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  1. #1
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    Aug 2013
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    Default Air gap between chimney and exterior wall when reinstalling brick

    Unless someone has the gift of intimate experience with chimneys in this area, diagramming the situation might be a good idea.


    Description of the situation

    1. A brick on the angled exterior flank of the chimney dislodged.

    1. The old lime mortar on the flank of that brick was broken where it went under the stucco of the house wall and then met with the 15 pound felt paper facing the exterior studs of the house wall.

    2. The 15 pound felt paper on the exterior studs has a U channel flashing at the bottom which it overlaps vertically a bit on the inward side. The flashing runs diagonally and upward toward the center of the chimney and appears to function as a drain for water collected from the felt paper and the chimney mortar which extends down into the U.

    3. There appeared to be a 1/8" or less air gap between the felt paper and the outer brick mortar face. Was this the unimportant result of natural shrinkage of the old lime mortar? Or is this air gap a design element that must be maintained when replacing the old lime mortar?

    4. If this air gap is an important design feature, how do I establish it when putting in new type S mortar when I only have restricted access from the side of the chimney where I can remove the old lime mortar under the dislodged brick and through the top where some of the stucco wall has been broken? Is there any material I can temporarily insert between the wall felt and new mortar to establish the gap and then remove without disturbing things?

    5. Note that the felt paper in place was ripped when removing some of the old lime mortar. Although the old felt paper is no longer tight, I have repaired the tear with waterproof caulking and it seems fairly strong. If I can maintain an air gap between the wall with its 15 pound asphalt felt and the new mortar, a problem is unlikely. However, if the weight of the setting mortar bears directly on the patched paper which is no longer taut, I am more concerned.

    6. There is about 3/4" room to push a new patch of 15 pound paper under the old paper beneath the upper section of opened stucco wall. There are no pieces of felt in place for this patch to side lap under. The sides of the patch would have to be caulked and stapled to upright studs and the bottom left to overlap the top part of the U channel flashing below. Gaining access to the the two upright studs would require extensive removal of the old lime mortar bed.

    Any and all thoughts and observations would be appreciated. If someone is familiar with the best approach in this situation from intimate experience, I would be more than grateful for any advice.
    Last edited by yourfriend; 08-08-2013 at 02:33 AM.

  2. #2
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    Nov 2012
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    Default Re: Air gap between chimney and exterior wall when reinstalling brick

    well actually you should have a 1" air gap but that almost never happens so I have a few questions is the chimney lined if so with what material and what is venting into the chimney. also you should use type n mortar not s n is softer and will work better with your old brick. many people will tel you that you need to use lime mortar but in our 35 yrs experience in the chimney business whe have not had problems when we use type n.

  3. #3
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    Default Re: Air gap between chimney and exterior wall when reinstalling brick

    Quote Originally Posted by junkout View Post
    well actually you should have a 1" air gap but that almost never happens so I have a few questions is the chimney lined if so with what material and what is venting into the chimney. also you should use type n mortar not s n is softer and will work better with your old brick. many people will tel you that you need to use lime mortar but in our 35 yrs experience in the chimney business whe have not had problems when we use type n.
    The chimney is from the 1940s and has a thin inner coat of what I am told is mortar parging on the
    bricks inside the chimney stack. There is no flue. A brick fireplace vents into the chimney from the living room.
    The fireplace has seen little use over its lifetime.

    As to the damaged building paper on the studs, I figured out how to place a 2 piece 15 pound asphalt felt patch in the constricted open area today. On the top, one part of the patch slips under the old paper. On both horizontal sides, I can staple or nail it (which is better?) to the studs below the old paper. I am going to put silicone caulk under the sides of the patch as there is no adjacent free felt surface to slip the patch underneath. Hopefully, I can draw the felt patch tight enough so it resist bowing inward from the weight of the mortar although, without breaking out a lot more of the stucco wall, I will only have 2 points of attachment at most on either side of the felt patch unless I break back the stucco over the studs a lot more.

    I am still at a loss as to how to establish an air gap between the 15 pound asphalt felt on the outside of the
    house studs and the new mortar which will abut the dislodged brick and fit into the rising diagonal metal U channel
    at the bottom of the repair area. Can I insert a loose piece of 15 pound felt over the patched area and into the U channel, put in the mortar and brick and then withdraw the loose piece of felt once the mortar has set to get the air gap? Do I need the air gap or can I mortar right up to the fixed 15 pound asphalt felt and filling the U channel with the mortar in the process?
    Last edited by yourfriend; 08-09-2013 at 01:05 AM.

