We have been asked to design the attachment of pressure-treated wood window bucks to 18 gauge cold-formed metal framing (load will be tension/pullout). Most all screws we can find either don't have load data for 18 gauge studs, are not long enough to get thru the 2x pressure-treated wood, or are not treated for use within pressure-treated wood.
The contractor currently wishes to use the S-MD 12-24 7/8 HWH#4 collated screw. From page 218 of this excerpt, a #12 screw has a pullout capacity in 18 gauge steel of 495 lbs. However, page 221 notes a "minimum drilling capacity" of 0.175".
Does this mean the S-MD 12-24 screw cannot be used in 18 gauge framing, despite having a listed load capacity? Or, does it mean that something like pre-drilling may be necessary to use it?
I would also appreciate any suggestions for a screw or fastener that:
- is treated for use in pressure-treated lumber
- long enough to get thru 1.5" of lumber + 18 gauge stud
- has rated pullout load capacities for 18 gauge metal framing
It seems like this screw does not exist, but should be a recurring condition at CFMF projects with window bucks.
Hi Conor,
When attaching pressure-treated wood to steel, the recommended fastener is our Kwik-Flex screw for low and medium corrosive environments and our Bi-Metal Kwik-Flex screw for high corrosive environments. This information can be found in Section 3.6.1.6 of Hilt’s Direct Fastening Product Technical Guide. The minimum drilling capacity of a screw is based upon the thickness of the base material plus the thickness of the top material combined. Therefore, a base steel thickness of 18 gauge can be used if the top material thickness is such that the total thickness of the two materials meets the required minimum drilling capacity. This can be seen in the attached screenshot from page 213.
Regarding the 1-1/2” of pressure-treated wood, two options can be considered: the use of a winged reamer screw or pre drill the wood to not damage the threads of the self-drilling screw. When considering either of these methods, it is important to remember the maximum total thickness that is listed for the screw is maintained.
Regards,
Ashley
