Anatomy of a Soundboard Split

Acoustic soundboards are crafted from quarter-sawn spruce or cedar planed down to roughly 0.100" (2.5mm) in thickness. These plates expand and shrink laterally across their width.

When relative humidity drops below 20%, the top wood wants to shrink by 2 to 4 millimeters. However, it is securely glued to the rigid mahogany or maple rim, the neck block, the tail block, and internal cross-braces. As tension builds, the weakest wood cells shear, opening a split along the grain lines — most frequently extending downward from the bridge to the lower bout edge.

Professional Cleating & Stabilization

  1. Gradual Rehydration: A dry crack should never be glued open. We place the guitar in a humidity chamber until the separated grain edges naturally swell back together.
  2. Glue Injection: High-tack wood glue is worked deep into the seam using suction cups to pull adhesive through the full thickness of the spruce.
  3. Cross-Grain Spruce Cleats: From inside the guitar, we install diamond-shaped spruce cleats with their wood grain oriented perpendicular (90 degrees) to the crack. The diamond shape minimizes mass while maximizing glue surface area across the crack seam.