What would be the proper product to use in rebuilding a chimney crown?

 

Concrete, steel, tin, aluminum, stainless steel. Lotta products, but we need to know what you need. Got pictures?

 

I am attempting to attach 2 pictures. The top layer of brick has about a 1" overhang. Would the new crown need to extend even farther out? The house was built in the early 60 so I am sure that this is what is left of the original crown.

 

A proper crown should extend to the end of the brick.

 

Definitely concrete, not mortar, 1-1/2 - 2" thick, sloped away from the flue, and to the outside edge of the top bricks at an absolute minimum, but I like to let them hang over an inch, or usually two, and incorporate a bevel at the bottom of the overhanging edge, to create a drip point. You can hold your form in place with a few bar clamps, and form the bevel with wedge shaped pieces inside the form. I also like to wrap the flue with a single wrap of sill seal, which will compress as the concrete shrinks, reducing the chance of the new crown scracking. You can then slice any of the seal that is sticking up, and seal it with bituminous caulk. While up there, I would also add a critter screen and rain cap over the flue.

 

Using pre-packaged concrete mix with decent strength is the way to go. I like Quikrete 5000 myself, having used it on many projects with no performance problems. As others have mentioned, but not shown in the pictures submitted previously, cantilever the entire concrete perimeter such that it overhangs the brickwork by at least 2". Remove all dirt and loose concrete/mortar before starting your formwork, making things squeaky-clean to promote good bond with the new concrete. I prefer using 2 x 6s for the edge forms, as they are stiff enough not to deflect excessively when beating on them to consolidate the new concrete, and deep enough to provide good fastener purchase on the underlying formwork. I only use bar clamps to initially hold forms in place, then remove them after all nails/screws are driven home, to avoid having to dodge the protruding ends when working the concrete up there.

You might give some thought to replacing that top flue tile section first, as the vertical cracks in it look significant enough to cause a fire hazard. Use a hand-held mirror on a sunny day to check the condition of other flue sections farther down the hole, too, replacing any/all with serious cracking issues. Uncontrolled chimney fires have caused many house fires.

 

Step one: Move the cars back

Since it looks like it's needed--how does one replace that top section of flue? I need to replace one where a large chunk has broken off the corner & a bird got caught in my flue damper (just one of many strange things that caused my heat to go out this past winter).

 

The flue liners come in 2' sections so you should be able to chisel out the top section and mortar in a new one.

 

Thanks for all of the input. I am starting to gather all of the stuff that we will need to complete this project. When I discovered the problem a couple of weeks ago I had the foresight to tarp the chimney. I am glad I did as we had a nasty downpour yesterday. Once the prject is done I will post a picture.

 

Just a note....that also make per-made crowns that can be cut to the size you need if you provide the measurements. Then all you have to do is put mortar underneath and in the joints on each side. I think you can even use adhesive caulk. If your chimney and flue happen to be a common size it could be one piece.

I'm sure they cost more than a do it from scratch method...but I can't imagine much more. I see advantage in a good slope and overhang without any forms of any sort.

I've read there should also be a small expansion joint between the liner and the crown, filled with a flexible sealant to prevent cracking.

Just more things to ponder now that you are already in the buying stage...lol.

 

Regarding tile flue replacement: If there is damage to more than just the top section of flue, other options should be considered (such as metal or CIP concrete liners), because access to the lower flue sections can only be achieved by completely removing the outside of the chimney (usually mortared bricks) and then rebuilding same once the replacement flue sections have been installed. Doing so will get expensive very quickly, unless you have the masonry skills and patience to relay all of the bricks yourself, after chipping off all of the old mortar still attached to them.

Also, be sure to use refractory mortar for bonding new tile flue sections to existing sections below. Using any other mortar will result in failed joints, as ordinary mortar cannot withstand the high temperatures inside of a typical chimney. If more than one flue occupies a single chimney, it's important to alternate the tops such that they aren't level with each other--at least a 6" difference between adjacent flues is necessary to achieve best draft conditions and minimize the chances of any cross-drafting.

