Guitar bridges lift off the instrument top for several reasons. Leaving a guitar in a hot environment can cause glue to melt or dry the glue out. Dry glue becomes brittle and the high tension of the strings breaks the glue seal. Internal causes such as bad or broken braces distort the guitar top and cause bridge lifting. A poor quality instrument may have the bridge glued to an already-finished top, creating a poor bond. Whatever the cause, failure to repair the problem damages the guitar.
A guitar with a lifting bridge needs inspection to determine the nature and severity of the problem. A sheet of paper pushed beneath the bridge reveals how much separation exists between the bridge and guitar top. Luthiers insert small mirrors, similar those used by dentists, into the soundhole to inspect the internal bracing. Braces glued to the underside of the guitar top support the wood. Broken or loose braces weaken the top, causing the top to shift and separate from the bridge.
All guitar bridge work requires string removal. In some cases, repairing the lifted bridge is as simple as re-gluing it back into place. The bridge remains on the guitar top, reducing time and cost. Fresh glue is inserted into the gap between the guitar top and bridge. The glue dries while pressure is applied on top of the bridge to close the gap, but re-gluing is a temporary fix. The old glue still lies under the bridge, preventing the new glue from making a wood-to-wood bond.
To create a more permanent bond between the guitar top and bridge, reset the bridge. Heat applied to the bridge loosens the remaining bond. The bridge separates from the guitar, allowing removal of all the old glue. After sanding the contact surfaces to create a clean bonding surface, the bridge is glued and clamped onto the guitar top for 24 hours. Resetting the bridge works best because any damage shows easily and can be repaired before replacing the bridge.
Broken braces and damaged guitar tops require complex repairs. A lifting bridge sometimes stresses the guitar top and damages the wood. Broken braces require removing the guitar top to repair them. Repairs of this nature involve a high degree of skill and should be left to a professional. Someone with no experience may ruin a guitar that could have been fixed. At the very least, a beginner should work alongside an experienced luthier for advanced repairs.