Is There A Minimum Airspace Behind A Veneer?
As the saying goes, “I am glad you asked that question.” The short answer is “Yes,” there is a designated minimum air space in the code.
Many years ago when I was still in school, I had to fight for good grades like everyone else. The problem in my math classes was not just getting the answers correct, but providing the necessary proof for the answers. The teachers did not seem to appreciate my knowledge (or maybe it was my explanation) that if I gave them my proof, I would have to shoot them. I could see their displeasure by the drop in my grade. They finally wore down my resistance, and I decided to see reality and provide the proof with my answers.
To find the entire answer, we go to Chapter 12 of TMS 402-13. The actual name is Building Code Requirements for Masonry Structures, but you will find I use “TMS 402-13” or the “Code” because it is shorter. Now some of you are asking, “Why is the Veneer Chapter now Chapter 12 in the 2013 Edition, but it is listed as Chapter 6 in the Edition you are using?” Because in this day of high tech, everything changes quickly and between the 2011 and the 2013 editions, the layout of the code changed. The 2016 Edition is available from the MCAA.
So, now back to Chapter 12. For veneer with wood backup, see Paragraph F22.214.171.124.3. Veneer with steel stud backup is Paragraph 126.96.36.199.4, and veneer on masonry is Paragraph 188.8.131.52.2. The minimum is one (1) inch for all backup systems. By using one inch there is very little, if any, mortar bridging. I was raised in the time when we just made sure our finger tips could fit between the back of the brick and the backup system. Wait, you want to know about bridging back then? Back then, mortar was spread in the proper amount and not ‘slung’ in all directions. You didn’t have to tell the young ones to spread the mortar right or chop it with a hoe!
One inch allows for the use of mortar collection device as well as air movement in the airspace if the top of the veneer is open behind a soffit/fascia has vents placed at the top. If the backup system is not straight or consistently flat, you will have to find the outer most point on each wall and use that for your base dimension to maintain the one-inch minimum. Some builders will want you to keep it small because their trim at openings doesn’t cover the one inch. Use the code to your advantage, and require them to add trim or move whatever is in the opening outward.
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You can come around the corner a little ways.