Tibbaryllis2 t1_ja8l68e wrote on February 27, 2023 at 5:26 PM

Reply to comment by Highlander2748 in Do I need to replace my deck joists? by RobotKevinSpacey

Thanks for the reply. The dock decking is mostly covered by a tin roof, but the dock perimeter and ~40 feet of catwalk from the shore is not.

The house was built in the 50s and this will be only the third time we replace the decking, but I’m not shy about incorporating new best practices.

The way the decking is built, the joists (?) underneath are laid horizontally (2x6s, 6 side up) for attaching the deck to, so there is quite a bit of lateral surface to collect water. So we do have some issues with it rotting out where we screw in the decking. While the exposed decking suffers the most from 24/7 full sun (house on the north side of the lake so the decking is south facing).

Edit: I think we’re also victims of the pressure treatment regulations change as we replaced the decking only ~15 years ago and it’s already pretty soft in some places compared to the original decking in a pile on the property still having some pretty sound pieces still.

Highlander2748 t1_ja9bp9q wrote on February 27, 2023 at 8:13 PM

Your description of the framing off the dock is concerning because I can’t think of a reason where joists would work as intended when installed as you describe. Their strength comes from being installed with the narrow edge used to support the decking/floor. You’re correct about updated pressure treated wood regulations. The ACQ lumber does not seem to be as hardy as the older CCA lumber. You may want to see if any dock builders supply a different grade of PT lumber? You could also use Mahogany/Ipe/Teak or any number of more exotic lumbers that stand the test of time better. I also suspect the decking failure may be connected to the joist positioning as that is a lot of surface area to have moisture trapped and it’s just as capable of damaging the underside of the decking as it is the top face of the joists. In your instance, the application of the joist tape may actually trap more moisture and hold it on the underside of the boards.

Tibbaryllis2 t1_ja9q0sx wrote on February 27, 2023 at 9:43 PM

Oh sorry for being misleading. It’s a welded steel frame with 2x6s laid over them to give area to screw to when laying the decking. All the decking ends overlap steel crossbars and tie into the underlying 2x6. (Edit- I assume it’s a standard way of building them because all of the neighbor docks are the same).

So it’s structurally sound, but the boards underneath are the first to rot (due to the lateral surface area available to hold moisture). Followed by the decking board ends where they’re in contact with the underlying wood.

Re: your last statement, that’s what I was curious about too.