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  1. #1
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    What material do you use for the roof of a masonry firebox?

    Hi!

    Any advice much appreciated. What material do you use for the roof of a masonry firebox?

    The discussion and comments on this website "livingstonemasons" here: /instructional/how-to-build-an-outdoor-fireplace seem to indicate that you can just use cement for the firebox roof.

    If regular cement is used for the roof, not refractory cement, should the cement be painted with high-temperature paint such as Rutland or AW Perkins?

    Thanks!

    p.s. The bottom and sides of the firebox would be fire brick with refractory mortar. It is the roof of the firebox that I am concerned with. I am referring to the ceiling (the gray, curved, top of the firebox) of the firebox, and I have attached a picture. Any help appreciated!
    Attached Images Attached Images  

  2. #2
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    I would use refractory brick for the roof of the fireplace. You lay it over a form to create the arch and each refractory cement motor-joint serves as a keystone. I would use a spacer - hemp rope will work - to make all the joints / keystones the exact same size. After the internal form is removed you just pull the rope and point the joints with furnace cement.

    PHM
    ---------


    Quote Originally Posted by Stigwort View Post
    Hi!

    Any advice much appreciated. What material do you use for the roof of a masonry firebox?

    The discussion and comments on this website "livingstonemasons" here: /instructional/how-to-build-an-outdoor-fireplace seem to indicate that you can just use cement for the firebox roof.

    If regular cement is used for the roof, not refractory cement, should the cement be painted with high-temperature paint such as Rutland or AW Perkins?

    Thanks!

    p.s. The bottom and sides of the firebox would be fire brick with refractory mortar. It is the roof of the firebox that I am concerned with. I am referring to the ceiling (the gray, curved, top of the firebox) of the firebox, and I have attached a picture. Any help appreciated!
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

  3. #3
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    It seems like the arched surface would be considered hot face just like the firebox walls.

    Try this link:http://www.traditionaloven.com/tutorials/concrete.html
    Good rule of thumb: If you're not sure, err on the side of too much engineering.

  4. #4
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    Thread Starter
    Thanks. Is there anything I can do at this point to salvage the situation? The cement ceiling is already installed.
    It is an outdoor wood burning fireplace.

    Quote Originally Posted by Poodle Head Mikey View Post
    I would use refractory brick for the roof of the fireplace. You lay it over a form to create the arch and each refractory cement motor-joint serves as a keystone. I would use a spacer - hemp rope will work - to make all the joints / keystones the exact same size. After the internal form is removed you just pull the rope and point the joints with furnace cement.

    PHM
    ---------

  5. #5
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    Thread Starter
    Thanks for the link. Anything I can do to salvage the situation? It is a wood burning outside fireplace, and that ceiling is already built. Would the paint help?

  6. #6
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    Quote Originally Posted by Stigwort View Post
    Thanks for the link. Anything I can do to salvage the situation? It is a wood burning outside fireplace, and that ceiling is already built. Would the paint help?
    The paint will help camouflage smoke stains, but add nothing in the way of structural integrity. Use the concrete arch as your base, as suggested by PHM, and install firebrick splits: https://www.amazon.com/US-Stove-FBP6...rds=fire+brick

  7. #7
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    Thanks, PHm and rmdingler! I think you are telling me to add bricks underneath the ceiling. And I think I see what you mean from your description. Are you aware of a step by step or diagram anywhere on how to do this? It is kind of boggling my mind given the way the firebox currently is.

    Quote Originally Posted by Poodle Head Mikey View Post
    I would use refractory brick for the roof of the fireplace. You lay it over a form to create the arch and each refractory cement motor-joint serves as a keystone. I would use a spacer - hemp rope will work - to make all the joints / keystones the exact same size. After the internal form is removed you just pull the rope and point the joints with furnace cement.

    PHM
    ---------

  8. #8
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    Oh! Is that picture you posted of the already-completed fireplace? That you already have built? I thought it was just a general example of the concept / shape / style and so forth.

    If that pic is of the actual unit we are talking about - are the walls actually fire / refractory brick? I ask because they look like standard red brick in the picture. Are they to be faced with firebrick? If not; what kind of firebrick are they now?

