Another top thickness question.

[Victor Seal]

To the parlor builders. I am starting a parlor build for a tiny young lady at church and am trying to get opinions on top thickness on a parlor compared to a dread. I have read opinions and articles that contradict each other. If you have built parlors, is there a general rule of thumb that you use to get in the top thickness ballpark.

[Alan Carruth]

The only way to really know how thick to make a particular top is to measure the stiffness somehow. There are various ways to do this. There's so much variation within any species that you can'y go by that with any real confidence. The Young's modulus (E) of softwoods along the grain does 'track' the density pretty well in most cases, and since it's the E value that determines the stiffness you can use density as a surrogate for that stiffness measurement if that's the best you can do.

I feel that since most guitar boxes are similar in length there's not much to be gained by scaling the thickness of the top on that basis. Smaller boxes do tend to be narrower, so that you can use tops that are not as stiff in the cross direction on those. I know suggesting that low cross grain stiffness might not be a problem, and could even be a benefit, is heresy on these lists, but I've found that it works for me.

[Victor Seal]

I have been building dreads with the help of a deflection jig. Maybe I should phrase the question like this. If I arrive at the desired stiffness for a dread, would a parlor, made with that top have to be thinned ?? Or use the same thickness for a parlor as for a dread???

[Bob Gramann]

Because the spans are smaller, a smaller guitar top does not need to be as strong to support the same string tension. But, there is a limit to how thin a top can be before the braces telegraph through and before it becomes too fragile to handle. So, I save my floppier tops for the smaller instruments I build. The weaker wood can be left a little thicker and still give the appropriate strength for a smaller guitar. I work for the appropriate stiffness for the entire top including the braces, the top, the bridge, and the rim attachments. The whole guitar works as a system.

[Michael Lewis]

Much depends on the strings you will be using. I find it interesting to look at some of the old Martins that used gut strings; the tops are about .120" (3mm) but the braces are very light. If you will be using steel strings you will likely need heavier bracing.

[Alan Carruth]

Michael's point is well taken. They probably used lower tension strings in the old days: steel only came in as a factory option on Martins in the '20s, iirc, and they beefed up the bracing over time as heavier strings became more popular.

Torque is what kills tops, and torque is the product of tension and string height off the top. Lowering the string height a little can help quite a bit. Flamenco guitars use thin tops and light bracing, but the string height off the top is low: usually around 7 mm instead of the 11 or so that's more normal on Classicals. The whole thing forms a system that facilitates the sound and playing qualities that are desirable for the style.

[Victor Seal]

Okay. Little thinner it is. Thanks.

[Michael Lewis]

New information: I just took in an old Martin for some fairly serious repairs, and it has a top that is only about .085", and has 3 fan braces, not an X as we so often see. The model is 2 1/2 - 17, and is from somewhere between 1868 and 1898. I'm still looking for any information inside the instrument, but it's pretty dark with age and dust. The top doesn't look like it has been refinished, so I assume the thickness is factory original.

[Barry Daniels]

That is certainly made for gut strings. I hope the owner never tried to put steel strings on it.

I once worked on a Ramirez classical that truly could fill a concert hall with a deep resonant sound. It had a cedar top that was 0.065" thick. It also had about a half dozen, full length cracks in the soundboard. It was obviously a bit too far.

[Bryan Bear]

New information: I just took in an old Martin for some fairly serious repairs, and it has a top that is only about .085", and has 3 fan braces, not an X as we so often see. The model is 2 1/2 - 17, and is from somewhere between 1868 and 1898. I'm still looking for any information inside the instrument, but it's pretty dark with age and dust. The top doesn't look like it has been refinished, so I assume the thickness is factory original.

The obvious question here is: Were the serious repairs needed on the top?

[Michael Lewis]

Serious repairs . . . . well, yes. The bridge had rotated and created an s bend in the top, and in the process it split the central fan brace that spans the bridge plate. It had steel strings on it, I'm sad to say. It also has damage to the headstock joint and the neck joint at the "ice cream cone" heel. This one will be an interesting puzzle, but should be in reasonably good condition when it leaves the shop. That won't be for quite a while.

[Simon Magennis]

I once worked on a Ramirez classical that truly could fill a concert hall with a deep resonant sound. It had a cedar top that was 0.065" thick. ….

I have never heard of a cedar tops being less than 0.1" (2.4mm) before. Gives me food for thought if I ever get round to using the cedar top I have stashed a way. (I swore never again after the only one I did).

[Barry Daniels]

Be careful though. That top was in the process of self destructing.