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My mother-in-law has a deck 8 feet off the ground that is is built with 4 X 4's for support, with only one notched 2 X 6 rim joist on the outerside of the deck which also supports the 2 X 6 joists that are spaced at 24 inch centres. The deck is at least 24 feet long by 8 feet wide.

There are no deflections. It's just old and had some joist wood rot along with top decking rot. I've replaced the three joists that had wood rot. The posts were set on patio blocks about 6 inches max into the ground and surrounded by crushed limestone and interlocking brick. Three of the six posts have wood rot below ground level.

The deck was built 25 years ago and does not meet current Ontario building code which calls for all decks to be built with 6 X 6 posts with 2 X 8 joists, and at least two, 2 X 8 rim joists joined together. Since I'm not building a new deck, you're not required to meet the newer Ontario building code, but I want to at least make sure that it is secure. It seems the 4 X 4's stood up okay for the past 25 years without requiring 6 X 6's.

I could remove one post at a time with temporary side supports, remove the patio stone and dig down four feet as required by code, insert a sono tube, fill with cement and then attach a saddle with a new 4 X 4 or 6 X 6 post. The current floor of the deck is covered with 2 X 6's. Quite a few need replacing. I was going to remove these.

I wanted to enhance the structural integrity of the deck by turning the 4 X 4's into 6 X 6's by adding a 2 X 6, vertically screwed onto one side of the 4 X 4, and then a 2 X 4 vertically screwed into the side that overlaps the 4 X 4. This would then make the post a 6 X 6. Then, for added rim joist support, I was going to lag bolt a second rim joist to the one that is already there. This rim joist would sit on top of the 2 X 6 that I would add to the 4 X 4. I was also going to add an additional 2 X 6 joist between all of the current joists thereby moving from 24 inch centres to 12 inch centres.

Please let me know whether this is a futile effort to enhance the structural integrity of the deck, or would I be better to spend her money and replace the current 4 X 4's with 6 X 6's, still adding a second rim joist attached and notched into the 6 X 6 post? I've already added joist hangers and lag bolted the deck to the 2 X 6 header that is attached to the house. Originally, this deck was built only with 4 inch nails.

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What's the highest compression load you have one any of the posts? –  Vebjorn Ljosa Jun 25 '12 at 12:38
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Before I answer, I'd like to know what structural instability you are trying to correct? Perhaps a quick MS-PAINT drawing of the deflections that are bothering you would be in order. Depending on the deflections, you might be better served with bracing, rather than thickening members. –  Chris Cudmore Jun 25 '12 at 16:56
    
@ChrisCudmore The edits by Vebjorn are from migrating an "answer" from Terry that was posted to respond to your first comment. –  BMitch Sep 26 '12 at 18:36
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1 Answer

If I understand your description correctly, you have a maximal load area of ~38 ft². If so, 4x4s are more than sufficient for the posts. (Check your trusted load tables; don't believe some random guy on the Internet.) If you replace some posts there is no harm in upgrading them to 6x6s, but I don't see a reason to sister onto the existing 4x4s.

However, it sounds like there is so much rot that it will be easier to tear down the entire deck and build a new one. If you do that, the new deck will of course need to meet current code requirements.

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Vebjorn, do those load numbers include supporting a heavy snow load? Given the OP's location I would be concerned about that. –  Alex Feinman Jul 27 '12 at 17:40
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@AlexFeinman: Depending on the grade of lumber, an eight-foot 4x4 can probably support 5000–8000 lbs, so at least 130 lbs/ft² for a 38-ft² load area. I don't know the snow loads for his area (I think they are defined per town in Toronto), but here in Massachusetts it ranges from 25 to 40 lbs/ft². –  Vebjorn Ljosa Jul 27 '12 at 18:03
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