Uneven String Height: Fret Rocker or Neck Bow

Why String Height Varies Between Frets

String height, often called action, is set by the combined effect of the neck curvature, the height of the bridge and saddle, and the evenness of the fret tops. When you press a string at one fret and then another, the distance between the string and a given fret changes if any of those parts is out of spec.

A curving neck, usually controlled by the truss rod, makes the middle frets sit higher or lower than the outer ones. Uneven fret wear makes one fret stand taller or sit lower than its neighbors, so the string height changes only at that fret. Understanding which pattern you are seeing tells you what to fix.

To diagnose the pattern, press each string at the first and last fret and look at the gap under the string at the 8th fret. That gap is the relief. If it is too large or nonexistent, the neck is bowed. Then run a straightedge along the frets to see if any fret stands proud. This simple two-step check separates neck issues from fret issues.

• If the height changes gradually from the first fret to the last, suspect neck bow.
• If the height jumps at one or two specific frets, suspect uneven fret tops or a loose fret.
• If the height varies from string to string at the same fret, check the bridge or saddle heights.

Check Neck Bow First

Before touching the frets, confirm that the neck has a proper amount of relief, the slight forward bow that allows the strings to vibrate without buzzing. Place a straightedge along the fret tops and look for a small gap around the middle frets. If the gap is too large or nonexistent, the truss rod may need adjustment.

Turning the truss rod changes the neck curve and affects string height across the entire neck. A simple adjustment can even out an overall trend, but it will not fix a single high fret. For more detail, see our guide on truss rod adjustment.

A good rule is to adjust the truss rod in small increments, usually a quarter turn at a time, then let the neck settle before rechecking. Over-tightening can strip the rod or crack the neck, so if you are unsure, ask a luthier.

Inspect the Frets for Wear

Even if the neck is straight, worn frets can create uneven string height. When you fret a note, the string should touch only the intended fret. If a neighboring fret is higher, it can cause buzzing or make the string feel like it is sitting too high. Run a fret rocker, a small straightedge with a rounded middle, across three frets at a time to see if it rocks, which indicates one fret is higher. If you notice buzz only at certain frets, see our guide on fret buzz on specific frets.

Fret wear is natural, especially on the lower frets where you press most often. If the frets have flat spots or grooves, they need to be leveled or crowned. Learn more about fret wear and leveling.

Fret rockers are inexpensive and available at most music stores. Place the flat edge across three frets and rock it back and forth. If it rocks, the middle fret is higher than the outer two. Mark each high fret with a pencil and check the whole neck. This method quickly locates problem spots.

Fret Leveling vs. Truss Rod Adjustment

The distinction is simple: the truss rod affects the curve of the whole neck, while a fret leveling affects individual frets. If you measure the string height at every fret and find a steady increase or decrease, the neck bow is the culprit and a truss rod turn is the first step. If the height is inconsistent from fret to fret without a clear trend, the frets themselves are uneven.

A fret leveling removes a small amount of material from the top of all frets so they sit in one plane. This is a job for a luthier or an experienced hobbyist, as it requires specialized tools and crowning. If you only have one or two high frets, a partial level or a fret dressing may be enough.

Always set the neck relief correctly before judging the frets. A neck that is bowed backward can make the fret tops appear uneven even when they are perfectly level. Adjust the truss rod first, then reassess.

If you are comfortable with minor adjustments, you can try a fret leveling yourself with a radius block and sandpaper. But a poor leveling can ruin the frets and make the action worse. For the majority of players, a professional setup is worth it.

Other Factors at Play

The bridge and saddle heights can also change string height, especially from string to string. If the low E is high and the high E is low, the saddle may be tilted or the slots worn. Nut slot depth affects the height only at the first few frets. String gauge and tension also affect how the neck responds, as heavier strings pull more on the neck.

The nut slot depth also affects string height, but only on the first few frets. If the low E buzzes on the open string but not fretted, the nut slot may be too deep. Similarly, a worn saddle slot can lower one string's action.

For a full overview of how action and setup work together, see our guitar action and setup article. The materials used in the neck, such as mahogany or maple, have different stiffness and response to humidity, as noted in Martin's wood guide, but the adjustment process remains the same. Strings are the first point of contact between your hands and the guitar's voice, as Martin's string guide puts it, and their tension and core design affect how the neck behaves. Different body shapes also influence volume and tone, which is why a dreadnought may feel different from a parlor under your fingers, but the neck and fret issues are diagnosed the same way.

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Questions

Is a slight variation in string height between frets normal?

Yes, a very small change is normal because the neck has a slight bow. But if you can see a clear difference when pressing at a high fret versus a low fret, or if one fret buzzes, something is out of adjustment.

Can I fix a high fret by sanding it myself?

You can dress a fret with careful filing and sanding, but it is easy to remove too much material or flatten the crown. Unless you have experience, take it to a luthier. Leveling all frets is more involved and is best left to a professional.

How often should I check my neck relief?

Check it whenever you change string gauge, after a change in humidity, or when the action feels different. A quick check with a straightedge takes less than a minute.

Can a loose truss rod cause uneven string height?

If the truss rod spins freely or is stripped, it cannot hold the neck in position, which can lead to instability and uneven height. In that case, the rod needs repair or replacement.