 

Not to add anything, but rather to reiterate, first off, I said concrete, but the Quikrete 5000 that Bridgeman mentioned is what I would use as well. It's not necessarily concrete in the truest form, but a good substitute in many cases, and definitely the right choice for a crown, as you don't want larger agregate popping up. And the refractory mortar, which I couldn't think of the name of, so was waiting for that to pop into my head before posting, is definitely what you want for the flue. I have personal knowledge of two house fires that started from the chimney, and both were attributed to mortar failure; you want to use the right stuff. Finally, I mentioned bituminous caulk around the flue, when I meant butyl, but frankly a good silicone caulk should work fine at that point. For longevity though, you definitely want something between the concrete and the flue, as the concrete will shrink, and will generally ultimately crack. As I mentioned, I use sill seal, but there are other things that will work, and then you want to caulk the joint.

 

In the process of putting on a new crown. We are following a design that has a 2" overhang. Looking at all of the chimneys in our area and I see, as well as our old crown, that 99% of them do not have an overhang. The botton of the crown is flush with the top layer of bricks with a slope above the bottom of the crown. Is having the overhang a new and improved thing?

 

I don't know about "new and improved", but yes, a crown that pitches away from the flue and extends beyond the brick a few inches is a good thing.

 

I merged your threads for continuity of the subject.

 

Can anyone explain the form building a little better or maybe post a drawing of the cross section of putting the bevel on the form. The way all the sites explain it is to rip a 2x4 with a 15 degree angle then screw it to a 1x6. I've only come up with one way to make this work but im not sure its correct and then once i lay it on the top row of bricks and then pour, once I remove it wouldn't I have a void where the wood was laying on top of the bricks just below the new slab? I've been searching like crazy but i just keep finding the same explanation that I am having trouble following to a T.

 

If you have local availability to precast concrete crowns/caps that fit your flue size, they are hard to beat. - Well finished, air entrained concrete (2-3" thick) and an over hang with a drip edge (very important).

They are a little heavy and bulky, but are available in and around many cities for the standard flue sizes like yours. Certainly a lot easier and limits the trips up the ladder. - As a supplier of masonry materials (mainly block), we allowed a couple of employees to make then in a corner on site using their own materials on their own time and bought them from the employees for resale to individuals and contractors.

Dick

 

snoop,
As I understand it the form would be built and wrap around the sides of the chimney like this:

 

I've never tapered the bottom forms upward like those shown, but rather just install them horizontal, with inverted 3/4" chamfer strips about an inch in from the edges to serve as drip grooves. I also either lay a heavy bead of caulk along the bottom outside corners, or install smaller corner chamfer strips, to eliminate the sharp edges that will otherwise be present and prone to cracking/breaking off when the forms are stripped.

 

I don't think snoop has a problem with building a form (cut wood, screws, nails, etc)...but I can't figure out how you would keep it in place. And how do you prevent drips and runs of cement down the chimney? As he said, supporting it with the top bricks leaves wood under the form.

 

Easy to temporarily clamp in place, tight against the brickwork. Followed by Tapcons or Ledgerloks into the bricks' mortar (easier drilling, easy to fill the holes later, but will only work if mortar is tight and strong), and a few long deck screws into adjacent form members (at the corners), such that clamps can be removed. Caulking the top of the forms where they touch brickwork will seal any openings, to prevent mortar from dribbling.

All of the formwork is stripped when adequate concrete strength has been reached, such that no wood is left in place. The concrete cantilevers out past the brickwork on all 4 sides.

 

My second row of bricks down is offset about half inch maybe a little more. I was going to cut a couple of blocks to support it on end/middle/other end and then maybe use a ratchet trap to hold the whole form tight to the chimney. I'm not sure if the ratchet will work or not... But back to the drip edge, in the last day since posting i thought about it some more. What if theuy meant by putting the bevel on one edge to mean sort of cutting one corner off the length of the 2x4 so you are not getting a bevel on the entire bottom edge but just a little at the edge of the form. See sketch. I was reading it the same as you posted in the last graphic initially as well. I did also think about just putting a strip of wood about an inch back as well... Also do you use any release agent on your forms to strip the wood off easier? Everything I found points to diesel fuel. But is it necessary? How long do I wait to pull the forms off and if I pull them off right on time will they come off easier negating the need for a release agent?

