Failure to understand the principles appropriate to a particular roof makes it all too easy to introduce condensation problems, often serious ones. A distinction between surface condensation and interstitial condensation is made. Before attempting work on any roof it is necessary to determine how the roof is designed to work. If the principles are wrong, the whole design should be checked and if necessary corrected. Approaches to various types of roof are described: cold or warm deck roofs with vapour control layers or vapour barriers, flat, pitched or lean-to roofs, giving suggestions for reduction of risk of condensation.