Chimney Joint Reinforcing
(Conversation with John Bufford and John Swink on Rumford facebook group)

John Bufford



You commented on the Rumford facebock group page a while back about joint reinforcement...

    John Bufford You can always use #3 rebar for ties.. deformed bars.. as long as they are not in the mortar joints and are in a grouted section.. max size bar in mortar joints should actually be 3/16", which is 1/2 the thickness of a 3/8" mortar joint.. Jim, you might actually want to look into changing the 1/4" rebar tie requirement in the code to be consistant with MSJC code on max size of wire in a mortar joint..
Of course it's too late for me to get a code change for the IRC in this year but I wonder if staff could editorially change it to make it consistent throughout the code. Or maybe clarify it in the Commentary.

My question had been about lap splicing and I couldn't find that in chapter 10 or the referenced Section R607 which deals with wall ties, presumably in single wythe veneer walls - not seismic reinforcing.  Here is the current code.

    R1001.3.2 Horizontal reinforcing. Vertical reinforcement shall be placed within 1/4-inch (6 mm) ties, or other reinforcing of equivalent net cross-sectional area, placed in the bed joints according to Section R607 at a minimum of every 18 inches (457 mm) of vertical height. Two such ties shall be installed at each bend in the vertical bars.
I don't even know what "placed within 1/4-inch (6 mm) ties" means. Must have something to do with wall ties which are not generally used in chimney construction. I think the UBC used to specify 1/4 pencil rod in the joints. We added "other reinforcing of equivalent net cross-sectional area" to allow K and ladder web. And there is no mention of lap splicing or overlap.

The vertical reinforcing section refers to Section R609 where ...

    R609.3.1 Construction. The thickness of grout or mortar between masonry units and reinforcement shall not be less than 1/4 inch (7 mm), except that 1/4-inch (7 mm) bars may be laid in horizontal mortar joints at least 1/2 inch (13 mm) thick, and steel wire reinforcement may be laid in horizontal mortar joints at least twice the thickness of the wire diameter.
That sounds more like the what R1001.3.2 should have referenced instead to R607 and supports your point unless the mortar joints are 1/2". Still no mention of lap joints.

Section R606 a big confusing section on general masonry, also has some language on masonry wall reinforcement - all confusing to me and not constant with the chimney section. And no mention of bar splicing or overlaps.

R611.5.4.3 Lap splices is the clearest I could find on lap splices but it's in the concrete wall section - not unit masonry. There it requires laps of 20 to 30 bar diameters or about 10" to 15" for 1/2" rebar and 5" to 7.5" for 1/4" rod. I didn't take the time to understand what is meant by "tension development length".

R703.7.3.2 on unit masonry lintels requires lap splices of joint reinforcement of a minimum of 12 inches when using double-wire 3/16" joint reinforcement.

Okay, in summary, I conclude that both the vertical rebar and horizontal reinforcement in a chimney should have lap splices of at least 12". And horizontal reinforcement should be required to be 3/16" in diameter or one half the mortar joint dimension as you suggest.

You made the comment that larger bars could be used "as long as they are not in the mortar joints and are in a grouted section" but since the code was changed to require an air space or insulation around the flue, the space between the flue and the enclosing masonry is not grouted solid as it used to be in California. Virtually all horizontal reinforcement in unit masonry chimneys is in a joint.

I think that these requirements should be in Chapter 10 and in Section R609 on Grouted Masonry. I also don't think I can do more than call attention to the problem since it involves so many other seemingly contradictory sections of the IRC and other codes.

One final worry. At several discussions at WSCPA it was noted that joint reinforcement tends to corrode in a surprisingly short time, as I remember about 20 years, especially if it's small diameter reinforcement and near the surface. Do you think 3/16" joint reinforcement is a good idea in a chimney that we hope to last 100 years?

Jim Buckley

John Swink


to Jim, John, Chip, Paul, Eric, Jason, David


  Thank you for the context. My experience and reading on joint reinforcing, including ladder wire and 3/16 wire is mostly negative. Let me cite a three examples:

  1. When we use it for crack control in CM veneers, it seldom does that.  Invariably we find that cracks occur anyway. They just occur at the splices, which are almost never staggered, as they are with rebar in concrete.

2. In the UCSD shaking table tests of brick veneer with and without joint reinforcing and seismic brick ties, the veneer with joint reinforcing performed worse than the wall without it.

3. I tested a number of brick fence panels with joint reinforcing used to resist wind loads at our Denton lab.  Most of the panels failed unpredictably from bond failures at the bottom of the bed joints. Wire was laid before the mortar, as it often is, instead of being tapped into the bedding mortar.

  Conventional wisdom would suggest that the 6" laps specified for joint reinforcing would work for chimneys. I am not sure that would work.  But if 6" laps work for 9 ga, we should at least maintain the 40d ratio with 8” and 11” laps for 3/16 and ¼” wire respectively.

  Item 1, however, suggests that the 6” lap lengths for joint reinforcing are not adequate for the 9 ga ladder wire used. Staggering joints would be nice, but difficult to achieve and inspect.

Items 2 and 3 may have similar causes. If you lay the wire and string the mortar on top of it, a gap is formed under the wire, which can propagate into a crack, separating the mortar from the brick at the bottom of the bed joint.

  Finally, the whole idea of horizontal reinforcing in the brick appears to derive from attempts to create ties to confine the vertical reinforcing in grouted chimney cavities. Vertical reinforcing would clearly be needed for some taller chimneys. But if there is no vertical reinforcing, there is no need for the ties. I think the best approach for possible future code revisions would be to set conservative aspect ratios for chimneys, beyond which they would require seismic structural design. But otherwise eliminate the special horizontal joint reinforcing in chimneys, and simply follow chapter 6 for "other than running bond."

John Swink

Training Index
<<< BACK - Codes - NEXT - Performance>>>

Buckley Rumford Fireplaces
Copyright 1995 - 2013 Jim Buckley
All rights reserved.