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Use of Plywood Outside

I am remaking a set of porch rail boxes for my 100 year old house. The boxes hold up the railings around a porch. The old ones rotted out from the bottom up by wicking up water (and just being really old). The boxes are of the post and panel construction type. I want to use regular 3/4 plywood as the panel. It would be more stable than solid wood as I could glue them without fear of expansion/contraction issues. I have lots of 3/4 interior plywood and don't really want to buy expensive exterior. The rail boxes would be painted. 40% of the plywood panel would be exponse. The rest is covered with trim. The rail boxes have a top and are open on the bottom. They are screwed to beams that protrude from the porch roof. The bottom edges of the wood have a vinyl edging (to prevent future wicking).

I would appreciate any thoughts with respect to the use of interior plywood in this application.

Re: Use of Plywood Outside

I think you have a good handle on this.

1) Make sure each box can drain excess water. You wouldn't want to trap water in the box.

2) Inspect each box in the spring to see if it needs any paint or waterproofer.

3) Probably not a good idea for your cat to sleep in it. My cats love to sleep in my window boxes!

3) Enjoy for many years.

Re: Use of Plywood Outside

marine grade plywood is the answer. around here it runs about $50 per sheet for 3/4". expensive, but a hell of alot cheaper than replacing them every 3-5 years.

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