Replies

1. 
   MikeSmith | Oct 23, 2009 04:22am | #1

   the most important thing is the fireplace / chimney footing... it has to support an exrraordinary load... make sure it is  going to be sized and bears on good soil

   typically..... the firebox /chimney will be outside the foundation line... if the firebox is say 3' deep...it might be 12" inside and 24 outside

   use a membrane  against the sheathing.... and brick ties  thru to the framing... these tie the chimney  to the wall

   detailing the flashing vertically is always problematical.... there is no seal between the brick and the flashing /sheathing.. so this gets flashed very similar to the way a corner board is flashed...

   rake overhangs have to cut back to allow the chimney thru... and the roof flashings have to be let into the  brick

   Mike Hussein Smith Rhode Island : Design / Build / Repair / Restore
1. 
   bradh | Oct 23, 2009 04:40am | #3

   Thank you for the info Mike.  Is there a 2" gap between the membrane covered sheathing and the chimney?  What about a gap at the rake?  How does the chimney intersect the mud sill, band etc.?

2. 
   Brickie | Oct 23, 2009 04:39am | #2

   Local codes will dictate some of that.  I don't know where you live, but if it's in an area of seismic or wind load concern, then the chimney stack will be reinforced (rebar and grout in the core) and strapped to framing at every floor and roofline.

   In general, a rough opening at the floor the firebox will be is made and then above that header, the exterior side is sheathed and papered as normal.  The masonry of the chimney stack will be right on the other side of the building paper.  The opening in the exterior wall at the firebox and possibly a notch in any soffit or roof overhang are generally the only framing changes to be made for a traditional fireplace and chimney.

1. 
   bradh | Oct 23, 2009 04:48am | #4

   Thanks for the info Brickie.  What is the 2" air space i keep hearing about?

1. 
   Brickie | Oct 23, 2009 05:10pm | #6

   There are clearances to framing (or anything else combustible) that need to be maintained, along with minimum thickness of masonry adjacent to combustibles.  Again, check your codes, and enjoy your fireplace when you're done.

   You are hiring a mason to actually construct it, correct?

1. 
   bradh | Oct 24, 2009 04:27am | #10

   Yes I am hiring a professional brick mason.  I will do the rest with some help from the FHB crew.  thank you for the info.

3. 
   BoJangles | Oct 23, 2009 05:47am | #5

   That's an unusual remodeling job.  You don't see many people actually building functioning masonry fireplaces anymore. 

   We are the only contractors in our whole area that still build them. It is a dying art, and if you are building the whole thing including the firebox, it really is an art.

   I would do some research on the dimensions and methods etc. of fireplace construction before you tackle this project.  You could end up with a non-functioning smoke generator if you don't do it right.

   The problem with an outside chimney like yours, is that they get cold and don't draft well.  I avoid outside chimneys if at all possible.  But there are a few things we do to improve the performance in all of our fireplaces.

   First, we always use round flue liners instead of the square or rectangular ones.  They are much more efficient and help with the draft. 

    The second thing we always do is insulate around the flue linings to keep them as warm as possible.  Using Perlite around the flues to fill the space between the liners and the block helps a lot.  You can also insulate block cores if you want.

   Make sure you have a good damper.

   Put a signature brick somewhere so people recognize your artwork!

1. 
   bradh | Oct 24, 2009 04:19am | #9

   Thank you for the helpful info.  I'm using a good mason, I just want to make sure everything gets thought about.  What do you think about a bee-hive detail on top?  will it affect performance?  Iwill be responsible for the siding(1x8) ,trim 2x material and flashing.  How will the flashing detail be done?  What should it be made of?   Thank you for the help.

4. 
   john_carroll | Oct 23, 2009 05:13pm | #7

   The footing is a real concern here. The soil around the foundation is backfill and may still be fluffy after decades. The safest thing to do is to dig down to undisturbed soil at the footing level of the house. Make sure there is no loose soil in the footing and pour a reinforced pad 12-in. thick and 6-in. wider than the chimney base dimensions.

   Incorporate a drain system for the chimney footing with the perimeter drain system of the house.

   In most cases, the chimney needs to be a minimum 24-in. out from the foundation. With brickwork you need to think in terms of full unit sizes. Three bricks with two mortar joints equals 23 5/8-in. To that add the required air-space (usually 1-2-in.).

   The width of the footing varies with the size of the firebox. At a minimum it should equal the width of the opening plus 14-in. on each side.

   At the footing level, you also need to think about how you'll support the profile of the fireplace and the hearth. These extend inside the exterior walls. You can pour a footing inside the foundation wall and bring up a foundation inside the basement or crawlspace.

   Alternatively, you can pour a reinforced slab that cantilevers into the house from the chimney foundation. This occurs during the construction of the chimney. The top of this slab should be below the finished floor.

   Find a professional mason with a track record of building quality, code-compliant masonry fireplaces and chimneys. A lot of mudslingers don't have the knowledge and layout skills required.

   
   

   ---

   Edited 10/23/2009 10:23 am ET by Mudslinger

1. 
   CapnMac | Oct 23, 2009 09:55pm | #8

   Excellent post

   you can pour a reinforced slab that cantilevers into the house from the chimney foundation. This occurs during the construction of the chimney. The top of this slab should be below the finished floor.

   Do not forget to allow for the hearth which may be required by code as well.  This can be 16-18" in front of the face of the firebox.   This is more of a problem with flush-to-floor fireboxes.  But, will sneak up an bite a person with raised fireboxes, as it's easy to forget something has to hold up that knee-ledge.

