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I have a cantilevered deck that was needing new decking boards. As it turns out it also has some dry rot on the joists that are cantilevered from the house (14ft high, extending 6ft from the house - a maximum of 2" X 12" joist according to OR building code bottom of page 7). I am hoping most of the damage is surface, but in order to to repair the joists w/o full replacement (as recommended here), I would follow the below laid out plan. In addition to the joists, the header joists also has small dry rot area. That would be a straight replacement of the said header joist. I would appreciate any feedback on the process. My brief research shows that best practice is to repair the dry rot, and sister the joists, hence the following plan.

  1. Remove dry rot
  2. Clean and re-fill the dry rot area with appropriate wood filler / epoxy (found it on This Old House)
  3. Add sister joists. [Should new sister joist be attached to house face with 4" X 12" joist hangers around existing cantilevered joist and the new sister joist?]
  4. Replace header joist [Shall I join this header to sister or the original joists?]
  5. Cover all joists/header with water barrier (joist tape or such)
  6. Install railing blocking
  7. Install new decking and railing

I have seen this advice:for sister joining, which has the OP wanting to repair some joists, while others advised replacement. In my case replacing the joists would probably mean much more work than having the small deck is worth, I think that joist sistering may be a way to go.

So any advice when tackling this task would be appreciated, especially on topics of sistering and maximum allowed joist notching.

Some additional resources:

When repairing joists - amount of removed material is of importance (of course), but what is the rule of thumb for sistering to be successful without compromising integrity? Notching is bad - what is allowed?

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  • Is adding support to the cantilevered end an option?
    – JimmyJames
    Nov 10 '20 at 17:17
  • 2
    Most of what folks call "dry rot" isn't dry at all. It's a deck. It's just rot. Wet, juicy rot. :)
    – isherwood
    Nov 10 '20 at 20:09
  • @isherwood, Could you explain the difference? But in structural integrity dialog I think it makes little difference as both need to be addressed in some way and both leave less material for structural integrity. Nov 14 '20 at 17:36
  • @JimmyJames, that is one consideration, but this would not completely address issue of sistering the joists, as they would need to go back into the house, if I understand correctly. In that case I would need front and back support and in that case it is now a free-standing deck with its own design challenges. Nov 14 '20 at 17:50
  • @user3671165 Baseed on my own (basic) understanding and what the other answers seem to confirm, the need to go back into the house is a requirement of cantilevering. If the outer end if supported, it's no longer cantilevered. If the wood is OK at the edge of the house, you can use that as one end of the support. I don't think you even need to sister. A ledger board should be sufficient.
    – JimmyJames
    Nov 15 '20 at 15:14
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You should consult with an engineer they can give you options.

Sistering joists typically means sistering all the way back to where the joist starts. If your plan is just to laminate the portion of the joist that is cantilevered that won't add much if any strength.

If your joists are cantilevered 6 feet from the house and have zero supports underneath then they should be ~18' long to meet with the 1/3 cantilevered rule. Your link talks about replacing the joists but really they are laminating the joists but only 2' for every 1' of cantilever so that is assuming some strength from the original assembly.

Steel can be a good option. Steel is much more rigid than wood and cantilevers of steel can be done from the house face. You might be able to laminate the joist cantilevers with plate steel. Again you'll want to actually get an engineer in to consult and give you an idea for a plan. A house call for an engineer - find an engineer that works for himself and not a large engineering firm - shouldn't be more than $1000.

Check out this article at FHB it presents several options for steel balconies including cable supported, face cantilevered assembly, and knee braced. Your engineer should be able to use one or a combination to strengthen your assembly without having to sister the 18' joists or having to go back into the house 12'.

https://www.finehomebuilding.com/2013/05/16/second-story-balconies

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  • Thank you for taking time. I did visit the page and located the engineering company, and looked at some of their work. It looks good. All of the solutions in here are leading to complete redo of the deck support (joists) be it with a replacement or build-up of support from the ground. Nov 14 '20 at 17:56
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The method you found in the This Old House article - treating the rot in the original joists and adding new joists - is pretty simple stuff. I am pretty sure the author of the BayAreaContractor.com is well aware of that option for repair. It works fine with joists supported at both ends of the span.

I agree with that article - that it's a much more complicated job with cantilever joists, and to make a structurally sound retrofit repair is really tricky. You can't simply repair the cantilever portion. You're going to need a very capable contractor to pull this off without risking an unsafe deck and without doing further damage to your house.

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  • batsplasterson Thank you for taking time to answer. I should have better explained myself. I was trying to illustrate the opposite extremes of addressing dry-rot, but was not proposing it as and either/or proposition. I am sure that BayAreaContractor.com are consummate professional(s). My question is where is that tipping point of addressing some portions of the cantilever beam being rotten and addressed with repair patches vs. full replacement. Those are extremes and few other choices are there as Fresh Codemonger points out. All in all, I am looking into solving this issue fully. Nov 14 '20 at 17:44

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