Believe it or not the thickness of the fretboard has quite some significance. This is typical for several different and various reasons.
Classical Guitar Fretboard Thickness:
For the Classical Guitar the fret board thickness aids in the strength of the neck and when used in combination with center ebony reinforcement lamination in the neck construction itself, the 2 combined pieces of wood form a “tee”, or a structural tee (like that used in concrete parking structures). So the fretboard acts as the top of the “tee” and does add significantly to the overall neck strength.
Classical Guitar fretboards are usually in the neighborhood of about 3/8″ (9.5mm) thick or slightly less. The additional thickness also allows for a classical guitar luthier’s trick. That is to warp down the bass side of the fretboard from about the 12th fret to the end near the sound hole. This minimizes string buzz when playing the bass strings very loudly or forcefully.
Acoustic Guitar Fretboard Thickness:
As for the Acoustic Guitar – I prefer to build my guitars with a dead level top. I also like to have the neck dead level and in line with the top plate. This makes for an easy fretboard/neck/body intersection and cuts down on fitting time.
The significance that the fretboard plays is that I like to keep the fretboard and the bridge close to the same thickness. That way, when I string up the guitar with the appropriate string height, no additional adjustments are required. (Not always the case, but a good portion of the time.)
This would make for a fret board thickness of about 3/16″ to 1/4″, which is slightly thicker than a normal acoustic guitar fretboard. I feel the extra thickness is justified in adding strength to the guitar. It also allows for a radius on the fretboard without loosing so much fretboard thickness.
Also a slightly thicker fretboard gives additional strength were the fretboard cantilevers over the top plate. And another reason is if you bind the guitar and fretboard with the same materials, the heights are very close to or exactly the same, which gives a very nice look to the guitar/neck intersection.
Another great attribute for a fretboard that is a bit thicker is it allows for a certain amount of milling down the road. Let’s say someone does a botched fret job on an otherwise great guitar. Rather than having to go through a fretboard replacement, you have a little room to plane down a thicker fretoboard, deepen the fret channels, and do a proper fret job.
The Impact of Inlays:
Another consideration for a slightly thicker fretboard (for the acoustic guitar), would be the use of inlays in the fretboard. While only a millimeter or millimeter and a half thick, this certainly compromises the fretboard to some degree. If you use a slightly thicker fretboard, you are only loosing about 15 – 20% of the strength vs. 33%.
The Negatives for Thicker Fretboards:
If you construct fretboards from African Ebony, a thicker fretboard can be more costly than thinner ones, and at the currently cost of ebony, that can make quite a difference.
The weight of a thicker fretboard, especially Ebony, can make an acoustic guitar more neck-heavy and should be considered, along with the weight of the neck wood that is used. Generally speaking if you use Mahogany or some other lightweight wood for your necks, the extra weight of Ebony will not be a concern. If you use Walnut or Maple, that is a different matter.