Achieving requirement quality is a crucial factor for the quality of the final product. Requirements quality depends also on the way functional and non-functional requirements are organized in the requirement documents. In fact, it is very important to distinguish these two types of requirements in order to guarantee traceability, completeness and lack of contraddictions. In particular, the identification and distinction between functional and non-functional reuqirements is a a key issue for well written requirement documents. In this paper we describe a methodology for facing these problems based on the extraction of non-functiona requirementsl. The possibility to create clustering of requirements according to a specific factor facilitates their analysis aimed at detecting possible inconsistencies and incompletenesses. The methodology has been made automatic by means of a tool able to build clusters.