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I have built a small formal pond out of concrete block. I have also constructed a deck frame which part crosses the formal pond. Visually, I intended for it to appear that the water disappeared away under the deck.

There are 4 joists for the decking. Three are connected to the deck frame in the usual way. One joist however has to cross the pond. The height of the pond is the same height as the deck frame, so the deck boards are just above the water surface. On reflection, this was an error!

I am now stuck with how to support the deck boards where the fourth joist should be between the two sides of the pond. I can only think of two options:

  1. Submerge a joist and somehow fix it within the pond. However, this is likely to rot quickly and will spoil the illusion that the water is disappearing under the deck.
  2. There may be another material I could use to bridge the gap. I wonder if a steel flat bar would do the job, secured either side of the pond to the top of the concrete block.

Hopefully the picture below makes it clearer.

Diagram demonstrating where the fourth joist is missing

Picture showing the issue

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    Cut them off at whatever distance they will self-support past the 3rd joist, or install a diving board for formal pool dives (tuxedo or evening gown required, please! This is very formal!) Flat bar does not offer much support due to geometry.
    – Ecnerwal
    Commented Jun 2, 2022 at 15:06
  • Could you provide a picture of your actual situation? Your picture does not match your description of "The height of the pond is the same height as the deck frame, so the deck boards are just above the water surface."
    – MonkeyZeus
    Commented Jun 2, 2022 at 15:39
  • @isherwood no railings or anything above. The end result will be that the deck is level with the ground around it. I've added a picture. Commented Jun 2, 2022 at 15:40
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    @MonkeyZeus I've added a picture to make it clearer. The top of the concrete blocks is at the same height as the top of the deck frame. Thus the deck boards sit just above. Hopefully that makes more sense? Commented Jun 2, 2022 at 15:41
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    Does your location legislate that open water to be fenced, covered, or protected? If so that may factor into your solution.
    – Criggie
    Commented Jun 3, 2022 at 10:08

4 Answers 4

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I think I'd use a length of aluminum box tubing as a joist. You can hang it on the concrete using segments of aluminum angle as joist hangers. You can then screw the decking directly to the tubing.

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    I think this would work nicely. Rather than fixing it to the concrete, I could take out a small chunk of the concrete block for it to rest in either side which would give a very sturdy support. Commented Jun 2, 2022 at 15:51
  • @ChrisEvans, how would one notch concrete squarely and precisely for that? A couple expanding anchors would make quick work of it.
    – isherwood
    Commented Jun 2, 2022 at 17:50
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    @isherwood concrete block and will be hidden. I imagine I should be able to cut out a small section with an angle grinder and back fill with cement to hold in place. I won't be able to put expanding anchors on the inside as the pond liner has to go in, so would result in a hole through the liner. Commented Jun 2, 2022 at 21:15
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    Make sure your screw metal is compatible with your Aluminum box material, otherwise you will get corrosion.
    – Forward Ed
    Commented Jun 3, 2022 at 1:06
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You could support each deck board individually by taking advantage of the strongest shape in construction, the triangle.

enter image description here

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Steel, even if galvanized will have limited life. However, something like 1/2 thick plate will last longer than your wood deck. Find an independent weld shop; they will have a variety of odd scraps of steel, likely not expensive. Aluminum would last longer but it would be important that no other metal be in contact because of galvanic corrosion. Stainless ,like 316, would be good but costly.

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    How wide would a 1/2" steel bar have to be to properly support decking for 80cm?
    – isherwood
    Commented Jun 2, 2022 at 15:29
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Shorten your concrete pool.

Either terminate the concrete pool such that:

a) The outside of the concrete block is essentially flush with the face of where your wood joist is supposed to be. Your deck boards will over hang and the end of the pool will not be visible from above. You deck boards can be fixed to the wood joist just like everywhere else.

b) The concrete end wall replaces the joist. Your deck board would need concrete nails to attach to the block below. If the concrete is not at the right height you can add a wood nailing plate of shims to support the deck planks and nail into or through. You can then either place a hanger on either side of the concrete pool so support the remaining portion of the wood joist. Alternatively you could build out your block wall to the wood post since it seems like a relatively short distance. If you have the material you could also cantilever the previous joist across the post yo reach the edge of the pool.

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  • A) is a viable alternative. B) is essentially what's happened at the next joist in from the edge of the pond.
    – FreeMan
    Commented Jun 3, 2022 at 11:26

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