I want to build 12-foot long benches on two opposite edges of my deck. They will function as pseudo-guardrails, but guardrails are not required since it is a low deck. The bench seating should be around 12” deep.

What kind of composite decking boards should I use for the bench tops?

I am leaning toward 11-12” wide fascia boards, but some are not very thick. I had trex woodland brown fascia on the side of the deck of my last house in 2009. It warped badly. It was installed vertically over solid pressure treated wood fascia board. That deck was in the sun and got very hot in the summer. Also there were daily temperature fluctuations all year round.

Where I live now, it gets very hot in summer and moderately cold in winter. It is much more humid and rainy. Daily temperature fluctuates less in summer and about the same in winter. My current deck is partially shaded.

I can create whatever structure I need under the bench to support the composite bench top. Feel free to advise on the underlying structure. There was some talk on the internet about how the boards needed to breathe (or something). I can also use two 6” boards instead of one 12” board for each bench.

The trex woodland brown color is perfect, but I may be able to go with a lighter gray-brown color. The yellow pine decking will probably be stained cabot pewter gray semi-transparent (IIRC), which is like a beige-y gray driftwood color but not gray.

Do the composite boards need to be a certain thickness? Would pvc decking be a bad idea? There seem to be new technologies now. I have seen fascia boards less than 1/2” and others more than 0.6”.

I have built a composite low deck before, but not a bench or railings.