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Loose braces can affect a guitar’s tone and structural integrity. Here’s how they’re repaired.

Tools to check for loose braces: a small light, a flat mirror, and a telescoping mirror.
An acoustic guitar’s braces allow the body’s top and back to withstand the pull of the strings, and they can also have significant impact on the tone of the instrument.

Braces are usually glued on using aliphatic resin (white glue), hot hide glue, and sometimes epoxy. Although many guitars never develop any problems with their bracing, fluctuations in humidity, excess heat, glue joints that were “starved” (too little glue) during construction, and even unseasoned woods can cause braces to become unglued.

Sometimes this will result in rattles or string action that is too high or too low. In this article, we’ll take a look at how you can identify loose braces, along with some approaches that a repair person might take to fix them.

A word of advice: the actual repair can be trickier than it appears, so I’m not suggesting that you practice DIY brace repair on your own guitar, unless you’re willing to run the risk of sacrificing the instrument. However, it can be helpful to know how brace repairs can and should be done in order to communicate with the tech of your choice.

Outside Inspection: Tap Test

An easy and often-effective inspection is the tap test. Simply hold the guitar by its neck with one hand while sharply rapping the top and back with the fingertips and thumb of your other hand. Loose braces usually give themselves away by rattling and buzzing. After performing an initial tap test, conduct a quick visual inspection of the back and top exteriors, looking for cracks, bulges, and hairline cracks that may occur if the wood “lets go” from a brace.

The tap test tends to be more effective for identifying loose braces in the back than in the top, because the back is free of string-pull stress (string pull on the top can cause a large enough gap between the top and a brace that they aren’t making enough contact to vibrate and buzz). Plus, it’s easy to look for loose back braces on a flattop guitar because you can peep through the soundhole and see any gap between the brace and the back.

Another way to identify loose top braces from the outside—especially in the bridge area, where braces often come loose—is to shine a light on the finish and look for the following signs: a hump that rises like a shallow bubble behind the bridge, a wrinkle or “pucker” in the finish around the ends of the bridge toward the rear, or a recessed dip between the bridge and soundhole (a result of the top pulling up behind the bridge and causing a dip in front).

Also check for a sag in the top between the front of the soundhole and either the bass or treble edge of the top, where it meets the sides. This can mean that one or both ends of an X-brace have come unglued. This is more common on the pickguard side of the top, because pickguards are usually made from plastic that, as it shrinks over time, can curl the top underneath.

Archtop Diagnosis

Archtop guitars are a little different. Most sought-after archtops don’t have back braces at all. And while some great archtops are X-braced, most have just two long top braces running lengthwise, because the carved top would have enough strength even without a brace. Additionally, the bridge exerts downward pressure on the top rather than pulling up on it, as it would on a flattop guitar.

Because of these factors, inspecting an archtop guitar from the outside probably won’t reveal much, since a loose brace won’t cause the back or top to rise or sink. Archtop guitars are also more difficult to inspect than flattops because most have f-holes rather than round soundholes. While you use the same inspection tools and techniques described below, your view will be more limited.

Inside Inspection Tools: Mirror and Light

To get a good look inside your guitar, you’ll need two things: a mirror and a light. Any small mirror will do, but I prefer a telescoping mirror that reaches far into the body, and I also use a three-inch by eight-inch flat mirror (see photo on page 88 of the August 2009 Print Edition).

A normal flashlight or penlight will work, and the flexible tube lights (also called “flexible impact lights” or “rope lights”) available at hardware stores are great, safe, and easily inserted or removed through a soundhole or f-hole. It’s difficult to find what you can’t see, even if you know where the rattle is coming from, so shake any dirt or fuzz inside the guitar toward the neck end of the body and vacuum it up. Now you’ll have a clear view.

Here are some things to look for on two of the most well-known vintage acoustic brands, Martin and Gibson. For starters, Martin guitars are far less inclined to develop loose braces than Gibsons are. Although the top bracing of some vintage models may come loose, the ends of Martin top braces seldom come loose from the kerfed lining they’re tucked into, and their back braces just don’t come loose without mistreatment.

Vintage Gibson guitars on the other hand—flattops in particular—are notorious for loose braces, top or back. The ends of vintage Gibson braces often come loose because they are extremely thin (compared to a Martin) and are barely tucked into the kerfed lining (if at all). It’s also not uncommon for Gibson braces to split at the ends, with or without being loose, and for the center reinforcing strips along the back’s glue joint to loosen!

More Inspection Tricks

If, after careful checking, you still can’t find the loose brace that you think exists, don’t put the light and mirror away yet; there are a couple of tricks left. Try pressing on the questionable area (gently, yet firmly, with the ball of your thumb or heel of your hand) from the outside while watching for movement on the inside. You’ll probably be able to see it, but only momentarily.

As the pressure is applied, the gap will open, but the movement may be almost imperceptible. If you can reach the area that you suspect is loose, try to probe into the (alleged) gap with a thin blade, like a .004–.008-inch feeler gauge—anything thinner will probably be too flimsy to go into a loose joint. I always tie a piece of fishing line to any probe so that if it gets stuck I can pull it out. Don’t use thin paper because if it tears, the paper will be stuck in the gap and difficult to remove.