Double Rim Joist vs. Extra Blocking?
Hi,
I’m new to this site and just recently got a subscription to the magazine as a result of the great info I have been reading. I’m hoping I can get some good feedback on my deck.
I have what I guess would be called a free standing, redwood deck on a slope. In short, the existing outer sides of the deck, which are perpendicular to what would be the ledger, currently have double rim joists. They have since rotten away and I’m looking to replace them. In case it matters, all the joists including the outer, double rim joists are cantilevered across a beam, which is supported by several cross-braced posts and footings (see photo).
However, I have been debating whether there’s a reason to go with double rim joists again vs. a single rim joist with several pieces of blocking instead. It’s my understanding that double rim joists are typically used for the load due to people standing near the edges to admire a view. My concern is whether using double rim joists, which seems like scissoring two pieces of wood together will just result in similar rot a few years from now.
My idea is to use a single joist with several pieces of blocking and Simpson ML angle hardware instead of double rim joists and a HL connector/machine bolt combo (which is what I have now) – see photo.
Does anyone have any strong argument one way or another for replacing my existing double rim joists with two new pieces of lumber vs. a single rim joist with extra blocking? Thank you in advance. I’ll be happy to answer any other questions that arise.
Replies
I want to guess that the reason there is a second joist is that the first is cracked.
The deck is at least 30 years old. I think the crack happened over time similar to the rot. Additionally, I believe this to be the initial design because the decking sits atop both double joists - it's not picture framed. And, it's double joisted on both sides.
https://www.awc.org/pdf/codes-standards/publications/dca/AWC-DCA62015-DeckGuide-1804.pdf
While this guide is prescriptive only for decks which are attached to a house, the details are worth reviewing. you might want to review your railing attachments as well, while you are upgrading the deck.
I'll definitely review this and see if it provides an answer whether to go with a double rim or not. And, Yes, the railing attachments are next on my list of upgrades. Thanks.
You need double rims to get a strong post attachment anyway. We put the inboard one on first, install the joist and face nail them through the rim joist, then install the second rim joist. Cover the top of the rim with https://www.homedepot.com/p/Grace-Vycor-Deck-Protector-4-in-x-75-ft-Roll-Fully-Adhered-Joist-Tape-25-sq-ft-5003029/302451944 to keep water out.
https://gcpat.com/en/solutions/products/vycor-weather-barrier-flashing-tapes/vycor-deck-protector
Not a fan of double rims. They trap water and make it harder to detail the railing posts. The only things the rim does on a cantilevered section is keep the joists vertical and provide attachment for railing posts. Check out Mike Guertin's stuff on this.https://www.finehomebuilding.com/videoseries/2019/05/shop-class-critical-deck-framing-connections