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I have a few bad deck joists that I replaced with new PT joists. These repairs are great.

Now I have a few more not too bad joists that will be rotted in a few years. I am thinking to add new PT joists sistering with the old ones. In addition, I will add carriage bolts to strengthen the two sistering joists together. My new joists can rest on a beam at one end, so the carriage bolts will be act like joints hangers.

Is the sistering repair an OK practice? Shall I go the complete repair to take the old joists off?

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    Personally, I would replace old ones without sistering. If You make it just how You plan to do, You will add a dead load. In addition, if You support only one side on the beam and the rest will be 'hanged' on old joist, You will make this section act unnormally and risk that both (old and new ones) will crack near the bolts. If You post any graphical reference we may try to consider other way doing that. – Marek Oleszczuk Dec 2 '15 at 8:28
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Sistering board on the exterior is problematic. Without a gap between the two boards, water gets trapped and rotts them from the middle out. Same reason ledgers without spacers are a problem spot on houses. If you sister the joists, make sure you put in a spacer to allow air flow between the two boards.

Or do not sister and just install another joist nearby. Not pretty layoutwise from the bottom, but not really noticeable from the top for the layman.

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Sistering is fine, with one exception: if you have deck boards meeting at your crummy joists, those could present problems in the future.

But even if you don't, cutting out the old crappy joists and putting new in is just about the same amount of work. You won't have rotten/rotting wood in contact with your decking. It'll look tidier. It's a good excuse to get a reciprocating saw if you don't have one already. Go for it.

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