  4. #4
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    central pa
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    Default Re: Air gap between chimney and exterior wall when reinstalling brick

    well first off if you are planning to use the fireplace the chimney should be lined absolutely. if it is lined with an insulated liner then you will need no air gap. if you don't line it then you should absolutely maintain any air gap you have your idea of putting in some sort of filler that is removable if probably the only way to do that. But again by modern standards the setup you have is unsafe and I would strongly recommend an insulated liner we use stainless but there are also the pour in place liners that would work well in your situation. either way code says that an insulated liner inside a nominal 4" masonry chimney can have zero clearance to combustibles otherwise you need a minimum 1" clearance.

  5. #5
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    Default Re: Air gap between chimney and exterior wall when reinstalling brick

    Quote Originally Posted by junkout View Post
    well first off if you are planning to use the fireplace the chimney should be lined absolutely. if it is lined with an insulated liner then you will need no air gap. if you don't line it then you should absolutely maintain any air gap you have your idea of putting in some sort of filler that is removable if probably the only way to do that. But again by modern standards the setup you have is unsafe and I would strongly recommend an insulated liner we use stainless but there are also the pour in place liners that would work well in your situation. either way code says that an insulated liner inside a nominal 4" masonry chimney can have zero clearance to combustibles otherwise you need a minimum 1" clearance.
    Junkout, thanks for staying with me on this. I appreciate your help. Here is my latest concept. Cut back the mortar
    bed for the shoulder on the inside to give some working room to install a barrier for the new mortar. "Working room"
    means there will be a greater space between the building felt and the mortar and more area to insert and withdraw
    the barrier.

    It may be that installation of a temporary barrier to retain the new mortar may still be impractical once I cut out
    the old mortar bed. Nonetheless, the U flashing in place may have been damaged by motion of the grit which I
    worked to and fro to remove from the tight space between the flashing and the intact mortar. Painting it with cold
    galvanizing compound seems a good idea given how it must have been scratched. That last is my fault.

    Again, thanks from me to you.

  6. #6
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    Aug 2013
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    Default Re: Air gap between chimney and exterior wall when reinstalling brick

    I removed most of the supporting rubble and bricks under the top shoulder course nearest the wall.
    What I noticed is that all bricks on the inside of the wall facing the paper are all covered with a layer of mortar. The mortar is what faces the paper inside the wall.

    Is there a reason for this? Is the mortar a protective coat for the ends of brick inside the wall?
    If so, could I lay a coat of mortar on the side of the bricks such that it goes under
    the face of the stucco wall but extends a lesser distance toward the asphalt felt paper,
    therefore providing that air gap between itself and the felt paper? Whether this is physically
    possible given the tight space and limited access is controlling but my question is--if it is possible--
    would there be any reason not to?

    As to the type N/ type S mortar issue, the chimney is in quake country which the upper section
    where it passes the roof line shows. The flexible strength of type S is higher than type N. However, I am
    concerned about face spalling on the older brick ready for repointing. If the comparative softness of type
    N--or any other reason--supports type N, I am ready for education.

    To repeat myself as I will again, thank you for your experienced intelligence guiding me here. If you ever
    get to quake country for a vacation, we have our points of interest and you will find a guide ready at your
    service.

  7. #7
    Join Date
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    Default Re: Air gap between chimney and exterior wall when reinstalling brick

    The brick was pargeed on the back as an extra layer to ensure against flue-leakage. Without being there I can't say, but if the mortar failed in one place the rest may be weak too. In any case you'll need to line the chimney to make it safe to use. If the rest of the mortar isn't good have the chimney demo'ed and reconstructed in accordance with code. If this is the only place the mortar failed I'd be wondering why it did that when the rest didn't. Parts don't fall off of houses without a reason!

    Phil

  8. #8
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    Default Re: Air gap between chimney and exterior wall when reinstalling brick

    Quote Originally Posted by Mastercarpentry View Post
    The brick was pargeed on the back as an extra layer to ensure against flue-leakage. Without being there I can't say, but if the mortar failed in one place the rest may be weak too. In any case you'll need to line the chimney to make it safe to use. If the rest of the mortar isn't good have the chimney demo'ed and reconstructed in accordance with code. If this is the only place the mortar failed I'd be wondering why it did that when the rest didn't. Parts don't fall off of houses without a reason!

    Phil
    Hello Phil:

    Thank you for that insight. It is logical.
    I'll refer the issue to the mason doing the repair.

    I appreciate your help.

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