Crack Behind Bridge

[Lukeu]

Hey, so I'm a little newer to luthier work and this is what I got from my recent build. It cracked behind the bridge all the way to the end block and it did it right on the glue seam. My question for y'all is what did I do wrong and what do I do to fix it. I'm guessing it's either a poor glue seam or improper finish result. Thanks!

LUKE

[fazool]

Most likely a low humidity symptom

[mirwa]

Originally Posted by Lukeu

My question for y'all is what did I do wrong and what do I do to fix it. I'm guessing it's either a poor glue seam or improper finish result. Thanks]

I don’t like being a downer, but I suspect the original joining of the two plates could have been better.

That aside.

To repair, couple of options, the worst one is remove the top and start again, the issue you have at the momment is the x brace and tone bars are holding the top in its current position, trying to squeeze it back together is going to be fraught with ongoing issues.

Best option is doing what I call a splint repair, I will pm you a link showing it done.

Steve

[charles Tauber]

Originally Posted by fazool

Most likely a low humidity symptom

+1. Looks like it dried and shrunk.

Have you controlled the humidity during the building and finishing process?

[Lukeu]

Originally Posted by charles Tauber

+1. Looks like it dried and shrunk.

Have you controlled the humidity during the building and finishing process?

I do now. I struggled with keeping the humidity level steady over the humid Illinois summer. I have the room at 35 right now. Which maybe to dry?

[charles Tauber]

Originally Posted by Lukeu

I do now. I struggled with keeping the humidity level steady over the humid Illinois summer. I have the room at 35 right now. Which maybe to dry?

It is relative. If you glued the top together and assembled the guitar at, say, 70% relative humidity, then, yes, it's probably too dry at 35%.

[LouieAtienza]

It's probably better to be a little dry than wet while building. This could also happen if the jointed edges had the slightest of hollows, and the joint was forced closed with clamps/wedges.

I also like to keep the grain lines as parallel as possible at the seam. It helps when the wood does move. An extreme example would be a picture frame molding. You can get the miter 100% perfect, but if the frame goes somewhere where it is very dry, if biscuits or corrugated nails were not used, the miter would open up, and always on the inside of the miter.