Correct Galv Deck to Heavy Timber Truss Fasteners???

[coatsandrew]

Does anyone have experience with connecting 20 gauge galvanized roof deck to heavy timber trusses? The major concern is excessive corrosion from the galvanizing/pressure treatment interaction. I've been told to spec a screw with a stalguard coating, but Elco does not have shear or pullout values for this condition. I can provide them with sample materials, and they will conduct the tests, but I thought I'd ask around here first. Thanks

 

[Ron]

I would be more concerned with the deck corroding than the fasteners. Be sure to isolate the deck from the timber, using either a membrane (No. 15 felt at the least). Corrosion resistant coatings on screws should not affect the mechanical properties of the screw (shear, tension) but can certainly affect the frictional characteristics of the screw/timber interface (pullout).

 

[dik]

I've done this using EPDM strips and Hilti PAF fasteners for shear and normal screws where not needed.

Dik

 

[MiketheEngineer]

Simpson has "double" galvanized and even stainless fasteners that might help you out. Strongtie.com

 

[TVP]

I think SFS intec has off-the-shelf technology for this type of application. Here are some links for more information:

http://www.sfsintec.biz/internet/sfsinten.nsf/PageID/Fastening_Steel_FR

http://www.sfsintecusa.com/aia/index.html

 

[CoryPad]

Ron,

Could you discuss your first sentence more? I would think the opposite - the deck has a thick (at least 40 [µ]m) zinc layer, while the fasteners have a thin layer (usually less than 20 [µ]m). The deck is in partial planar contact with the timber, while the screw shanks are completely surrounded by the timber into which they were drilled.

Thanks.

 

[Ron]

CoryPad,
Condensation is usually the culprit. The deck is relatively thin, although it has a better coating than the fastener. I have typically seen deck corrosion at the support interface in these applications before the fasteners tend to fail.

If the deck is isolated (a membrane or mastic coating) this condition does not present an issue. Then, the fastener protection prevails.

Ron

 

[CoryPad]

Ron,

Thanks for the description. The originator mentioned the pressure treating chemicals, so I was limiting myself to corrosion induced by it. I understand your points regarding isolation and condensation.

 

[dik]

have to be careful... with some pressure treating processes, only stainless has any longevity.

Dik