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What causes a cracked guitar top?

Cracks aren't bad luck. They're wood snitching on you — telling you exactly what stress it wasn't built to handle at that moment. The two big culprits are almost always humidity and physical impact, and they leave very different fingerprints.

Humidity cracks

Wood shrinks as it dries out, so much more than you could imagine. To the tune of 100 grams or more in extreme situations. It’ll feel noticeably lighter in your hands. If the guitar top loses moisture faster than the wood can gracefully deal with, it will form cracks right along the grain lines. That's your tell for a humidity crack versus an impact crack.

Vermont creates the perfect conditions for this. Relative humidity deep into the high 20’s for extended periods of deep winter will leach the water out until your guitar is a shriveled husk of its former self. Solid tops are more prone to this than laminate, one of the rare moments laminate construction actually wins. But in these extremes, it’s not a guarantee.

Impact cracks

I have a regular customer whose ex-girlfriend put her stiletto heel right through the case and the guitar on the lower bout. Old beautiful Guild acoustic. But drops, bumps, a dive off a stand… they all produce cracks with a totally different personality: irregular, sometimes radiating from a specific point, often with other visible damage hanging around nearby as evidence.

Shipping is a big culprit as well. Bad packing can be catastrophic. My own Epiphone Dot came with the neck detached. A customer of mine had a beautiful Taylor classical that was cracked in transit, but this one was also kind of a manufacturing issue. The bracing did not extend out to the kerfed lining (i.e. the edge). It ended short by an inch. Guess where it cracked? Right along the edge where the bracing ended.

Why cracks matter more than they look

A hairline crack looks cosmetic and is not. Any crack chips away at the top's structural integrity, and it can slowly widen under ongoing string tension if you ignore it — especially humidity cracks, which open and close with the seasons like a tiny, annoying tide. A crack near the bridge or bracing is a much bigger deal than one out on the lower bout, because that's where all the structural heavy lifting happens.

Surface cracks in the finish can be bad if they are fragile or deep. It’s important to protect the finish and the wood underneath from further insult and injury. Making them look perfect is difficult and expensive. Guitars earn their scars, in my opinion, and so I focus more on making feel invisible and providing protection.

What to actually do

Don't ignore it, and for the love of tone, don't glue it yourself with whatever's rattling around in the junk drawer. A proper crack repair means cleaning the crack, gluing it with the right adhesive, and usually cleating it from the inside for reinforcement. Left alone, a small crack almost never stays small.