i know adhesive caulking isnt going to hold the weight of the letters
Why do you think this method will not hold the weight?
Concealed stud mounting is the commonest way to install letters, even heavy cast bronze. A 16-inch tall reverse channel is not overly heavy that a 1/4-inch aluminum stud buried in silicone cannot hold it. Obviously, a reverse channel letter that stands off the wall two inches will require studs longer than the typical three inches. I have used studs as long as six inches when needed. Studs are also available in stainless steel which are considerably stronger than aluminum.
It's possible that there may be code requirements in some areas that require something more elaborate, but I have never run into this.
I once installed some very heavy porcelain-on-steel letters that came with specialized hardware that had nuts and expansion elements. I couldn't have pulled the installed letters out if I wanted to. But that was unusual and I have never seen that hardware since.
Keep in mind you are mainly concerned about shear strength. That is, you are dealing mostly with simple gravity pulling straight down. As long as the stud does not bend from the letter's weight, the likelihood of the stud pulling out of the hole is remote. Of course, the stud must have sufficient penetration. And the silicone must fill the hole completely. So never rely on squirting the adhesive into the hole, as often the stud, upon insertion, will simply push the wad further back into the hole without being firmly gripped by it.
Always coat the stud itself as liberally as necessary by inserting it into the end of the tube of adhesive and triggering the gun as you withdraw it.
I might do differently on very large reverse channel letters.
On very large reverse channel letters, such as three-foot tall letters, for example, I expect I would remove the channel faces and install the polycarbonate backer first using whatever attachment I wanted, perhaps an expansion bolt of some kind or a rust-resistant masonry screw, and then re-install the faces last. But I've never actually installed reverse channels of this size, so I would definitely ask someone experienced to help me do the job.
Brad in Kansas City
.
