Outdoor new masonry fireplace help needed!

[HeatherL]

I am having a mason build an outdoor wood burning fireplace for us. He has 18 yrs masonry experience and I have seen slot of his work. I am concerned he doesn't really know the proper sizing for our fireplace. The actual opening is 6' (feet) wide 24" deep and 32" high. How high should the chimney be? The fireplace is rectangular, how should the interior of the chimney be sized? I know it needs to be tapered, but to what extent and how narrow /wide should chimney opening be? Should we do 2 openings up top for such a wide fireplace? What materials should he use to accomplish this chimney taper from such a large opening, an insert, anti fracture concrete?

I know these dimensions are critical to insure we do not have the issues many have written about. Any help is much appreciated!

 

[cmonSTART]

Hi Heather. Is this going to be attached to the house or separate?

1. Please check with your local building code officials. Codes for outdoor fireplaces aren't really spelled out clearly anywhere and in many places and often no one cares what you do if it's not attached to the house. For some good general guidance Chapter 13 of the International Residential Code (IRC2006 or IRC2009) will have a lot of good, useful information about sizing, dimensions, and materials to use. Note: These apply to a normal "in house" fireplace structure, but should give you guidance.

2. Sizing: This applies to normal "in house" fireplaces, so apply it to your purpose as you see fit. I cannot recall a specific sizing requirement for height. However, there are clearly spelled out ratios for flue cross sectional area vs. firebox opening size. Basically for an average shaped fireplace opening, the flue cross section should be at least 1/10 for normal square/rectangle flue tile, or 1/12 for a round flue. So your 24x36 fireplace opening you will need a square/rectangle flue of at least 86.4 square inches. This is based on an "average" height flue (14 or 15 foot minimum height maybe?) Usually a 13x13 OD flue tile will meet most requirements.

Now, outdoor fireplaces generally have shorter chimneys which generally don't draft as well (smoke out the firebox). You can cheat this and make them perform better by doubling the size of the flue.

3. Also a good, smooth smoke chamber will be important to help the thing not smoke out the door. Some good dimensions and information are spelled out in IRC, but generally a nice, even, and smooth transition to the flue is important. Make sure any corbeled brick (stepped) is parged smooth with mortar.

I hope this helps a bit. Good luck with your project! And remember, check with your local building officials because in most areas no one cares what you do with an outdoor fireplace.

 

[HeatherL]

Thanks for info. The fireplace is NOT attached to the house. I wiil check codes but believe they do not apply. Where would I find the irc? Info you mention? So the 6 foot wide opening does not require any special consideration? The mason is planning on about a 8 foot total chimney height so you suggest doubling the suggested dimensions in that article you referenced? Two openings is a bad idea?

Thanks again! I feel like I'm running blind here.

 

[begreen]

The large opening will need to be designed for. Sizing and the proper ratio to the flue are important. The big opening may need a larger throat to the flue. If your mason is a good professional, he should know this. Have you checked his references for other work of this type that he has done?

For more information I would do a google search on outdoor fireplace design for some additional tips and caveats. Here are a few to get you started:

http://www.concretenetwork.com/outdoor-fireplaces/
http://www.outdoorfireplaceplan.com/category/designs-and-plans
http://www.standout-fireplace-designs.com/stone_outdoor_fireplaces.html