 

The crown should be sloped downward from the flue. It should have a 2" overhang and concave half round used to make a drip. The drip is important because prevents the runoff water from migrating back toward the chimney and possibly migrating under the crown into the void between the flue and brick(big long term problem) and common to many major problems historically.

Anchoring a crown down is not a problem because it has continuity and mass. If precast, a generous bead of silicone adhesive will seal the joint and still have the flexibility to adjust to any temperature situation.

Dick

 

Probably any pointy edge will chip & crumble as years go by. I like the idea of caulking the form corners to create a round-over. Neat.
On wood sills & steps you often see (if the worker knows his stuff) a small groove on the bottom just back from the front edge. Water will hit this groove & fall off. Maybe a small bead of caulk (or hot melt glue) in the bottom of the form 1/4" back from the edge would create such a groove & act the same?

 

The plans I used I got from HANDY Magazine | DIY | Home Improvement | How To | Plans | Power Tools. It is on page 3 of Masonry Projects. We poured the crown on Saturday. We assembled the form on the ground, screwed the corners and cinched 2 tied downs around it. Bring a screw driver on the roof when you mount it. You may have to loosen a screw or two. We didn't need to use the nails to hold it on the chimney as the tied downs held it in place well. We happened to have, and no longer need, a quart of Clutch Saver. That is what we used as oil to put on the wood, and the cardboard around the flue. When we removed the form 24 hours later it came off without a hitch. Couldn't have been easier. We used 1 bag of concrete but could have used another 1/2 bag more. Knowing what I know now, I would have been a little pickier about how it was poured. Less gravel in the area that creates the overhang. There is a little touch up that needs to be done on the outside edge of the overhang. Overall the project turned out just fine.

 

Now that the crown is poured, can anyone recommend a high temp silicone sealant to fill the expansion joint around the flue and the base of the crown? Thanks

 

It is best to keep one thread going on a subject. I merged your new post to the original thread, since, standing alone it didn't make any sense.

 

Chimney Crown > Handyman Club of America

Ya never know how long these kinds of links will work but at least for now this takes you to the project.

 

Touch up needed because of exposed aggregate? I don't have much concrete experience but I've read the steps to creating a concrete countertop. A tip was to vibrate the outer edge of the forms with a sander, handle of a reciprocating saw, etc to drive the aggregate away from the form. Hammer taps might also work. Cement will flow in to fill any voids and leave a smooth edge.

 

Erica,

Your summary of the concrete placement has a few hints regarding what could have been improved. Not intending to criticize, but just some constructive comments for others who may be considering a similar placement.

It sounds like you didn't adequately mix the concrete before placing it in the forms ("less gravel at the overhang" comment). A properly mixed batch will be completely uniform, with all of the "gravel" dissipated by the Portland cement and water in the mix. If there are any voids present after stripping the forms, it's possible that you also didn't adequately consolidate the mix after it was placed in the forms. For small placements, just "spading" the mix (vertically jabbing it) repeatedly with a hand shovel, followed by vigorously beating (not just tapping) on the forms themselves, is usually adequate to remove any entrapped air pockets. Doing this requires that the forms are firmly built, and not prone to moving or falling apart when struck with a hammer.

Your comment about "could have used another 1/2 bag of concrete mix" indicates that your placement wasn't thick enough in the center, adjacent to the chimney flue. This means that there is probably very little pitch on the finished concrete surface, which would enable water to freely run off instead of collecting on top. Freezing water can do a lot of damage to any concrete surface, especially newer placements with little cure time, so you may want to check on the condition of your cap from time to time, to make sure it's performing adequately.

 

FWIW, the Handyman presentation shows a few glaring mistakes, that should be avoided by anyone wanting to build a chimney crown. Most important is the fact that the tile flue in the background, third from the front, clearly has what appears to be an open, vertical break or crack in it. Good way to start a house fire. If that particular flue was no longer being used, it should have been removed or permanently capped. Secondly, mitered corners are great for picture frames, but not so good for concrete forms. Properly-built forms should never have mitered corners where members join each other, as is shown in the photos. Rather, the ends of formwork should be cut square, and overlapped, enabling adequate bearing and fastener purchase without fear of the fragile, mitered corners breaking when compressed by nails or screws.

