Flue And Flashing

6stringmason

Im tearing down a chimney and rebuilding it and pouring a new cap tomorrow. Its a decent size. 5'x2' roughly. Well anyway, Im wondering what you guys use to "glue" the flashing to the flue before your pour your cap?
edit: and im doing it the way the site Tscar posted shows. I was taught differently but the way that site shows looks like it is the best way. I was never taught to put exp joint in or make a drip edge.

__________________
Kamps Masonry & Concrete Countertop Design Green Bay, WI
920-680-3195
Visit our website @
http://www.concretecountertopsdesign.com/

denick

6,

We don't put a flashing between the brick and concrete. We rebuild quite a few chimney tops a year to withstand the New England weather. On many of them the style is not the same as in the picture on the web site.

I feel the important factors are, A good concrete mix, not an over hard mix. The thickness of the concrete at the edge is 2 1/2" minimum we pour 3 1/2" most of the time. That there is a good pitch on the concrete. The concrete has a smooth finish BUT not a hard toweled finish. You want air in the concrete and if you push on that trowel your pushing out the air. That the joint where the flue touches the concrete is slightly V'd out and caulked. On chimney's where the detail doesn't allow for an overhang of the concrete the joint at the brick and concrete is caulked. If there is an overhang it is 1 1/2" minimum 2" preferably. Many times the detail work of the top dictates the cap and thats an owners or design person's call even when we recommend differently.

You shouldn't have to use an adhesive to hold the flashing in place. But we have had excellent luck with pure silicon bonding anything to masonry. You don't want a real solid bond between the two.


Nick

red_cedar

The flashing on all 4 sides should'nt move off if they are soldered together.
I have also tried wrapping rope around the clay flue pipe before the cap is poured. I read where the caps crack due to the flue pipe expanding when heated. Dont realy know if it worked because I have never had to go back.

Did you also work the counter flashing pieces into the brick as it was built?

6stringmason

Thanks for the replies guys. I tore the chimney down today. Man was it rotted. The cap was cracked in about 4 different spots and someone had tried caulkin the cracks. The brick were very rotted on about the top 4 courses.

As far as counter flashing red_cedar I didnt build the chimney. Im also not tearing down to the roofline so I dont have to worry about it. Im tearing off the top 6 feet or so and rebuilding. The rest is pretty sound and in good shape. A few joints to grind out and tuckpoint back in but not much.

Im think Im going to try denicks approach and pour a nice solid cap without the flashing. I am going to use expansion joint between the flue and concrete though.

Thanks again guys.

__________________
Kamps Masonry & Concrete Countertop Design Green Bay, WI
920-680-3195
Visit our website @
http://www.concretecountertopsdesign.com/

Tscarborough

If you pour a good cap, I doubt you would need the flashing anyway, so long as you detail the concrete/flue joint.

denick

My thought on the flashing is an apprehension of a place for moisture to condense into water and in the winter with the sun hitting the cap during the day and any moisture condensing on the flashing pools as water and when the sun goes down it freezes on and on every day. If the cap is solid the condensing moisture would be absorbed by the larger mass of the chimney and dispersed as to not have an effect.

I could be looking at it wrong, it's just my view of it.

Nick

I just edited this to distinguish between moisture that is almost always present in masonry and water that you hope is never present in masonry.

Tscarborough

My view on flashing is to use it where you expect to get water. Done right, I would not expect to get water where it shows that flashing.

denick

Tscarborough's comment made me edit my post to differentiate between moisture and water in masonry.

Nick