New chimney crown - my experience

[kolyur]

Last fall I put a completely new concrete crown on my chimney. This was in preparation for a new PE Super insert and insulated SS liner (for which this forum was an invaluable resource). Most of the Internet articles I found on the subject just involved slapping a layer of mortar around the flues, which was not the route I wanted to take. So I figured I'd document my project here in the hope that it may help someone in the future. I relied heavily on (broken link removed) for the basic process.

My old crown was in very bad shape. I’m not sure if it was original to the house (built in ’74), but it was poorly constructed and allowing water to seep into the brick. My chimney is quite large, about 6 ft x 2 ft, so I wasn’t able to capture the entire thing in most of the pictures. Removing the old crown was quite easy; most of the chunks came off without much effort. I also took off the top course of brick because the top of the flue needed to be several inches above the top course so the crown could be sloped down for drainage.

I built a welded steel frame from angle iron to support the new crown. The ends are notched to allow the pieces to rest on the brick. Building and mounting the concrete form was probably the most time-consuming part of the whole project. It is constructed of 2x4 and 1x6 material, providing a 1.5” overhang around the entire chimney. I beveled the top side of the 2x4’s at a 15 degree angle in order to create a drip edge, so that water could not flow around the crown and end up running down the brick. Once the form was in place, supported on my scaffold, I attached sections of waterproof cement board to the steel frame and covered all the joints with flashing tape. The article I referenced above emphasizes that is important to create a “bond break” between the brick and concrete, so they can shift relative to each other. I also wrapped the two flues with cardboard.

I had never worked with concrete much before, but it turned out to be fairly straightforward. Since I was working 20 feet up in the air, I pulled up my garden hose and mixed about 400 pounds of concrete in a 5-gallon bucket using a paint mixing attachment on the end of my cordless drill. Not the best setup but it got the job done. When the form was about half full, I added a layer of 1”x1” steel wire mesh for reinforcement. As you can see from one of the photos, the unfortunate thing about working with concrete is that once you start, you have to keep at it until the job is done. I did my best to angle the surface away from the flues for proper drainage.

I removed the form about four days later. Everything looked good except that there was some bad pitting around the sides where the concrete had not filled in very well. It was mainly a cosmetic issue, but in hindsight I probably would have used a smaller-aggregate concrete and/or vibrated the form during the pour. I ended up applying two coats of Thoroseal, which is a thick cement-based coating that can be applied with a stiff brush. It fills in all the pitted areas and also acts as a sealer. At this time I also removed the cardboard from around the flues and filled the gaps with backer rod and silicone sealant.

Overall I was happy with how it turned out. Hopefully it will outlast me… it’s not a job that I want to repeat! If others have done similar projects, I would love to hear your experiences.

-John

 

[Yagminas Masonry and Wood-heat]

Nice job! I'm curious about the lack of dividing wall between flues though....

 

[weatherguy]

Are you coating the crown with anything? I was thinking of putting this product on my crown

(broken link removed to http://www.chimneysaver.com/crowncoat.html)

 

[fossil]

Yeah, you did a beautiful job...but I'm wondering the same thing as Yagminas. I'd expect to see brick between those two flues. NFPA 211 says a 4" masonry separation is required between two adjacent flues in a single chimney structure. What it doesn't say is whether or not that separation must extend the full height up past the roof to the cap. I just always assumed it did. Dunno, I'm not a professional. Again, very nice work on that project. Rick

 

[kolyur]

NFPA 211 says a 4” masonry separation is required between two adjacent flues in a single chimney structure.

Interesting... I had not even considered this. What's really strange is that the flues have no additional support along their 20-foot height. In other words, the flue tiles were just stacked on top of each other with no connection to the chimney wall at any point for stability. After I took the old crown off, they were quite unstable until I had the steel frames and flashing tape in place. The smaller flue is for my gas hot water heater... not sure if that makes a difference codewise?

Are you coating the crown with anything?

It has two coats of Thoroseal... I'm hoping that will be sufficient.

 

[burleymike]

Wow I always thought they built a chimney out of block then put the brick around the block. No wonder chimneys are the first do go down in an earthquake. Your new crown sure looks nice. That is something I need to get around to doing on my place.

 

[cmonSTART]

it looks fantastic! Nice work!

 

[kolyur]

Thanks... now I've been thinking about painting it. The rest of my house is brick and dark brown wood siding, so the drab gray of the concrete kind of stands out. I inspected the crown after one season of burning and there was a single hairline crack in the center. Not a big deal but I probably should have done a better job of reinforcing the concrete.

Wow I always thought they built a chimney out of block then put the brick around the block.

Never having done this before, I had no idea what to expect when I pulled the old crown off. I tend to be haphazard when laying my tools down and I'm just lucky that I didn't drop any down inside, or they would have been gone forever.