The surface roughness of hydraulic fractures reflects the fracturing process at the tip. We conducted an experimental study of hydraulic fracture propagation and observed a characteristic roughness pattern. The roughness developed without shear or torsional loading in the plane of the penny-shaped fracture and is determined by material properties and the externally applied stresses on the sample, which represent the effective in-situ stresses. We quantified the roughness and showed that it correlated well with a measure of the plastic zone size around the fracture tip. New explanations for fracture surface roughness that develops under Mode I loading conditions are presented.