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I’m looking to get some advice with sistering some deck joists. Usually I know you don’t want to sister mid span, however where the joists have hit the header are almost completely rotten out (just end rot for about 1 foot) the other supported side is in great shape however there is no external support as it sits on the interior sill plate though the crush glass stucco of the house.

The sister will be connected to the original joist coming out of the house with construction adhesive and peppered with 3” galvanized nails. It will have about 5-6’ of overlap and then a 1 foot section of the original rotted sister cut out before handing on the next support header. I’ll ten scab in a chunk of wood where the existing original header chunk was completely removed.

Will I be okay to cut the 1’ of rot out of the bad end and sister to one side a 8’ length with construction adhesive and lots of nails to make this repair and be confident it will correct the issue?

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    Will you be poking the end of the 8 foot sister all the way back into where the rotted end was removed and support and fasten that end the same way as before. You know it would be helpful if you could add a top down view sketch showing the layout, where the headers are, rotted portion, and existing support locations.
    – Michael Karas
    Oct 27, 2022 at 6:08
  • Suggest you merge your comment above into the original question. Oct 27, 2022 at 11:49
  • It’s hard to know how good the joist coming out of the house is as it passes over its supporting wall. (In my experience, it probably isn’t in great shape, which might mean an interior problem as well as an exterior problem.) Can you investigate deeper? Oct 27, 2022 at 11:52
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    If the span is 8 ft and you were going to overlap 5-6ft, why not just add a new 8ft joist glued and screwed to the old?
    – RMDman
    Oct 27, 2022 at 12:08
  • I have taken quite a good look at it . There seems to be no rot anywhere else. The only reason for the rot in the area currently is due to the previous owners fastening an structure though the plywood right above where the end rot is in the specific area with no measures to prevent water leaking onto the noisy. The rest of the span of the joists are very solid. As to the other question, the span is 12 feet, with no sill plate to rest onto so there for I was going to use 8 foot sister and only cut the 1’ of rot out
    – Graham
    Oct 27, 2022 at 14:17

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I'm not sure I can give you a full-on green light with the information presented. However, I'll offer some thoughts...

  • Expect more rot than you're currently aware of. You may find that screws run into the joists plunge deeper than expected. If this is true, your plan is not sound. Replace the joists entirely.
  • Construction adhesive is not to be considered structural. It's mostly use as a sound-preventing measure in modern construction. It cannot be used where its failure results in safety issues.
  • Screws, not nails. Nails work loose with seasonal movement. Screws do not. Most carpenters stopped using nails with almost any aspect of deck building decades ago. One notable exception is with joist hangers.
  • "Lots" of fasteners isn't the right approach. A random spray may leave critical areas under-supported. Use them specifically as needed for the local load. In your case, the highest stress is at the ends of the scab. I might use three screws, spread widely in the vertical, at 6" intervals near the ends and 12" intervals in the middle 50% of the overlap.
  • Use clamps as needed to get the joists tight together. Once you've added multiple screws it's nearly impossible to snug them up. Gaps dramatically increase load on the screws (and look unprofessional).
  • There's not much point to filling in where the rotted area is removed. Unless decking joints rest there or you want specific screw positions, don't bother.
  • The opposite end of the scab doesn't need to rest on its beam, but it should be close to it. The longer the patch the less force is transmitted through the individual screws.
  • I would have a local expert take a look with you. There may be nuances to this project that a rudimentary sketch doesn't reveal.
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    All good advice. I might add that the screws should be structural screws, not just any screws you have laying about. Bolts are an acceptable alternative, and using them might be required by your local building inspector. Oct 27, 2022 at 21:03
  • I've never quite figured out what "structural screws" are. Load rated? What's our load? I agree, but without more specific guidance the term doesn't mean much.
    – isherwood
    Oct 27, 2022 at 21:06

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