    PHM
    ---------


    Quote Originally Posted by Stigwort View Post
    Thanks, PHm and rmdingler! I think you are telling me to add bricks underneath the ceiling. And I think I see what you mean from your description. Are you aware of a step by step or diagram anywhere on how to do this? It is kind of boggling my mind given the way the firebox currently is.
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

  9. #9
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    I would have built the top of this fire chamber from the bottom up - a wood form / a layer of firebrick / two more layers of standard brick. But if it's too late for that now and I had to do it from the bottom down I would measure the highest and the lowest heights of the ceiling.

    Then I would rip out some small wood strips - say; 1/4" by 1"

    Subtract the thickness of the ceiling firebricks from the hi/lo ceiling heights to come up with two numbers. Then I would cut a dozen or so sticks from the 1/4" by 1" wood strips in 1/4" increments.

    Say the ceiling heights (less the ceiling brick thickness) are 24" and 30" - so I would cut the strips:

    24.0"
    24.25"
    24.50"
    24.75"
    and so on until 30.0"

    And get some interior cedar shakes or a dozen wood door shims.

    Then slather the top and side of the firebrick with furnace cement and stick it up in the far back corner. Hold it firmly up in place with one of the sticks wedged under it. If furnace cement doesn't squish out all around - you didn't use enough - so do it over. Then do the one next to it the same way - hold it up with a stick wedged under it. You will want a very Slight bend in the vertical stick for tension. When you are wedging - Plant the top of the stick against the firebrick and then Adjust the tension by moving only The Bottom of the stick. This to avoid having the firebrick slide out of place and require re-doing it.

    If you come to an awkward spot where no stick is exactly right - use a cedar wedge under the bottom of the stick. After each three bricks back-track and trim the squish-out off. 'Point' the jolnts smooth with a small trowel to make a nice smooth joint. To save time you should keep a separate trowel in water to do this with each time. There will not be time to pause as the cement dries pretty fast. So if you have to take a break make sure that the next-space is scraped completely clean before you break - especially the edge of the last brick you installed. And then clean all your tools before you sit down.

    Make every job a Masterwork - you will never regret it.

    PHM
    --------

    Quote Originally Posted by Stigwort View Post
    Thanks, PHm and rmdingler! I think you are telling me to add bricks underneath the ceiling. And I think I see what you mean from your description. Are you aware of a step by step or diagram anywhere on how to do this? It is kind of boggling my mind given the way the firebox currently is.
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

  10. #10
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    Thread Starter
    Yup, that is a photo of the actual unit. The walls are refractory brick.

    Quote Originally Posted by Poodle Head Mikey View Post
    Oh! Is that picture you posted of the already-completed fireplace? That you already have built? I thought it was just a general example of the concept / shape / style and so forth.

    If that pic is of the actual unit we are talking about - are the walls actually fire / refractory brick? I ask because they look like standard red brick in the picture. Are they to be faced with firebrick? If not; what kind of firebrick are they now?

    PHM
    ---------

  11. #11
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    Many thanks for your advice Mikey. Really appreciate you taking the time. I will have to sit down and think hard about the steps you describe to get it right. I am worried about what happens at the edges and at the front... Won't be straight forward, but I guess that is what you get for not doing it right the first time. :-(
    And I guess that high-temperature paint such as Rutland or AW Perkins would be just a stop-gap? Or not even a stop gap?

  12. #12
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    The arch would be best supported by some kind of mechanical-support at the edges - but that would require facing both side walls. Of course you can also use half-thickness fire brick on the walls as well.

    The front and back won't require any support as the arch will be self-supporting there.

    PHM
    --------


    Quote Originally Posted by Stigwort View Post
    Many thanks for your advice Mikey. Really appreciate you taking the time. I will have to sit down and think hard about the steps you describe to get it right. I am worried about what happens at the edges and at the front... Won't be straight forward, but I guess that is what you get for not doing it right the first time. :-(
    And I guess that high-temperature paint such as Rutland or AW Perkins would be just a stop-gap? Or not even a stop gap?
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

  13. #13
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    Thread Starter
    Many thanks for taking the time to respond, Mikey. So I did some research. Do you think that this will work on the firebox ceiling? Heat Shield Chamber Coat: Use Chamber Coat for hand-parging or trowel-on applications or where smoke chamber is easily accessible. http://heatshieldchimney.com/smoke-chamber-repair/



    Quote Originally Posted by Poodle Head Mikey View Post
    The arch would be best supported by some kind of mechanical-support at the edges - but that would require facing both side walls. Of course you can also use half-thickness fire brick on the walls as well.

    The front and back won't require any support as the arch will be self-supporting there.

    PHM
    --------

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