Bridging one deck joist over a formal pond

Question

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.

Answer 1

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.

Answer 2

You could support each deck board individually by taking advantage of the strongest shape in construction, the triangle.

Answer 3

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.

Answer 4

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.