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Old 01-06-2011, 11:32 AM   #1
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Rebuilding chimney cap


I have a chimney cap that was obviously made of mortar and has completely deteriorated. It is a single flue and there is atleast a foot length gap long ways to the outer brick. I want to cast a cement crown, but am unsure of what to use to support the base so i can build it up. I was thinking some type of wire mesh to go over the gaping hole.
any ideas how the pros do it?

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Old 01-06-2011, 11:58 AM   #2
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Rebuilding chimney cap


Pics?

I have not done this myself but I have researched it extensively as it is on my HoneyDo list for next Summer.

The gap you are trying to bridge is between the brick and the side of the flue (clay liner)?

Are you planning on overhanging the brick on the outside? I have worked enough with concrete and watched enough poor examples of this being done by "experts" (one in particular) on youTube to get a handle on what not to do.

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Old 01-06-2011, 12:15 PM   #3
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Rebuilding chimney cap


1 5/8" patio block laid flat on the top, cut to length of course.
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Old 01-06-2011, 06:29 PM   #4
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Rebuilding chimney cap


I made one up by building a plywood form supported by 2x4 blocks. I overhung the brick by a couple of inches and nailed a 1/2" half round to the base to form a drip groove. Then I used hardware cloth to reinforce the concrete. I also stuffed oakum between the liner and the concrete to allow for expansion then put backer rod and caulk to seal it from the weather.
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Old 01-06-2011, 07:06 PM   #5
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Rebuilding chimney cap


won't the plywood get too hot, and burn?
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Old 01-07-2011, 05:49 PM   #6
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Rebuilding chimney cap


the plywood is a form for pouring the concrete into! when the concrete sets, you strike the form and are left with a cured concrete cap
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Old 01-13-2011, 06:10 AM   #7
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Rebuilding chimney cap


Sounds like a very professional job mrgrins. The overhang and drip edge are correct details often overlooked by the "experts". Where and how did you form the inside of the cap (Closest to the flue liner)?

I'm thinking heavy gauge sheet metal
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Old 01-13-2011, 03:44 PM   #8
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Rebuilding chimney cap


The gap should be small enough that the oakum will fill it. If not, I suppose a strip of metal would work as long as you leave enough brick exposed for the mortar to adhere.
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Old 01-27-2011, 05:10 PM   #9
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Rebuilding chimney cap


sorry for the late reply but just got around to getting on the roof, here is some pics of it, the top two layers of brick stick out about 1/2 an inch over the column. And it appears the old cap was composed of pavers rested on the inside ledge and somehow stuck together with the mortar (poor job).


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Old 01-28-2011, 04:30 PM   #10
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Rebuilding chimney cap


anyone know how to make a form for this? it is a hollow chase. I was thinking of sheet metal over it and flashing on the side of the flue to caulk and allow for expansion.
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Old 01-30-2011, 09:49 PM   #11
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Rebuilding chimney cap


Bump.
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Old 01-31-2011, 09:09 AM   #12
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Rebuilding chimney cap


I don't think you need any sheet steel on that at all. The concrete doesn't look like it would run through any of the brick & mortar as long as it's not mixed extremely runny. Hopefully this helps a little:

Rebuilding chimney cap-cid__dsc00089r.jpg

Yellow is a 2x4. Blue is a 2x8. Measure all sides of the top course of brick. Make form on ground (if you have help to get it up there, just install it on chase as one piece), form should be 1/4" or more bigger than the chase dimensions. It's best to bevel the sides of the cap about 5 degrees as well, because than you can simply slide the form over the brick and it will rest on the brick via friction. On the longer sides, you will likely need a "stiffback" (2x4 screwed into 2x8 sideways to add lateral strength to resist bowing of form) so avoid alot bowing. This is kind of hard to explain over the web, and may take a few tries to get it to fit correctly on the brick, but it can be done with little or no fasteners into the existing brick & mortar.

