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One thing some people don’t realize is that an acoustic guitar can go through a tremendous amount of movement over time, depending on temperature, humidity, string tension, and other factors.
That can lead to problems, including parts coming loose, leading some people to wonder:
“Why is my guitar bridge raised?”
Guitar bridges can be raised because the strings are too tight. One reason is a belly bulge, where the wood of the top actually bulges out, usually caused by temperature and humidity changes. Additionally, the glue on the back edge of the bridge might’ve melted causing it to separate from the top of the guitar.
Let’s look at what can cause a raised or lifted bridge, what problems a raised or lifting bridge can exacerbate, and how to deal with a lifting bridge.
How Can I Tell If My Guitar Bridge Is Lifting?
One of the first signs of a belly or a bridge that is lifting is action higher than you expect, especially if the action height changes at different points along the fretboard.
If you think the bridge on your guitar might be lifting, there are two things you can check pretty easily using just a piece of paper.
First, use one edge of the paper as a straight edge and rest it against the top of the guitar. While most acoustics are called flat tops, they all have some curve to them.
But in the case of a belly, instead of a gentle curve from one side of the top to the other, you’ll find a pronounced bulge in the center of the soundboard just below the bridge.
Using the edge of a sheet of paper should give you an idea of how severe the belly is.
The other test is to use a corner of the sheet of paper and see if it will fit under the back edge of the bridge, as shown in this photo.
You shouldn’t be able to get anything under a bridge if it’s properly glued down, so note both how far the paper slides in and how big the gap between the bridge and the soundboard is.
If you have a set of feeler gauges, this is an excellent place to make use of them. They can show you exactly how far the bridge is lifting from the soundboard.
Why Is The Bridge On My Guitar Lifting?
Let’s take a closer look at the several reasons that a bridge might lift on a guitar.
Reason 1: String Tension
The first and most obvious is the tension that strings can put on the top and neck of a guitar.
That pressure can cause the neck to bow and it can also cause the soundboard to develop a bulge or even for the bridge itself to come loose from the guitar.
Reason 2: Temperature And Humidity
Another common cause of lifting bridges is major changes in temperature and humidity. Humidity changes can affect the tone of the guitar itself, so it shouldn’t be surprising it can do more than that, also.
The swings in both humidity and temperature cause the wood of the soundboard and the bridge to shrink and swell, but because they are made from different species of wood and are of different thicknesses, they don’t react in exactly the same way.
That can cause the top to swell and develop a bulge.
The temperature and humidity cycles, with the different rates of expansion and contraction, can also cause two parts that are glued together to slowly start pulling apart.
This normally happens as the glue is weakened after being heated, and can cause internal braces to come loose, as well as causing both a belly and causing the edge of the bridge to come up from the top of the guitar.
Reason 3: The Bridge Was Glued Incorrectly
If the person handling the finishing and the person handling the bridge installation aren’t careful, then the bridge might end up glued down incorrectly. That’s because adhesives work best when they are making a clean, wood-to-wood connection.
The addition of any finish or paint, whether nitrocellulose lacquer, polyurethane, or something more exotic, like shellac, can keep glue from forming the strongest possible bond. Add that to changes in temperature and humidity, along with string tension, and you have a recipe for a lifting bridge.
How Can I Fix A Top With A Belly Bulge?
A belly bulge might seem like a hard problem to fix at first, but it doesn’t necessarily have to be.
There are two main ways that professionals handle the problem:
Using something to help anchor the bridge and top together while also pulling them into better alignment, and using heat and clamping pressure to change the shape of the wood.
Products like the JLD Bridge Doctor are installed and use a screw through the bridge to keep the top and bridge aligned, then use a dowel against the tailblock of the guitar to help pull the belly bulge down and flatten out the top.
This video shows it being installed in a 12-string guitar.
Some people will complain about the Bridge Doctor, since it uses a screw to hold itself in place and includes hardware that remains inside the body of the guitar.
Those people argue that using a screw and leaving the dowel and body of the device in the instrument will deaden the sound it produces.
That is certainly possible, but so many players have used the product in the past several decades, that if such problems were common, we’d hear a lot more about them.
But there are clearly times when putting a screw into a guitar is a bad idea — if the guitar is particularly valuable, for example.
In that case, TJ Thompson Guitars offers a different system, the belly reducer. The system is made of sets of matching cauls that, when clamped together, will flatten the belly at the top.
To use the system you have to remove the bridge, then heat the aluminum cauls in boiling water. You clamp the cauls on the inside and outside and they pull the top into alignment.
This has the potential to cause finish damage, but it is far less intrusive than the Bridge Doctor.
How Can I Fix A Lifting Bridge?
If your problem is a bridge that’s coming up, rather than a belly bulge to fix, the process is more straightforward.
This video looks at some of the repair techniques professionals use to repair a lifting bridge.
The way to proceed is to gently heat the bridge until you can use a metal spatula or a palette knife to work the entire bridge free.
Be careful not to damage the finish of the guitar while you do this, and be careful not to dig your tools into the wood beneath the bridge.
Once the bridge is free, you’ll need to decide whether you are going to reuse it or make a new one.
Once you have the bridge you’ll use, carefully make sure you remove all the finish from where it will be glued. Use self-adhesive sandpaper to make sure the bridge fits precisely to the contours of the top, then glue the bridge in place and clamp it down firmly.
A lifting bridge is a very common problem, especially with guitars that are older or have been through a lot of temperature and humidity changes.
It’s a common cause of high action and can make a guitar feel unplayable at some points on the neck.
There are a couple of things you can do to prevent issues with a belly bulge or a lifting bridge, though.
First, keep your guitar in a hard shell case and keep the humidity between 45 and 55 percent. That will keep the instrument at a relatively stable temperature and humidity, and slow down any changes, keeping the wood from moving too quickly.
Second, you can make sure not to leave your instrument tuned up when you’re going to store it. You’re safe to leave it tuned for a few days, but if you are putting the guitar down for a week or more, you should tune it down to lower the tension on the neck and the bridge.
While a lifting bridge or bellied top might not change the sound of a guitar much, it can affect playability. That makes preventing it and knowing how to fix it essential if you want to keep playing as well as you can.
I’ve been a musician, particularly a guitarist, for more than 25 years. I love writing about guitars, gear, recording, music in general and more.