Screws into face of plywood

[jjl317]

I have an application (light exterior stone cladding) that the design team had decided should be attached to a plywood substrate. Does anyone know of a good product that can be used to screw stainless clips to plywood (preferably with an ESR report, or at least good technical data)? I was hoping of something like the Simpson HD Connector Screws, but Simpson doesn't seem to have data when attaching the plywood. In regards to other technical information, the only other relevant data I have found is the APA "Fasteners Loads for Plywood - Screws" - June 2011. Maybe there is another reference out there that I am unaware of?

Just not finding what I was expecting searching online. Any thoughts/comments would be appreciated.

 

[JStructsteel]

I would look at GRK info. They have a few ESR reports.

 

[jayrod12]

Generally speaking, I wouldn't. I'd be ensuring that the tie-back clips are installed at the stud locations so your screws penetrate into something substantial. Think of how the screws are shaped that would be small enough to have good bite in the plywood. 1/2" long screws would have almost negligible threads actually engaged in the plywood once you've discounted the tip length. Also, the diameters of screws of that length are generally quite narrow, not always but often.

And having long enough screws that the reduced diameter tip portion penetrates fully through the plywood, risks having the non-threaded shank portion engaged in the plywood, again reducing capacity.

Lastly, since there's only 1/2" of material (I'm assuming since it's wall plywood) stripping the threads is extremely likely. Especially so when the fastener is going through a steel plate first that won't allow the screw to continue to penetrate. A fraction of a second too long on the trigger of the screw gun and you've fully stripped the hole negating any tensile capacity.

 

[CANPRO]

to add to jayrod12's concerns, I'm assuming your wallboard will be OSB and not plywood. I've pulled handfuls of OSB out of walls that have had the envelope fail. Doesnt take much water to turn OSB to mush. I wouldn't be relying on any connection to OSB to retrain the facade.

 

[jjl317]

For what it's worth, it's 3/4" plywood (not OSB), and we are based in the US (neglected to mention previously).

And generally, we do try and transmit the loads directly to the studs, by adding a horizontal light gage track on top of the sheathing, to provide a continuous anchorage location. Unfortunately, the stones typically have specific anchorage locations relative to the exposed joints/face, and trying to coordinate anchor locations with stud installers (different trade, on site much earlier) is logistically impractical/borderline impossible. And for this particular project, the cavity space is very small, and adding a horizontal member would be a challenge (though maybe not impossible).

I did check out the GRK fasteners, and the reports only appear to reference use in wood, not plywood.

I agree that the thread of these fasteners would likely need to be full length, without a non-threaded shank.

In regards to the installation issues, I agree that care must be taken, but my thinking would be that these same issues occur when screwing into wood also.

 

[Brad805]

I am not a fan of this, but it is a common problem for those working in the SIP world. One example I found SIP Screw

 

[XR250]

Perhaps use 1" plywood

 

[jayrod12]

How often are your anchors? What sort of demand are we talking. Where I am, the design load on each masonry tie-back is in the range of 75lbs. I wouldn't stand underneath a fero-tie into 3/4" plywood with 2 screws holding a 75lb weight. Would you?

The logistics of getting the studs aligned properly are almost easier than controlling the installation torque. Just because the guys running the screw guns, i.e. the masons, don't give a shit about that stuff.

 

[jjl317]

Attached are pdfs of two elevation views for this project. Current thought (not necessarily shown correctly in these elevations, but close) is 1/8" thick x 3" long stainless bent plate, with (2) 1/4" screws @ 1 1/2" c/c into the plywood, loads per screw are 20 lbs shear, and 25 lbs tension (ASD wind and dead). Shorter / taller stones are different materials and thicknesses. Hopefully this illustrates the challenges with trying to hit the studs (in this case, light gage).

IMO, loads are light, just trying to find the best fastener option I can.

Thanks to all for the comments


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[NorthCivil]

what happens when that plywood rots? all that moisture sitting in the grooves between stones, keeping that plywood moist. wet dry cycles reoccuring. how long until that plywood goes soft?

 

[GC_Hopi]

Can you do an adhere stone veneer? if it is exterior you need a weather barrier, drainage mat, etc. Check with the manufacturer of the stone. They likely have an installation guide and preferred/recommended method.

 

[jjl317]

Marine plywood behind a air/water barrier is used to help protect the plywood, plus there is an airspace from back of stone to face of plywood.

And adhered stone could be an option, but it because of the height (above 30'), it exceeds IBC adhered stone criteria, plus the project is in CA, and adhered stone plus seismic gives me some heart-burn.

Thanks for the responses.