Expert: Bart Ogden - 11/6/2007

Question
QUESTION: I have a Regency Z2510 fireplace. With it I purchased a masonry retainer kit and installed cultured Stream Rock (smaller than River Rock) all the way to the ceiling (22 ft). After several fires in the fireplace, we noticed cracks forming around the rocks directly above the fireplace door. The design of this fireplace is such that the heat is directed to the front and up, and the rocks between the door opening and the top louvers get quite hot. Eventually, one of the rocks in this area fell off, and I was able to remove 3 others. The rocks in other areas of the face don't exhibit the same cracking, but also don't see the high temperature cycles that this area does.

I used a modified mortar for the base coat, but I found the modified mortar dried too quickly, and so I used regular N-type mortar to set the stones. It appears that the bonding between the N-type mortar and the porous surface of the manufactured stone failed (presumably because of temperature cycling), with small pieces of stone being left behind with the mortar.

SO what did I do wrong, and how do I fix it?



ANSWER: Hi John,

Cement Board should have been installed over the face of the unit followed by stucco lathe, followed by a scratch coat of mortar, then setting the stones in a bed of mortar.

The only way to fix it that I would know of is to remove what you've got and re-do the stone in the manner I've described.

Thanks for using AllExperts.Com and good luck with your project.

Sincerely,
A. Bart Ogden, President
Home Safe Hearth & Chimney, Inc.
Wichita, KS
Voice: 316-265-9828
Fax: 316-265-9890
www.homesafeks.com


---------- FOLLOW-UP ----------

QUESTION: The use of cement backer board is not possible in this case. The masonry retainer kit consists of expanded metal welded to 3/4" angle iron which is then fastened directly to the steel exterior of the fireplace.

But I believe I have found the reason behind this phenomena. Natural stone has a linear thermal expansion coefficient of 4.5 - 6.5 x 10-1 x F-1. Dense aggregate concrete masonry has a thermal coefficient of about 5.2. This makes rock and mortar fairly compatible over normal temperature excursions. I could not find thermal expansion specs on the particular stone that I used, but literature suggests that light weight cultured stone has a thermal expansion coefficient about 4 times that of natural stone. Over normal indoor or outdoor temperature excursions, this difference is not enough to cause problems, but the relatively high temperature excursions experienced in this one area (too hot to hold your hand on), is breaking down the bond between the stone and the mortar.

I have looked at 2 possible solutions so far; both involving adhesives. One is to use an epoxy, and the other is to use high temperature silicone. Both are capable of withstanding the temperatures, but I am leaning towards the silicone because it will remain elastomeric. Both the mortar and the epoxy are stronger than the cultured rock, and I am concerned that the hardened epoxy would simply break away the back of the stone. What are your thoughts?

J.A. Coutts

Answer
Hi John,

I recommend you contact Regency (Fireplace Products Inc.) or your local Regency dealer where you purchased the fireplace.  They would be able to tell you exactly what is approved for use on the front of that unit.

Many times, silicones and adhesives will off-gas in these situations causing more problems.

Thanks for using AllExperts.Com and good luck with your project.

Sincerely,
A. Bart Ogden, President
Home Safe Hearth & Chimney, Inc.
Wichita, KS
Voice: 316-265-9828
Fax: 316-265-9890
www.homesafeks.com