Seasonality versus fastener strength

Well here we are again ... mid summer July accompanied by mid summer heat that is creating 30+ degree temperature swings overnight. This is when a year's worth of seasons catches up with our outdoor structures that were put together up to a year ago. The pressure treated sub framing has contracted, expanded, shrunk and twisted to it's final "resting position", with the decking surface forced to withstand up to 2100-2300 lbs of shear force while doing so. If under deck fastening devices were used, you can only hope that the screws used to secure them still remain intact and holding under this much increased stress common to this natural movement in the deck's first year of life.

I was called up to British Columbia earlier this month to assist in resolving this very matter of fastener failure, where the deck fixture manufacturer specified #8 diameter screws for this underside fastening requirement (as do most of them). With summer heat arriving, all composite type decking moves laterally "both directions" everyday, and the fasteners need to be able to withstand these ongoing loads resulting in substantial shears. This is not a surprise that any new deck owner needs to deal with long after the contractor has finished your project, and the lumber dealer where the materials came from simply refers to the "installation instructions" for the answers to your dilemma. It's a no-win situation for everyone involved. In the cases in B.C., yellow zinc coated fasteners
are not compatible with contact in type ACQ treated lumber either, so they would have failed structurally and corrosively within a two year period at the most as well.

We see this situation somewhere every year, and it can be avoided if contractors and/or users are properly educated and knowledgeable on proper fastener selection for the particular application, and the "compatibility" of the materials being installed "together". "Knowing your materials" is a priority, and selecting the right fasteners and proper technique of installation will save everyone involved big headaches down the road ... both physically and financially.