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One day I intend making an archtop. I have the Benedetto book. I have a slab of 9" x 2" spruce for the top - exactly 2" thick. When I get it sliced, it will obviously be a bit under 1" depending how it is done.
The Benedetto book says to start with a 1" thick piece.
So my question is, how crucial is it if it's a bit less than 1"?

Hi Frank,
It should not be a problem.I have made a bunch of archtops with 7/8" thick tops and or backs.
Good luck,
BradBrad Goodman39049.1007407407

Frank, I agree with Brad. If you end up carving to a 7/8" height you'll want to make an adjustment to the 4.5 degree neck angle Benedetto recommends. Otherwise you may end up with a bridge that is higher than the 7/8" to 1" height recommended. Put another way, if you go with a 4.5 degree neck angle and a bridge higher than 1" you'll be increasing the downward pressure on the top plate which could alter the sound by driving the top plate to hard.

....well I certainly would not have thought about that. Thanks for making me aware of it - valuable insight.

Guys, I may be wrong but 7/8" overall height for the top should be about right with a 4.5 degree neck angle to produce a 1" standoff at the bridge. It depends, of course, on how much material is removed from the underside of the neck extension when fitting it to the top plate contour.
Nelson

npalen39050.4192708333

I've never done an archtop guitar, but I have gotten by with different arch heights on mandolins.

There is no consensus on the issue, except that if you change one thing, you usually have to change another. In this case, neck angle and string break angle over the bridge will be some things to look into.

I hesitate to even mention it, but I've heard arguments that the arch height is more important than graduations, the converse of that and even that graduations should be replaced by an even thickness throughout the plate. Granted, I'm not sure of the pedigree of all the people holding these positions, but they are out there.

Bottom line, in my mind, is to do what you need to get the top ringing how you like it. Anything can be changed to achieve that, so long as it will still work structurally and that other things can also be changed to compensate.