top/back radius

[Jason Brown]

Can the top and back both be the same radius?

[Ryan Mazzocco]

I don't see why not. Although, make sure you understand the reason for the radius and the (sometimes) unintended consequences of radiusing your top and back plates.
I could be wrong, and I probably am, but I believe that most people put a smaller radius on the back because most hard woods are less stable across the grain than the softer top woods. The purpose of putting a radius in the first place is to give the instrument available material to shrink when dried out without cracking. This radius will also stiffen your top making it let free to vibrate. Thus, in most guitars the back radius is smaller than the top radius. There is also a thread on here somewhat recently about making true flat tops with no radius. Most reported they had no problems with them. I have built one true flat top (though accidentally) and it has not cracked after being tuned to full tension for the last year and a half or so.
I've just seen this post on here for the last few hours unanswered, so I thought I'd throw out something for you to chew on until the real experts log on later this evening to tell you what's really going on.

[Michael Lewis]

Ryan, you're doing fine.

[Chuck Morrison]

I don't see a problem using the same radius on top and back arches. I do it all the time.

I believe the initial rationale for the back arch was to deflect sound out the sound hole. Whether that is what is actually happening is perhaps debatable. Similar arching can be seen in guitars built as early as the 1500s, where some builders used multiple sections/staves, like that in a lute back, and elaborately decorated them. Joachim Tielke (German, 1641–1719) is one such builder. see: http://www.metmuseum.org/toah/hd/tiel/hd_tiel.htm

J. Tielke guitar showing arch

Now we can point to the advantage arching has with regard to wood movement with humidity change, but I doubt that was the original intent. Not sure that this is all the germaine to the original question, but it's always fun to post a photo of Tielke's work.

[Long Vu]

Arching top and back is a must. Though, less arch for the top is prefer to allow top movement. More arch to the back than the top is usually done since the back contribute less to the sound performance, but enhance internal air volume, thus, enhance the bass .

[Barry Daniels]

Arching top and back is a must.

More of a preferred option. Quite a few guitars with flat tops out there.

[Rodger Knox]

Arching top and back is a must.

More of a preferred option. Quite a few guitars with flat tops out there.

I'll agree building with an arch is a preferred option, but there are quite a few builders that build them flat.
There aren't many flat tops once they get string tension on them.