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Old 06-02-2017, 09:23 PM   #1
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Dock-Deck-Loose :)

My cousin and I are about to replace the 130 foot long dock at our families' cottage on the St. Claire River in SE Michigan. As you can see in the attached sketch currently the design is 12 inch pylons with 6x6 timbers, two 4x6 timbers span the 8 feet from one pylon to the next, and 2x10 boards make the decking. The 4x4 posts are half-seated on the 6x6 and the 2x10s are cantilevered off of the opposite edge. Everything is held with Bolts, nuts, and washers; the deck boards are held with screw/bolts/nails/lags...etc whatever grandpa could find in his garage (it has been a while waiting for grandpa to hand the place over to us). The result has been that after a few years the railing became wobbly and the deck boards became see-saws and catapult people who stand too close to the edge (a huge liability with guests and children trying to get in and out of boats). The 2x10 boards and 6x6s are about 33 inches long and the space between the 4x6s is about 26 inches.
In order to avoid this catapult and wobbly railing effect in the future, and since we need to replace everything above the pylons anyway, I proposed that we extend the 6x6 to 39.5 or 40 inches so that the 4x4 posts will be fully seated and we will get a full 36 inch wide deck, enough that two people could pass by turning sideways, with no cantilevering. My cousin is concerned that by having the 2x10s supported only on the ends at 36 inches long the boards will be springy or bouncy. Is that a serious concern or is he just paranoid? Obviously the easy solution is to use a third row of 4x6s down the middle, or perhaps joist each 2x10 with a 2x4 or 2x6, but those require additional material and this is going to cost us quite a bit as it is. But always remember Rule #3.

Rule #1 "See #3"
Rule #2 (Doesn't count)
Rule #3 "Saftey"
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Old 06-02-2017, 11:31 PM   #2
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Re: Dock-Deck-Loose :)

I'm not sure if you have colds you need to comply with in your area and if you do have to comply with codes what the codes are going to require but overall your design has several flaws that I can see.

Obviously, how the vertical 4 x 4s are attached is a major concern. However they attached to the 4x6s? One idea would be to put the 4x6s right at the end of the 6x6s and then you could lag the vertical 4x4s into both the 4x6s in the 6x6s (every 8') having the additional 6" of contact area would make a significant difference.

Yes I would be worried about the span with the 2x10 decking even at the span you have presently. Putting a center 4x6 is probably the easiest way out of that.

Not that it would make any difference of her strength goes but I would consider using 2x6s or even 2x8s instead of the 2x10s. The narrower area would allow water to run off quicker and probably help delay deterioration of the deck boards.
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Old 06-03-2017, 12:06 AM   #3
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Re: Dock-Deck-Loose :)

No matter the cost of any extra material to meet rule #3, it is less than losing any small child or Grandma into the river.

Spend it now or have a guilty conscience in the event of a disaster.

Yes 36 inch span is pushing the limits of safety, so run another 2X down the center of the walkway for support.

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