What causes fastener heads to become visible as small bumps in a drywall surface?

Nail pops are a wood movement phenomenon. Framing lumber loses moisture after installation and continues to gain and lose it seasonally, and as a stud or truss shrinks in thickness the fastener stays where it is while the drywall face moves relative to it. The fastener head then stands slightly proud, breaking the compound over it and appearing as a small bump or a flake of paint. The distribution is what identifies it — pops appear along framing lines, often in bands across a ceiling or up a wall, because that is where the fasteners are. Truss uplift produces a related pattern at ceiling-to-wall junctions and is seasonal. The finding is almost always cosmetic and correctable with refastening and refinishing, and the inspector reports it as such rather than as a structural indicator, unless it accompanies other evidence of movement.

Framing shrinks as it dries and moves seasonally, so the fastener stays put while the drywall face shifts relative to it, pushing the head proud of the surface. The pops appear along framing lines because that is where the fasteners are.

Nail and screw pops result from dimensional change in framing lumber as it dries and cycles seasonally. They are typically cosmetic, correctable by refastening adjacent to the pop and refinishing, and are distinguished from cracking associated with structural movement by their distribution along framing.

The Wood Moved, the Nail Did Not. Bumps in a line follow the framing — that is shrinkage, not settlement.

Distinguish pops from truss uplift, which lifts the ceiling away from partition walls seasonally and produces a gap rather than protruding fasteners.

• •Reading fastener pops as evidence of structural movement
• •Attributing them to finishing or installation errors
• •Confusing pops with seasonal truss uplift