A mechanistic approach for pavement and overlay design is based on stress and analysis of layered structures. Deflection analysis is a valid instrument in determining the elastic parameters of soils and pavement layers. Deflection depends on the load stress and climate factors, such as moisture and temperature. A large study analysing deflection sensitivities to drainage, as well as climate factors, was undertaken. Different structures, specially regarding bituminous thickness, in different seasons were evaluated. Deflection regression models linked to seasonal climatic factors were developed. Both overlays and new pavements were analysed. In fact, the main conclusion focuses on the influence of drainage conditions, as well as climate factors, on deflection values.