[Sage Runner]

Hey Robin. Good Question. On Flat tops Using spruce primarily-You have so many variables due to the Spruce variety—Sitka,German,Red,Engleman etc. But each individual top will have varying degrees of Stiffness regardless of the thickness. You will find flimsy top wood-all the way to extreme stiffness. That is why Tops are Graded and calculated in the rough by the Luthier before the Tops are Chosen. Typically Stiffer the better so the luthier can brace the guitar lightly. Less Stiff Spruce top wood typically will be left a tad thicker or maybe braced a tad heavier to get a desired result! Everything is considered though—what size of guitar—tiny parlor? Or huge Jumbo? A Luthier will take all things into consideration. Are they crafting a primarily Responsive Finger style guitar? Match up a Super stiff Adirondack top —with very light Sitka brace-Wood for a particular tone and response —they might be trying to Achieve. Matching a certain Spruce or maybe Cedar/Redwood type to a certain Back and sides to achieve a desired tone. The same concept applies to Solid Spruce Arch-top builders — but no doubt Boutique Arch -top makers are even more particular about the Top wood they will use.