Solutions for a Sunken Top

seangil · #1 08-26-2019, 04:17 PM

I'm working on my first acoustic build and have completed the body and attached the neck. However, I have run into a problem in the top has sunk down and has become slightly concave as opposed to convex. The bracing was built with a radius, which is now gone. The shop environment underwent a drop in humidity during the course of the build by about 10% and will not be going back up to its original level. I don't know if this is the sole cause of the change in the top (or how to assess). Does anyone have good techniques for how to fix this or otherwise ensure that it doesn't wreck the build?

Two other details that might matter...First, the guitar will be stored and played in an environment closer to the original workshop humidity (ie. 50+%) AFTER it is completed. Second, I have read re-hydrating advice using DampIts/sponges/guitar case tools. I can do this, BUT will still have to regularly bring the guitar into a lower humidity environment for the remaining work of (fretboard, bridge, setup) and finishing. So the guitar would be regularly moving between environments that differ by 10%. Any suggestions?

charles Tauber · #2 08-26-2019, 04:36 PM

One of the reasons to build in a stable environment is to prevent changes to the geometry of the components during the build.

I once braced a top with convex bracing. A change in humidity during the build changed the curvature of the braced top to concave. Restoring the humidity to its previous level returned the top to its original curvature.

My suggestion for your build is to return it to the humidity level at which the top was braced. If you have assembled the instrument components at different humidity levels, you’ve complicated the matter. Once the instrument is strung, string tension will help flatten the top or pull it convex.

Howard Klepper · #3 08-27-2019, 10:38 AM

A 10% drop in RH when you are in the area of 50+% should not be enough to turn a convex top concave. Lots more info is needed.

What were the RH numbers before and after? How are you measuring RH? Why do you say the RH will not come back up again in your shop? How much curvature did you build in? And how about a photo of how you braced the top and how concave it is now?

redir · #4 08-27-2019, 02:15 PM

Room humidifiers are pretty cheap.

seangil · #5 09-14-2019, 06:50 AM

Thanks for your questions Howard. I have been working in a shop owned by another person since I have not yet been able to set up space myself. The owner had been monitoring humidity using a digital hygrometer and had been controlling to a reading of about 50% on that hygrometer. Through a bit of luck (or not), he realized that it was a faulty device and has now gotten several in place. He does not know the exact difference, but I understated in my first post. It is more likely that the shop had been running somewhere in the upper 60%/low 70% humidity range and is now being controlled in the mid-40s as measured by the new devices. Now that there are better measurement tools in place, the shop humidity is going to remain at its current levels indefinitely.

A couple other guitars in the shop that had the same vintage also suffered from sunken tops. In my case, the top went into a very consistent curve that reached about 1mm at its low point.

Since making this post, I have tried to re-hydrate the guitar with a pair of damp-its. As of one week ago, the top had risen back, but not in a perfectly smooth curve. I will be in the shop again on Sunday and take a photo. I am also starting to wonder if I made the bracing too thin since there are some more heavily braced tops in the shop that did not show the same issues as mine. The ones that sunk had a design and internal bracing that was likely on the lighter side.

bostosh · #6 09-14-2019, 07:37 AM

Hi, i have a solution to the soundboard that is "sunken, cracked, ripply, warped, unglued braces, neck reset, etc....
Solves all the problems and provides a stable part to the entire guitar.
However it is so nontraditional and never been done so that i will show in P.M. only. It's too crazy to show in public here. Too many "knowledgeable critics"
And sounds good too. It's been to NAMM and been played by the best.
It reallly needs an experienced pro to hear and see.

JonWint · #7 09-14-2019, 07:55 AM

Quote:

Originally Posted by seangil

Since making this post, I have tried to re-hydrate the guitar with a pair of damp-its. As of one week ago, the top had risen back, but not in a perfectly smooth curve. I will be in the shop again on Sunday and take a photo. I am also starting to wonder if I made the bracing too thin since there are some more heavily braced tops in the shop that did not show the same issues as mine. The ones that sunk had a design and internal bracing that was likely on the lighter side.

You could remove some (or all) of the braces and re-apply after you are certain of stable humidity. I once removed all bracing and reglued after a similar humidity excursion problem. Top is fine after a couple years.

Is your bracing unusually light?

phavriluk · #8 09-14-2019, 10:22 AM

OP's problem seems to have showed itself after using wood that was excessively damp and it the distortion showed up when the wood moisture content receded. I think there's no temporary fix to this. The instrument can't live in an environment as humid as it was made in and the shrinkage resolved only when the moisture is returned. I can't see but what OP needs to start over following the advice above about the state of humidity when building. I know one builder who won't build during the summer just to avoid this kind of humidity problem.