Cantilever screws at 1/2: stucco Gap

[chorner26]

Does anybody have a good design example of bending of a screw at a stucco gap.
I've always calculated fb= v/a+ M/S
for a reduced circular section, which very rarely calculates for anything less than a 3/8" anchor.
Commonly contractors want 1/4-20 screws, any advice on how to make it work?

Thanks

 

[jdgengineer]

What's the application? Simpson Strong-Tie does make Drywall hangers that are intended to be attached to outside of (2) layers of 5/8" sheetrock. They are installed with SDS screws. Simpson may have some allowable values they can provide for this instance.

As an aside, from a waterproofing perspective it's generally a bad idea to screw through the stucco. The stucco really should be removed, a ledger hard mounted to the framing and proper flashing installed. Screwing through the stucco is a recipe for a leak in my opinion.

If this is for a deck installation, I personally would not allow the stucco to be between the ledger and the framing. I would require removal.

 

[Ron]

Agree with jdgengineer.....stucco is not a face barrier system for waterproofing. If you screw through it, you will breach the water-resistive barrier system and promote deterioration of the substrate. Further agree that structural applications should not be through stucco.

If you continue as you are apparently thinking, you will increase your liability tremendously!

 

[chorner26]

The application is an underside of a soffit condition for a window storefront(Wind Loads only no sustained loads). The architectural details show this all the time which I usually have them change, however many times the storefront is already installed and I'm hired to go back and make it work. So my question is specific about bending of a screw.

 

[Ron]

Ok...if you screw through the stucco, you cannot count on the stucco to resist bending in the screw. It will crush under the screw. Given that, you can compute the moment and allowable bending in the screw. As a common example, 1/4" dia. TapCons are used routinely to fasten windows. This allows up to a 1/4" gap between the window and the opening. If it gets larger than that, the bending in the screws takes over and the screws have to be increased in size.

1/4-20 screws are usually not very high strength....certainly not in the range of TapCons. If you have stucco that is at the thickness required by code, I doubt you can get them to work at all.