Use a smaller width like 4" - 4 1/2" to stop cupping and nail directly to the c-joists through the tongue on an angle for blind nailing. Typically a 1 1/2 fin. nail would do, I'd use a galv. variety for the exterior with my finishing gun.

If you do go with the 8" back priming may help if you decide to paint... but you will most likly get some with t&g that wide, dependent on the moiture variation & content of your area.

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Alter Eagle Construction & Design

Thanks for the advice. I did found some 1x4 beaded pine that I'm going to use. I live in Michigan so moisture can be a problem but it's covered completly so rain won't get to it. I'm going to stain and finish it with exterior grade products.

Thanks again.

I would stain it and apply one coat of a exterior urethane such as Minwax Helmsman on all sides before installing it and then two more coats on the face of the surface. You don't have to use an exterior stain on this as it is not exposed directly to the weather.

Thanks for the reply, that's what I was told form my local professional paint store too.