I need to put a hole in the roof and flash for a 8" double walled steel chimney.
My roof pitch is around 4:12 (18 degrees).
A good while ago I bought this high temp dektite flashing, which is made from rubber or silicon:
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I was going to use it. But now I start doubting whether the rubber flashing will work well enough for the type of roof I got. The info I can find online says these dektite stubs are for steel roofs. Yes, my roof is a steel roof, but it has parallel/horizontal seams (not vertical), and the panels are covered with small stones so any caulking would have a harder time sealing between the dektite and roof plate.

Alternatively I could try to find a steel or aluminum flashing like this one here:

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These types are pretty expensive, but surely I have to do this right, otherwise there will be more expenses and regret if the roof is leaking. The pros for the latter type of flashing is that there are not a ton of screws to hold it down and the top of the flashing's flat plate goes under the roof element above it, which makes me think it would hold up better.

Where we live it gets pretty hot in the summer, and we do have seasonal strong rains.

Any advice from the community here would surely help me in my fact finding before I decide the best course of action.

Thank you.
 

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retired framer
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The flashing on the high side would be the tricky part.
Have you figured where on the roofing the hole will be?
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Discussion Starter · #3 ·
The flashing on the high side would be the tricky part.
Have you figured where on the roofing the hole will be?
Hi Neal, That is a good point. No I have not. I guess I can find the probable spot by laying the elements flat in the same distance as the hole is from the lower eave edge of the roof. I can't do this today as the roof is covered in a blue sheet and we're expecting rain, but that would be a good next step to figure out. Do you think the flashing option would be influenced by where on the roofing the hole will be?
 

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Can you go out the side with chimney? I would try to overlap the flashing with roof but that kind of roofing commits you to once only kind of build. No change allowed once the roof is cut. Also look for steel flashing. Heat resistant rubber or silicone? I would not use it. Silicone can withstand high heat but time after time and into long term? Where did you find this? Even pro store, current push is for make them cheap and let the buyers deal with future problems. Other words, you must think conservatively. The photo flashing also does not have much side flanges. It looks more like glue on than physical barriers to water.
 

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Discussion Starter · #6 ·
Hello, thank you Carpdad and Randy- definitely some different opinions here.

I have thought about going out the side but abandoned that plan due to excessive cost for double walled chimney pieces - would cost me about a thousand USD just in chimney so...

Meanwhile I found a steel flashing that I have ordered and that was not crazy expensive -only 130 USD, still kinda expensive but much less than other similar ones I have seen for several hundred. I even debated to fabricate my own flashing out of a large piece of aluminum, but I am a lousy stick welder, and trying to weld aluminum like that would be really asking for unnecessary trouble haha... It has much more edge also than the rubber one, so that I think will help keep the water at bay. This is the one. And maybe I should try to get some aluminum flashing tape as well for the edges, to be safe.

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We have extremely strong sunshine in the summers here, and like carpdad mentions, I am also not convinced about the durability of the rubber-silicone boot there. So I better go for the steel option and add a storm collar over the steel flashing on the chimney.

So I plan to lay the roof first anyway, and then carefully (and hopefully) be able to cut such that part of the roof panel is cut around the upper part of the flashing, as you mention, and then use caulking between flashing and roof, and hope that will do it.
 

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Discussion Starter · #7 · (Edited)
The flashing on the high side would be the tricky part.
Have you figured where on the roofing the hole will be?
View attachment 681167
Hi again Neal. I now know where approx the chimney will come through. I think the flashing has to be on top for the bottom roofing sheet, and under the top one, or something like that. I feel this is kinda lucky, because the flashing has full roofing support on the part where it goes over the roofing, and then I only need to flatten the edges, and then cut half round hole in the top roofing sheet. If anyone has advice I am all ears. Otherwise its time to google this for me.



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retired framer
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So would you just flatten the lip where it is above and below it?
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Then a sealant under the lower part.
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And maybe a tape over the upper part.
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Or could you install it first with a half round out of both installed with a good sealant around the hole.

I have not found a sample of how they do it with that roofing.
 

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Discussion Starter · #9 ·
Thank you Neal.

You put forward two interesting options.

Initially I preferred the first option, as that was what I have in mind. I have some heavy duty flashing tape that would be positioned as you sketch it.

But the second option, well that might be even better. If executed well, it would have the least amount of hole where water could penetrate. I need to analyse the pros and cons of both options carefully.
 

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Discussion Starter · #10 ·
Thank you Neal.

You put forward two interesting options.

Initially I preferred the first option, as that was what I have in mind. I have some heavy duty flashing tape that would be positioned as you sketch it.

But the second option, well that might be even better. If executed well, it would have the least amount of hole where water could penetrate. I need to analyse the pros and cons of both options carefully.
Hi Neal,
Just to update what I ended up doing. Thank you for your advice. Maybe someone else will google steel chimney flashing on the DIY chatroom in the future and can get some info from my post.

I still need to put some heavy duty flashing tape on the upper side. Will do before the rain hits later next week.

Chimney is 3,2 feet above roof, around 2 feet above ridge. I have no roof chimney brace, but am considering whether that would be good, since we do get some pretty full on seasonal storms here.

Cheers,
Simon

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Hi Neal,
Just to update what I ended up doing. Thank you for your advice. Maybe someone else will google steel chimney flashing on the DIY chatroom in the future and can get some info from my post.

I still need to put some heavy duty flashing tape on the upper side. Will do before the rain hits later next week.

Chimney is 3,2 feet above roof, around 2 feet above ridge. I have no roof chimney brace, but am considering whether that would be good, since we do get some pretty full on seasonal storms here.

Cheers,
Simon

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You may want to check flashing details I have never seen flashing with the bottom edge caulked?
Flashing is designed to let any water that does get in a path to exit.
 

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Looks pretty good, only thing I would do still is caulk the top side. Way it is water will get in under the shingles there. What are you using for caulk?
I also use sealant under the storm collar and on top side all the way around it.
 

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Discussion Starter · #14 ·
Thank you for your advice gentlemen. I realised what several people have said that the bottom should not be caulked shut but allow any water to flow down and out, so cut some slits across the bottom caulking to allow water flow. Have then pressed a glue sponge packing around into the upper area between roofing and flashing. Would have been easier to do before the roof was on, but lesson learned. I then put wakaflex heavy duty flashing around the top. I think (and hope) this is as watertight as its going to get. How this will last over time through wind and weather remains to be seen.

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That looks better. I stuck some blue peal and stick to the roof of a van, 10 years later the blue in the top plastic had faded to clear but never leaked.
 
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