During the last decade massive wood constructions have become a popular building technique in the Nordic countries. Reasons for this are many, but it is generally accepted that use of wood in buildings helps to promote a sustainable development of the society and extensive use of wood indoor have been claimed to promote a good indoor environment. The architects seem to find this new technique interesting to work with, the flexibility of the systems is an advantage and craftsmen we have met have expressed a positive attitude working with massive wood due to its flexibility. The authors are involved in an ongoing project initiated by industrial parties producing and/or building with massive wood. One of the aims of the project is to develop and give guidelines on how to build well insulated, airtight and moisture safe roofs for different slopes and roof coverings in Nordic climate. The main focus has been on moisture performance. Whether a vapour barrier in massive wooden roofs is necessary is of great concern among architects, entrepreneurs and building owners involved in projects where massive wooden elements are extensively used. In order to give good answers to these questions, a study has been performed including hygrothermal simulations, laboratory measurements and measurements in a case project. Important parameters that are studied by simulations are the thickness of the wooden constructions, different insulation materials and different thicknesses of the insulation (e.g. necessary thickness by the outlet). The vapour permeability of wood is increasing with increasing relative humidity and moisture content. Vapour permeability measurements have been performed for various relative humidity levels. The influence of this has been studied in simulations. Moisture measurements in a building in the south of Norway have been performed. The building has roofs with and without a vapour barrier.