Mica-rich rocks (shales, slates, schists) are a major and significant component of the continental crust. They are often characterized by a strong seismic anisotropy, a key factor in the seismic interpretation of the architecture of the continental crust. Roofing slates are a group of natural rocks used in construction that must possess a continuous and undeformed slaty cleavage. This makes roofing slates an exceptional benchmark to study the relation between the rock microstructure and the development of seismic anisotropy in single-foliated low-grade slates. Direct measurements of seismic velocities using an ultrasonic wavemeter at room conditions were done on eight cubic samples from active quarries in the NW Iberian Peninsula. Seismic velocities were also calculated using the Hill averaging method from crystallographic preferred orientation (CPO) maps of slate-forming minerals on the same sample blocks using electron backscatter diffraction (EBSD), achieving unprecedented coverages of up to 87%. The seismic velocities estimated by both methods are similar parallel to the slaty cleavage but much slower using direct measures in the axial direction, producing anisotropies for Vp and Vs1 up to 7 and 5 times higher than the calculated velocities. The results are indicative of the highly anisotropic contribution that cracks, pores and fractures have in single-foliated slates, given the strong shape fabric that the rocks present. The samples selected also show that not all slates exhibit near-transverse isotropy, as it has been commonly assumed.