Why reglue a bridge?
The bridge-top glue joint on a flat top acoustic guitar will eventually fail. This typically happens slowly over time and often is not noticed until the glue joint is severely open. As a bridge lifts there is less mass dispersing the force of the strings and therefore the stress on the top becomes concentrated in an ever smaller area. If left unattended long enough a lifting bridge will most likely eventually break the top across the pin holes, warp the top and the bridge itself can become too bowed to be refitted to the top requiring a bridge replacement rather than reglue.
Why do bridges lift?
There are many factors in the failure of this glue joint. Some common causes of a lifting bridge are: repeated changes in relative humidity, exposure to high temperatures and finish left under the bridge by the manufacturer.
One of the most common is undue stress put on the glue joint by top expansion and contraction due to extreme fluctuations in humidity. As an instrument’s moisture content decreases as a result of a low environmental relative humidity the woods shrink. Conversly, as the instrument’s moisture content increases the woods swell. Over time this puts a large amount of undue stress on the instrument’s glue joints.
Heat Induced Glue Joint Failure
Exposure to high temperatures (such as a guitar left in a car on a hot summer’s day) can also cause glue joints to fail. Most glues used to construct factory built acoustic guitars today cannot withstand high temperatures, therefore highly stressed glue joints (such as the bridge, braces under the bridge and the fretboard) are the first to fail or creep.
Finish Under the Bridge
Often times manufacturers micro-roude the underside of the bridge around the edges to allow room for a small amount of finish to hide beneath the bridge. This allows for a flawless transition between the bridge and the lacquer. Unfortunately not all bridges are micro routed. This means that some instruments did not have proper wood to wood contact during the original assembly at the factory.
What can be done to fix a lifting bridge?
There are two ways to repair a lifting bridge. The simpler approach is to rub glue under the lifting portion of the bridge and clamp down the bridge. This repair may only help a modestly lifting bridge. Because old glue remains in the joint this repair is never a long term solution. Often times manufacturers fully or partially glue bridges to the finish on the top of the instrument. If this is the case rubbing glue under the bridge and clamping it will not help.
The second and most effective way to repair a lifting bridge is to remove the bridge, remove and underlying finish on the top, clean up and refit the bridge to the top and reglue it. Although more expensive, this is the preferred method of bridge repair.