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Can I cast a concrete drip edge onto my chimney?

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Hi - I need a drip edge on my chimney crown (which is in good shape) but I don't want to go through the hassle and expense of rebuilding the crown.  It is ca. 9 feet long and 3 feet wide.  I have been battling a leak problem for over a year now, and I am finally convinced that all my troubles can be solved by adding a drip edge to the chimney.  My question is can I set up a wood frame around the perimeter of the chimney and cast a concrete drip edge (with capillary groove of course) over and around the existing concrete crown?  I am not making a whole new crown or adding a new crown on top of the old, just the drip edge and maybe a half inch extra on top of the existing crown.  Would the newly cast concrete bond ok with the old concrete?  Thanks!  Gary
asked in House Chimney by garymc (120 points)
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1 Answer

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Hi Gary,
From what I saw over the years on many chimney crowns; any attempt of bonding new concrete to existing concrete fails sooner or later. It depends on workmanship and individual conditions / circumstances but it eventually separates, crumbles and sometimes causes more damage.
This is probably because it traps water in a seam between the old and new concrete, no matter how good this seam is, concrete is porous and absorbs moisture. If there are no cold winters in your area and ambient temperature is always above freezing, your project should definitely perform longer than if set-up in a mixed climate geographical location. 
Assuming that you currently have a concrete crown / slab on top of a masonry chimney and that slab is flush with the chimney walls, you can insert an aluminum drip edge, the same type as it is used for the roof, in between the concrete and the layer of brick, stone, or cinder block beneath it.
After that, run a bead of silicon along the concrete – aluminum seam to minimize water penetration. If there’s no space to insert the drip edge, you can cut a thin channel using regular concrete blade and a circular saw. 
I don’t know exactly how this chimney looks and if something like that wouldn’t compromise its appeal but it would definitely save you a lot of work, time, and probably money. Aluminum will not deteriorate either (you could also use copper for aesthetic reasons)… silicon will most likely need replacement every few years. 
answered by darekrudy (21,730 points)
Thanks a lot!  I'll check out the aluminum drip edge then, that sounds perfect.