Sealing the chimney

[dbhansen]

Because my 8" DuraVent chimney will go directly through the peak of my gable-house roof, I'm thinking about building a square frame around the chimney at the peak (I guess you call it a "chase"). The frame would be covered with Hardibacker and then probably stucco. My questions are:

1) How do I seal this thing? Freeze/thaw/rain is a big issue here. Do they make a flat flashing that slides over the chimney? Would the flashing cover the entire top of the chase and fold over the sides, over the stucco (like this one)? Or can I just stucco the flat part and use a hi-temp sealant where it hits the chimney pipe?

2) If I use some type of roof tile (like slate or cement tiles), would I need more flashing at the intersection of the slanted roof tiles and the vertical chase?

I'm beginning to think I need to hire a roofer!

Here's the basic style I'm thinking of, though the chimney would extend a bit higher: Link

[wlively]

Re: Sealing the chimney

Well since your reference is my oven, maybe I can help.

I cut a hole in the roof decking, 6" wider than the pipe. While burning my curing fires I monitored the outside temp of the duravent, and 6" is overkill, but since I used wood decking I went with an extra insurance factor. Then I layed pieces of flashing over the gap, cut to fit around the pipe. I built the box out of durock only, screwed and mortared together. Then filled the box with stucco base as I was stucco'ing (is that a word) the box, making sure to slant it so that any water would run away from the pipe. That box isn't going anywheree now. It turns out the chimney cap completely covers my box, so unless it is raining sideways, it won't be getting wet. I can't claim any forethought on that one, luck all the way. My outside stucco finish coat is acrylic stucco, so it is waterproof and flexible, I put on top as well. No need for high temp caulk as the insulated chimney stays cooler than the sun will get the roof.

I think that making sure your box is smaller than the chimney cap and that it is close, is a key to longevity. If there is no standing water on top, then there is no way for it soak in and without water, freeze cycles don't mean much. Make sure to use a waterproof coating on top.

Can't help with tile or slate roofs, no experience. I would think same principle, good layered flashing, sealing at the joint.

[dbhansen]

Re: Sealing the chimney

Thanks Wade! I didn't realize that was your oven. I might copy your idea of wood trim around the roof, too; I like the look of that.

Daren