Acoustic Guitar Construction

Hi guys,

This is my first post on that forum and I'm currently working on my second build, a dreadnought. I bought a plan online but I'm also following instructions from Cumpiano"s book for how to proceed with different steps. I was surprised when I realized the guitar from the plan I bought had a flat top rather than a 25 ft radius and after doing some researches, I found that many luthiers build dreadnoughts with a flat top. I guess it's a matter of taste how you want to do it but I would like to have a better understanding of the pros and cons of both ways of doing it. Any thoughts? Thanks!

Welcome to the forum! I've built most of my guitars with a 28' radius. I think it helps to prevent cracking if you have humidity changes where you live.

Hi, Welcome.
As Carl does,I too make all my guitar tops with a 28" radius. Such a radius provides more tension in the top and therefore is a little less free in movement. But since Dreads can sound boomy, I prefer the domed tops.

The biggest advantage is, as Carl pointed out, the ability to compensate shrinkage, when humidity gets real low. A flat top there tends to be more vulnerable to splitting in the middle.The advantage of a real flat top is an easier build. Make the perimeter and neck block just flat, and you're fine.

Good luck building,
Herman

Thanks guys, this is very relevant! I’ve heard that before and I still don’t quite understand why the pull tension on the bridge doesn’t compensate for this shrinkage.

Dominic

Hi Dominic,

When humidity changes top wood mostly expands and shrinks across the grain.
But the top is fixed to the sides.

So when humidity rises, the top wants to grow wider, but the sides inhibit that.
The plates gain pressure against the sides, but also against each other. The glue line in the middle is compressed.

On the other hand, when humidity drops, the plates shrink across the grain. And the plates want to re-track from the sides, but also from each other. Then a lot of stress comes to the glue line and with bad luck a crack can occur.

In case of a pure flat top, the plates have no other way to go than side wards from another. With direct stress on the glue line.
When you have a domed top, in the situation of shrinkage, The top first drops a bit in the middle. The top has a little room to compensate before real re-traction starts.
A domed top has more length of wood between the sides, than a flat one has. And that helps in dry conditions.

Hoping this helps a bit.
Herman

That's a very thorough explanation, thanks Herman!