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joelbuckley 11-07-2008 05:10 PM

Where a wall meets a chimney
 
I'm planning to partition an upstairs room that has a chimney in one of the walls. There's no fireplace in the room, but the chimney runs behind one of the existing walls. On that wall, the brick is plastered over smoothly, i.e. there's no framing over the brick. The chimney has three flues: one for a fireplace downstairs (which we do not use), one for the gas furnace, and one we aren't sure about. The house is 75 years old.

http://www.ohrob.com/roomsketch.gif

The new wall (conventional 2x4 wood framing) will meet the chimney at a 90-degree angle, about 8" from one end of the chimney. What's the best way to handle the junction? My inclination is to anchor the last stud in the wall to the chimney with a few tapcons, but I'm afraid that the fire codes would prohibit this. I don't want to frame out the entire chimney for various reasons, not the least of which is that space is at a premium already. Opinions?

bjbatlanta 11-08-2008 03:18 PM

As long as the tapcons don't penetrate the brick entirely, it shouldn't be a problem. You can also glue the stud in place.......your top and bottom plates will hold the stud while the glue sets.

Maintenance 6 11-10-2008 09:15 AM

I thought code required a 2 inch clearance between a chimney and framing members.

bjbatlanta 11-10-2008 09:32 AM

I think that may be for a fireplace insert?? You may be right though, but a metal stud could be fastened to it I would think........ I wouldn't use the glue though if combustion is a code issue in this case, just a tapcon or two. You could "float" the drywall the last two inches, but you may have cracking problems if it gets bumped. Unlikely in a corner though.

joelbuckley 11-10-2008 12:45 PM

My concern is the requirement for 2" separation between a masonry chimney and any combustible materials. Of course, none of the existing framing has this much clearance (1" at best) and there's a wood floor and wood base molding installed right up against the plaster on the chimney, but I still think it's best to do new work to current codes as much as possible.

A metal stud is a good idea. I can bring the wood framing for the new wall to within 2" of the chimney, and then bridge the gap with a metal stud and spacers made from 1/2" cement board.

bjbatlanta 11-10-2008 02:04 PM

I agree, but I'd check and see if that's, in fact, code. If I understand correctly, there's no open fire box, just the chimney chase? As long as there's no penetration into the actual flue, there's no danger of fire anyway. Or maybe I'm not seeing the whole picture.....

joelbuckley 11-11-2008 10:26 AM

It looks like there are some significant ambiguities in the codes with respect to chimneys, especially as they relate to historic homes. I found an article at rumford.com that talks about changes made to the 2000 IRC that try to address these issues (see below). The code calls for "air space" clearance of 2 inches. I don't think any of the listed exceptions apply to me.

R1001.15 Chimney clearances. Any portion of a masonry chimney located in the interior of the building or within the exterior wall of the building shall have a minimum air space clearance to combustibles of 2 inches (51 mm). Chimneys located entirely outside the exterior walls of the building, including chimneys that pass through the soffit or cornice, shall have a minimum air space clearance of 1 inch (25 mm). The air space shall not be filled, except to provide fire blocking in accordance with Section R1001.16.

Exceptions:

1. Masonry chimneys equipped with a chimney lining system listed and labeled for use in chimneys in contact with combustibles in accordance with UL 1777, and installed in accordance with the manufacturer's installation instructions, are permitted to have combustible material in contact with their exterior surfaces.

2. When masonry chimneys are constructed as part of masonry or concrete walls, combustible materials shall not be in contact with the masonry or concrete wall less than 12 inches (306 mm) from the inside surface of the nearest flue lining.

3. Exposed combustible trim and the edges of sheathing materials, such as wood siding, shall be permitted to abut the masonry chimney side walls, in accordance with FIGURE R1001.15, provided such combustible trim or sheathing is a minimum of 12 inches (306 mm) from the inside surface of the nearest flue lining. Combustible material and trim shall not overlap the corners of the chimney by more than 1 inch (25 mm).

http://www.rumford.com/code/IRCch.GIF

joelbuckley 11-13-2008 10:00 AM

Well, here's my solution using steel framing around the chimney. The studs don't actually touch the chimney, so there won't be any conductive heat transfer. For maximum strength, I might use metal lath and durabond (instead of paper tape and joint compound) on the inside corner where the partition meets the chimney. That corner will be prone to cracking.

http://www.ohrob.com/chimney-framing.gif

bjbatlanta 11-13-2008 12:28 PM

Don't think the wire lath is really necessary if you're only "floating" a couple of inches. You might want to tape with durabond. It will give some added strength.

joelbuckley 11-13-2008 01:15 PM

Thanks bjb. I'm only worried about cracks because the chimney is independent from (and won't move with) the rest of the structure, but then again, it's probably pretty stable after 75 years.

Durabond is great stuff!

bjbatlanta 11-13-2008 02:52 PM

New, old, or both you'll get cracks from time to time. In a corner as we're talking about, it will likely be a hairline crack in which case I'd caulk it. Caulk is a little more forgiving when it comes to movement. I don't think you'll have any serious cracks. And it would be hard to get a smooth finish with metal lath.....

joelbuckley 11-13-2008 03:04 PM

All excellent points. You talked me out of it (lath). Thanks again!

bjbatlanta 11-13-2008 03:19 PM

Glad to be of help.


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