   OP needs to be thinking about how the roof framing will be carried across, too. 

   From the description, this will be framed like a doorway, with the front of the chminey breast "pushing" into that opening.

   Retrofits like this can really make an insert and cultured masonry a lot more reasonable an answer (and wood-burning inserts are available).  Not having a heat sink shot through an exterior wall can be a good thing for efficiently conditioning the interior space.  Not having to engineer a foundation for 20± tons while also not changing the dynamics of an existing slab can be a benefit, too.

   As was said before, it's more art than science.Occupational hazard of my occupation not being around (sorry Bubba)

2. 
   bradh | Oct 24, 2009 04:30am | #11

   thank you for the info.

1. 
   Jeff_Clarke | Oct 24, 2009 06:14am | #12

   We did a beautiful new masonry chimney in conjunction with our 2007 renovation/addition.   There's nothing like a *real* fireplace.  In our case we also used salvaged antique delft tile and a turn-of-the-century mantel.

   Our chimney is completely within the building envelope - the siding runs right by the back.   Sure, masonry chimneys are heavy but over a footing area of say, 25 SF you can support 50 tons @ the (generally-accepted) soil bearing capacity of 2 tons/SF.   It isn't going to weigh 50 tons either.

   The 2" is the required clear distance to combustibles (from fireplace masonry).

   Flashing to brick, generally should be copper.

   Codes vary, so books like this one:   http://www.amazon.com/Masonry-Fireplace-Chimney-Handbook-Amrhein/dp/0940116294 - while they have a lot of good information don't necessarily conform to YOUR code.   It's still a good, if somewhat expensive, book.

   Spaces around terracotta flues should NOT be filled with anything (especially mortar).

   Hearth extensions should be 19" to 21" (follow your code here).

   As an architect I have to tell you that there is a LOT to designing and building fireplaces.   It's very easy to miss something important like the clear distance to wood trim at the mantel around the firebox opening or forgetting to provide dampered outside air to a fresh air inlet inside the firebox.   Our design tolerance around the firebox (tile layout, curtain, custom surround, etc.) was 1/16".

   Jeff

    

   Edited 10/23/2009 11:24 pm ET by Jeff_Clarke

1. 
   BoJangles | Oct 24, 2009 06:14pm | #13

   Spaces around terracotta flues should NOT be filled with anything (especially mortar).

   Are you referring to my suggestion in a previous post about using Perlite around the flues?

1. 
   Henley | Oct 24, 2009 08:53pm | #14

   I don't use perlite or vermiculite anymore out
   of concern for moisture. Also, the dead air space around the flue is quite effective
   on it's own. Especially if you insulate the cores as you suggested.

2. 
   Jeff_Clarke | Oct 25, 2009 12:51am | #15

   Yes - from my experience with chimney masonry, masons and sweeps (inspectors) you don't want any kind of fill around flue tiles.

   Jeff 

3. 
   BoJangles | Oct 25, 2009 01:46am | #16

   I agree with you about all the rest of the stuff, but Perlite solves a lot of problems when used for that purpose.  It certainly doesn't cause any problems that I have ever seen.

   Here's why I like to use it:

   - When you are constructing a chimney, especially when you are using round flue tiles in a square opening, it is a big help in holding the flue tiles in place while you are laying them.  If you fill as you go, it will hold the tiles right in place as the chimney is built up.

   - It keeps the flue warmer and increases draft flow.

   - It drastically reduces condensation inside the flue.

   - It allows for thermal expansion of the flues while doing all of that.

   - It prevents a chimney fire from breaking out of a damaged flue and using the outside of the flue space as a chase for the fire.

   What's not to like about all of that??

   BTW,  This is not my idea.  Fireplace contractors have been doing that for years.  I have never heard of it causing any kind of a problem.

4. 
   Henley | Oct 25, 2009 02:10am | #17

   Seriously, water content is an issue with perlite. Many fireplaces are used infrequently and/or not completely
   water tight. Nothing worse then trying to get a draft with the firebox and
   flues packed with damp perlite! Well maybe mortar :) A one inch air space will allow the the flue to heat more rapidly and thus draw more readily anyhow. Having taken down a few I can tell you that perlite is a sponge.

5. 
   BoJangles | Oct 25, 2009 02:30am | #18

   I can't imagine how it could ever get wet.  At least not the way we build fireplaces.

   It would have to come from a leak in the chimney cap or gaps in the flue liner.  If that's the case, theres a lot of other problems to solve.

   I would bet you that if you saw wet Perlite, it was not the right product for insulating masonry chimneys.  There are many kinds of Perlite.  In fact, they sell it for potting plants, which is probably the kind you saw if the chimney.

   Edited 10/24/2009 7:36 pm ET by BoJangles

6. 
   Jeff_Clarke | Oct 25, 2009 06:06am | #19

   The code not only doesn't prohibit insulation, it allows it so perlite should be OK.

2. 
   Piffin | Oct 25, 2009 10:13pm | #20

   "The 2" is the required clear distance to combustibles (from fireplace masonry)."That is to framing, not surface trim 

    

   Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

1. 
   Jeff_Clarke | Oct 26, 2009 02:58am | #21

   Yes ... I know ;o)

2. 
   Piffin | Oct 26, 2009 03:03am | #22

   hadda clarify 

    

   Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!