A standing seam roof assembly in which adjacent roof panels are supported by underlying support structure in overlapping edge relationship to form a standing seam between adjacent roof panels is disclosed. A two piece hold down clip having a hook shaped section and a clip base secures the standing seam to the underlying support structure. The hook shaped section includes a slidable radiused portion, a substantially vertically extending portion and a hook portion which hooks over and is compressively secured to a corresponding radiused portion of the male sidelap portion. The slidable radiused portion slidably engages a c-shaped beam portion of the clip base, allowing the hook shaped section to move relative to the clip base as the panels expand and contract as a result of environmental conditions, while still resisting the unfurling and unzipping of the standing seam as a result of uplift forces on the panels. The clip base has a flange that is secured by way of fasteners that extend through the flange and into the underlying support structure. The flange includes downwardly extending feet tabs useful in concentratingly compressing a layer of thermal insulation under the flange to ensure a secure and level attachment of the clip base to the underlying structure.