Ive recently run into an issue with installation of frameless mirrors in toilet rooms using mastic; in the past, Im quite sure Ive seen installations specified and detailed where the mastic was the sole means of support (no mechanical attachment), however, recently we have been advised by some subcontractors that they cannot install mirrors with mastic unless also provided with permanent mechanical attachment. A quick check of the installation instructions for several of our specified products (CR Lawrence, Gunther, Palmer) does indeed support this claim. While I dont doubt the wisdom of this, I cant recall it being an issue before (or maybe Im just really old?) Im curious of there may be some recent event (legal action?) or code interpretation (a strict reading of IBC 2403.2 Glass Supports?) that has led to various manufacturers to take a unified stance on this and if there is a consensus on just what adequate mechanical support constitutes. Are there any readers who have insight to offer on this?
When I was an active specifier until a few years ago, I never specified mastic-only installations on any project. I just did not trust it.
Sorry, don’t know the answer, but thinking out loud, could it be related to an OSHA requirement? Mirrors mounted with mastic can generally only be removed in pieces, sharp, nasty edged pieces.
Long ago, I did specify mirror installation with mastic only - at that time Palmer was the only show in town.
Since then, I typically specified mirrors with an aluminum channel at the bottom and mastic for the rest of the installation.
Never had call-back on either.
My understanding was that VOC restrictions reduced the holding capacity of compliant mastics.
Belts and suspenders.
My understanding was that VOC restrictions reduced the holding capacity of compliant mastics.
Belts and suspenders.
Working within a California Healthcare/Hospital project environment, mechanical support by J-mold or other means has always been required by the AHJ. With a support like that on the bottom edge taking care of gravity loads, we have sometimes been allowed to rely on mastic for the lateral loads, but usually use the slip track style J-mold trims at the top edge. We still include the mastic so that the assembly will not rattle or vibrate.
Thanks to all responding above. The low-VOC angle is one I had not thought of. I agree with those indicating a continuous J-trim at the bottom for gravity support, and always call for that on my own projects. My problem more recently is trying to convince designers (who are most concerned with visual effect) to “do the right thing”! Having a specific standard or code requirement helps in this regard.
MasterSpec states that “The National Glass Association recommends that where mastic is used to position mirrors initially on a surface, mechanical devices should also be used.” They also note that GANA recommends use of mechanical devices in a addition to mastics for all but the smallest mirrors.
“trying to convince designers (who are most concerned with visual effect)” Yes, that quarter inch wide chrome or stainless steel strip at the bottom of the mirror is going to stick out like a sore thumb (NOT!) I’ve had similar issues with designers who insist on a 20-foot tall equipment screen, set well back from the parapet on a 20-story building, so folks on the ground will not see the ugly HVAC equipment. Ugh! Being a life-long Houston resident I think HVAC equipment is beautiful. 
James, my wife insists that air conditioning is the single greatest invention in the history of humanity. She can’t sleep without it.
A plumbing engineer I used to work with used to claim that we design buildings for the purpose of housing pumps and pipes. Everything else was secondary.
Ken, in my part of the world, and many others, air conditioning has become as vital as air itself. Houston and many other cities in the south would not be what they are today without air conditioning.
Someone posted a picture the other day of downtown Houston circa 1927. It was obvious from the shadows people were casting that it was around noon on a sunny day in the spring or summer. All the men in the picture (and it was almost only men) were wearing coats and ties! That was pre-AC! How did they survive?
Yes, don’t hide systems, structure, and hardware like they are an embarrassment. Celebrate them.
Bless you Willis Carrier and Dave Lenox.
Depending on time of year, I can’t sleep without air conditioning.
As one of the “token architects” in a large engineering firm, I often DO “design buildings for the purpose of housing pumps and pipes.”
Vis-a-vis mirrors and mastic: A mastic-applied mirror is a Class I vapor retarder (vapor barrier), so be careful where you put them. Generally not an issue for your basic bathroom mirror, but can cause trouble with large mirrors (e.g. whole walls mirrored in a dance studio).
Dan, I wouldn’t think that mirror being being a vapor barrier would be a problem. Mastic is applied in beads several inches apart, to allow air to circulate behind the mirror. You’d need full mastic coverage for the vapor barrier properties to function, and that’s not how mirror mastic is applied.
Thanks Dave,
You beat me to the punch.
Discrete supports are also required to overcome gravity while the mastic hardens.
I assumed that Dan was talking about the vapor-retarding properties of the mirror itself (i.e. glass), rather than the mastic installation causing the vapor-retarding qualities. It seems a fully mirrored wall would/could act as a vapor retarder in either mastic or j-channel installation methods.
With that in mind, I figured Dan was concerned that if there was large amounts of humidity from the exterior or room on the back side of the mirror, that it might cause issues with a water-based mastic. The airspace behind a smaller mirror, regardless of installation method, would allow that moisture to ventilate into the room, but maybe not so much for an entire wall surface of mirrors.
That’s me reading between lines. Dan may have to clarify for himself what he thought was at issue.
Good point, Rosa, I hadnt thought of that. But whether full-wall mirrors are on an exterior wall, or an interior wall with a high-humidity room on the other side, how is that any different from a glass curtain wall? Locate diffusers above the mirrors to keep the wall surface warm.
And in thinking about it further, in the conditions mentioned, install full-height mirrors with a gap between adjoining mirrors.