Aluminum roof, to ply or not to ply?

[cuyeda]

madjack wrote (from another thread):
...if you have a substrate(ply) on the roof first, go with the thinner AL if not, go with the thicker AL...the AL skin can be floated(recommended), using the trim to hold it in place....

I wonder how many people building AL skins use the ply substrate vs. just the AL skin on top of the spars. Perhaps the ply substrate adds to the overall strength of the body.

If the ply substrate was skipped, I wonder if it would be significant weight reduction. What is recommended?

[Dean_A]

I struggled with this myself, and eventually decided to skin with 1/8" ply under the aluminum. I figured the extra weight was a good tradeoff for preventing (potential) dings.

I've read that plywood weighs (roughly) 25 lbs. per 1/4 in. of thickness for a 4x8 sheet, so assuming that's correct, I added about 20 pounds to my build, give or take.

[asianflava]

I actually used 2 layers of 1/8 ply on my roof. I did it to overlap the seams as I used the trailer before I was finished. One layer is good enough, it's not that much extra weight plus it gives the roof more strength AND better dent resistance for the aluminum skin.

[cuyeda]

snip... I did it to overlap the seams as I used the trailer before I was finished. One layer is good enough, it's not that much extra weight plus it gives the roof more strength AND better dent resistance for the aluminum skin.

Dean A.: I figured the extra weight was a good tradeoff for preventing (potential) dings.

That's all the reason I need to use a ply substrate. Thanks guys!