Acoustic Guitar Neck Resetting

Flat top guitar neck resets Neck resets are performed to correct (regain) the appropriate angle between the fingerboards plane and the top and bridge. The strings on a flat top guitar are pulling on the top with roughly 150 lbs. of pressure,  over time this tension begins to create a greater arch in the top and raise the instruments action. This arch* in the soundboard, in addition to other changes in the bodies shape can cause the strings to get further and further from the frets.
As the strings try to draw the neck and bridge together the action rises. As the action rises, lowering the saddles height is often the first method used to reduce the strings height above the fingerboard. But as you may have already guessed, we eventually run out of saddle.  Many guitars in need of a neck reset have high action and a saddle that is barely taller than the bridge.
On a flat top guitar the plane of the fretted fingerboard generally skims right over the top of the bridge. While there are numerous variables to the angle at which the neck sits, an instrument who's neck angle greatly under shoots the top of the bridge is likely to have high action and require a neck reset.

The need for a neck reset is not uncommon on flattop guitars.

How is done?  (Click here for pictures)

* It is normal for flattop acoustic guitars to have a slight arch in the top, in fact, nearly all flat top guitars are built with a slight arch. However, excessive arching or ripples may be an indication of loose braces or other problems and should be inspected and repaired immediately. A loose X-brace or cracked bridge plate can spell trouble for a flat top guitar.

Archtop guitar neck sets - Why is it that archtop guitars do not require neck sets as often as flat top guitars? Well it's not that an archtop will never need a neck set, but chances are, if it does, it's for an entirely different reason as those explained above.
First of all there is a fundamental difference in design and the way the string tension exerts pressure on the top. Archtop or carved top instruments normally have their strings anchored to a trapeze tailpiece that is mounted on the end block. The strings pass over the top instead of passing through it and create downward pressure on the top as opposed to pulling on it.

Classical guitar neck sets - Most traditionally built classical guitars are made in such a way that neck removal is not possible (the neck and neck block are one).  When that is the case other methods must be used to lower high action.

Loose necks The neck can come loose from a guitar for several reasons but to repair it professionally the neck must be completely removed, all old glue cleared, new shims installed and refit.

Gaps found around the heel of the neck may indicate a loose neck or simply a poor cosmetic fit right from the factory. If you see a gap between the heel of the neck and the sides of the guitar have it inspected.

How about when action has gotten too low ? If this is a brand new development on your instrument I would advise you to have it inspected. Except for slight differences that occur when there is a lack of humidity, action that has gotten considerably lower may be an indication of serious problems. A cracked or loose brace could cause the top to sink. Extreme dryness can also cause the top to flatten or sink.
Dryness and sunken top syndrome.