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Old 12-06-2009, 10:41 AM   #1
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Hardie board installation- roof and flashing gaps?


When installing Hardie board siding how important is the 1/2" or 2" gap on roofs decks flashings etc? I prefer the look of running it tight. Anyone have any experience installing it this way?
I just started my first Hardie job and I have butted the siding against the shingles and decks for a clean look, please inform me if I need to change this b4 it's too late and I ruin a job.

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Old 12-06-2009, 10:47 AM   #2
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Hardie board installation- roof and flashing gaps?


I always prefer a reveal to keep butt edge of siding off water, I agree tight to shingles looks sharp but i have replaced alot of damaged material sitting on roof. what does manufacturer say about absorbtion in that app?


Last edited by tpolk; 12-06-2009 at 10:48 AM. Reason: spellcrap
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Old 12-06-2009, 10:50 AM   #3
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Hardie board installation- roof and flashing gaps?


To my knowledge Hardie board is impermeable- based on that I can't understand the need for a gap- unless maybe the shingles/ decking material are somehow corrosive when mated to the Hardie?....
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Old 12-06-2009, 10:59 AM   #4
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Hardie board installation- roof and flashing gaps?


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Originally Posted by hammerjockey View Post
When installing Hardie board siding how important is the 1/2" or 2" gap on roofs decks flashings etc? I prefer the look of running it tight. Anyone have any experience installing it this way?
I just started my first Hardie job and I have butted the siding against the shingles and decks for a clean look, please inform me if I need to change this b4 it's too late and I ruin a job.
You already ruined it by not following the Hardie installation guide. It says to keep it off the roof a certain amount for a reason. Why did you not follow what they tell you to do?

http://www.jameshardie.com/pdf/insta...eplank-hz5.pdf
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Old 12-06-2009, 10:59 AM   #5
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Hardie board installation- roof and flashing gaps?


the other thing that happens I feel is you are offering easier access to water wicking up the back side. i had a commercial job that had vertical ribbed siding setting on a flashed curb, we had water showing inside and found several places where water was wicking up the 8" curb flashing and inside
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Old 12-06-2009, 11:13 AM   #6
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Hardie board installation- roof and flashing gaps?


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You already ruined it by not following the Hardie installation guide. It says to keep it off the roof a certain amount for a reason. Why did you not follow what they tell you to do?
It's not ruined- yet. As I said it's impermeable and it looks better tight, that's why I did it, and maybe I'm inexperienced
I'm aware they recommend to keep it off the roof, but I like to know what the reason for it is.

I think Tpolk's onto something with the wicking.
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Old 12-06-2009, 11:29 AM   #7
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Hardie board installation- roof and flashing gaps?


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It's not ruined- yet. As I said it's impermeable and it looks better tight, that's why I did it, and maybe I'm inexperienced
I'm aware they recommend to keep it off the roof, but I like to know what the reason for it is.

I think Tpolk's onto something with the wicking.
Were you aware BEFORE you installed it? I'm also inexperienced at installing Hardie because I've never done it before. I would follow their installation guide BEFORE I install it.
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Old 12-09-2009, 01:00 PM   #8
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Hardie board installation- roof and flashing gaps?


I've got a well respected colleague here who said it's absolutely not going to cause an issue, due in part to the low slope of the roof his idea is that water can not be blown in by a heavy wind, anyway I'm going to trust his experience on this one.
In the future I will maintain the gap, but only after adding about a foot of ice and water shield to the meager 4" step/ wall flashings.
Thanks for the posts- happy building.
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Old 12-09-2009, 03:07 PM   #9
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Hardie board installation- roof and flashing gaps?


A low slope is even worse. The water travels slower and stays wet longer. Believe it or not, Hardie will grow mold on it. If you want a cleaner look, use some coil and make a nice solid flashing of the same color. Keep it off the roof.

BTW, what would the slope have to do with not blowing water in with a heavy wind?
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Old 12-09-2009, 03:56 PM   #10
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Hardie board installation- roof and flashing gaps?


I think that the literature out there reads that Hardiplank is impermeable to the damage that water causes to wood products, not that it is actually impermeable to water or moisture.

For sure water will wick behind the roof/flashing/siding junction the way you installed it and even if not I think you voided the warranty in that area for any potential trouble you might have.
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Old 12-09-2009, 04:26 PM   #11
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Hardie board installation- roof and flashing gaps?


If you are in an area with a significant freeze-thaw cycle, Hardiplank installed with untreated exposed edges up against a roof surface is subject to damage.


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