 

How long can I expect the cure time to be? We have been keeping it covered with plastic and wet rags since, oddly enough, the weather has been pretty warm in the Seattle area lately.

 

For a rich mix (with adequate Portland cement), wet curing is usually needed for just a week or so. For most pre-bagged mixes such as yours, which are usually notorious for having low cement content, leaving the wet cure on for an additional 4 or 5 days would be a good idea. Doing so will optimize the amount of cement particle hydration taking place, resulting in maximum possible strength and durability for that particular mix. An additional "dry-cure" period of at least 4 days is also a good idea, before using the fireplace.

A good practice for most pre-bagged mixes is to "sweeten" them with some added Portland cement during the batching process. A few heaping shovels-full per typical (wheel barrow) batch is what I usually add. Any left-over Portland cement (the stuff comes in 94-lb. bags) can easily be stored in a sealed (with a lid), 5-gal. bucket for later use.

 

Should the curing of the concrete occur with the forms still on? I am concerned with getting a wood grain pattern on the portions touching the forms so I wanted to remove them as soon as I could but I also don't want to sacrifice the strength of the concrete. Also considering the pour is cantilevered over all 4 edges I was afraid to remove the forms too early just to have the cement sag or worse crumble when I remove them. So how long should I wait before removing them? Maybe I could remove the sides first but leave the bottoms on longer? I also have a strip of base toe molding nailed inside the bottom form to create the drip edge so I'm not sure if that will make a difference in your answers. I am using a bagged Sakrete Crack resistant fiber reinforced concrete available at my local HD. The data sheet for the product says to set for an hour before finishing but I'm afraid they are just referring to the top exposed side not the sides against the forms. I think the bag says 12 hr set / 24 hr walk on. Would I be safe removing the forms at the 24 hour time and would that still allow me the opportunity to smooth out the wood grain pattern?

 

Unless there's a strong reason to strip forms early, I prefer to leave them in place. Better cure, with less chance of losing internal moisture needed for hydration.With just short cantilevers all the way around, all forms could be removed following initial set. However, depending on the roughness of the forms, there will be some wood-grain showing on the new concrete surfaces.

If it really bothers you, buy a rubbing brick and use it on all surfaces to rub them smooth after stripping. Keep in mind that it's not likely that anyone on the ground will be able to see form imperfections, unless they're doing a critical quality control inspection, using a very good set of binoculars.

 

After all the talk about the ways to do it and problems, I am not surprised that most of the contractors we supplied used precast crowns to get it done right very quickly and easily instead of trying to make a "jury-rigged" combination of brick, forms and mixing and lugging around wet concrete for such a small, but important thing.

That was years ago and I took it for granted because precast crowns (in various sizes) were all the good contractors used. We allowed our union employees to make them off the clock in a corner of the shop and they also sold them to our competitors.

It might be worth checking around because there we many suppliers of fiberglass molds then to allow casting upside down with a good finish and a proper drip in different sizes.

Dick

 

A possible drawback to using precast concrete chimney crowns (assuming one is lucky enough to find one with the correct flue opening(s), both size and locations), is the fact that the things can easily weigh up to 300 lb. each, or more. Not exactly within many DIY's capabilities in terms of getting something that bulky lugged up a ladder and then set into place on top of a typical chimney. Quite a bit easier to go cast-in-place, hauling up whatever amount of concrete is comfortable, in partially-filled 5-gal. buckets.

 

Yeah I have 2 flue pipes, one is smaller and square the other is a rectangle and a bit bigger. The gap between them is only about 1.5 inches. I'm sure it would be difficult to make every combination of every possible opening. Then it makes it a "custom order" type of thing and then you hope you measured perfectly... I'm sure the pre-cast is the way to go when you only have one flue on a square chimney but start varying and forget about it. Plus theres always that weight problem...

I'm going to leave the forms on. I did think about it afterwards that no one is going to see it from the bottom however my roof has a very low pitch to it and its easy to walk up there so the inspections will probably not be by binocular...