What is the best approach to repair outside cantilevered wood deck joists with dry rot?

Question

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?

Answer 1

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

Answer 2

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.

Answer 3

A year ago we replaced our cantilevered deck with a ledgered and posted deck. To replace the rotted cantilevered joists, it would have been necessary to open the floor of the living room (or the ceiling of the room below) twelve feet. So we canned the cantilever and went with posts.

A problem I see, that I haven't seen addressed yet, is the fact that you obviously can't put a ledger on the side of the house and magically levitate the other end of a cantilevered deck.

You are starting to go down this road if you sister ONLY the part of the joist outside of the house, hanging one end to a ledger board and the other end to the... nothing. You're supporting the decking boards but doing NOTHING to prevent the ledger board from turning into a hinge.

So the question is-- how bad are the joists? Are we talking about an inch or two of rot on the top of the board, close to the railing? This will cause an obvious and annoying sag in the decking but not a serious structural problem, risking the deck falling off the house.

Or, is the joist mostly rotted through, close to the house? Then, the leverage of the deck load is a problem at that weak spot. If you replace it with a ledgered joist, you will support the deck boards only. But in this case, the support that the cantilevered joist provides to the deck, keeping the deck from hinge-ing at the ledger and falling down, is mostly gone already.

I am not an engineer. If it were my deck, to make this judgement, I would look at maximum spans allowed. For example, if you have a 2x12 with two inches of rot on top, three feet away from the railing, you basically have a 2x10's strength for those last three feet. Probably ok. Laminate away. But if you have 6" of rot on a 2x10 joist 6' away from the railing, you basically have a 2x4 trying to span 6', and that is probably NOT ok.

Of course, you should absolutely not do what I'm suggesting, and you should pay an engineer to tell you how to do it right.