One more thing. You need to cut off a few inches of the flue & add another 8-12" onto it to get it protruding above the new cap height. The reason for cutting first is to get the flue joint inside the concrete for stability. You'll likely have to buy a 2' section of flue. Measure carefully now, as there are many variations of flue dimensions out there.
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Old 01-31-2011, 10:31 AM   #13
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Rebuilding chimney cap


Quote:
Originally Posted by no_Wedge View Post
anyone know how to make a form for this? it is a hollow chase. I was thinking of sheet metal over it and flashing on the side of the flue to caulk and allow for expansion.
No_Wedge

I did a very similar project on my chimney a couple of years ago and think I did a pretty good job of it. The crown was in pretty bad shape and there was a crumbling step on the side. I had also raised my roof line around the chimney and needed to add a length a flue to get it back in code.

The side step was easy, just cleaned it up, prepped and re-cemented without forms.

Before:
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After:
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The crown was cracked and spalling. Low spots would collect water, even with a cap over it.
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I was going to cast a new cap as well, but had it covered with metal later on.
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To continue....
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Old 01-31-2011, 11:27 AM   #14
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Rebuilding chimney cap


....continuing

For the crown, I first taped some 3/4 foam board tightly around the perimeter of the chimney, flush to the top, to ensure a gap so the crown would truly float. I wrapped this with a double slip sheet of 6 mil poly to ease removal. I then built the drip groove from 2x4s. I made this just a smidge undersized so when it was screwed together it gripped very tightly and created a pressure fit around the foam board. I was also careful to place the screws where they would not be covered by the concrete, as they had to come out later. The next part of the form would create the drip ledge. Just more 2x4s tightly fitted and located maybe an inch and a half below the first set. I used silicone caulk to fill the joints and create a small cove in the corners.

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Then the side forms were made using one bys. I ran the short sides long to accommodate external bracing for the long sides.

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Then the external bracing and more caulk to round out the corners.

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I added legs to the bottom for support. The extended flue came next using flue cement. Thirty pound felt over the old crown provides a bond break for the new cement. I made the rebar grid to come up about 3/4" short of the forms on all sides.

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I then wrapped the flue and standoffs with a thick layer of corrugated cardboard for expansion space and then a double layer of 6 mil poly as a slip sheet. Next I packed sand around the base of each protrusion to ensure the bond break continued. I also added a ratchet strap with center blocking on the sides to prevent any chance of bowing.

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But wait! There's more....
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Old 01-31-2011, 12:59 PM   #15
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Rebuilding chimney cap


Let's pour! But first I greased up the inside of the forms with some old peanut oil from my turkey fryer. Go ahead and laugh, but it worked really well. (Ya know ya got good friends when they volunteer to carry buckets of mixed concrete up a ladder for you!)

I added more wire reinforcement as the job progressed.

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Finally, it's poured! I did the exaggerated slope because I wanted the new flue joint to be well below the surface of the crown, besides the obvious water shedding capability.

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Unfortunately this was done late in the year and after the pour there was freezing weather called for. So I wrapped the entire top of the chimney with a few layers of old blankets and covered that with some poly sheeting, all secured with a strap. I then put a small electric heater inside the fireplace to keep the new pour from freezing. This worked amazingly well, as whenever I went up to mist concrete the whole shebang was very warm and toasty! I kept it this way for probably a couple of weeks.

Now, off with the forms, and they really came better than I expected. The innermost 2x4s had to be split apart to come out and the foam was removed easily with only a few pieces that had to be extracted with a screwdriver.

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And the drip detail:

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I then slid the pipes out of their cardboard sheathing so I could put in some longer ones to accommodate a flue top damper (oops!). I first sanded them and gave them several coats of black paint and slid them back in place. The next step was to remove the expansion spacing. The cardboard slipped out with a bit of coaxing, especially around the flue, but the plastic kept the cement from getting a grip on it. The plastic came out easily tho. I then filled the gaps left by the cardboard with sand to within 3/4" of the top. The last bit was sealed with cement caulk around the pipes and special high heat caulk for chimneys around the flue.

And this is what it looks like today with the metal cover on the old cap:

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