Prime Coat vs Tack Coat: What's the Difference?
Two different bituminous applications, used at two different stages
Prime coat: base to first asphalt layer
A prime coat is a low-viscosity bituminous material sprayed onto a granular (unpaved) base course before the first asphalt layer is placed. It penetrates the base surface, binding loose particles and helping the base bond to the asphalt layer above it, while also reducing moisture entry into the base.
Tack coat: asphalt to asphalt
A tack coat is a thin, different bituminous application sprayed between two asphalt layers (for example, between a base course and a surface/wearing course, or over an existing asphalt surface before an overlay). Its job is purely adhesion — bonding one asphalt lift to the next so they act as a single structural layer rather than sliding against each other.
Why the distinction matters for estimating
Because the two materials serve different purposes and are applied at different rates and dilutions, they're calculated and ordered separately based on the specific surface area each one covers in a given paving plan — a prime coat area (over the base) and a tack coat area (between asphalt lifts) are not the same square footage in most projects with multiple lifts.