Water in a roof can increase its conductivity and decrease its insulation efficiency. A system has been devised by Gammie Nuclear Service Company for detection of trapped water in roof layers. A portable unit is wheeled along the surface of a roof to take readings at points in a grid that has been superimposed on a scale drawing of the roof. The unit measures hydrogen ions, factoring out the hydrogen ion content in other materials in the roof, the thickness of the roof, gravel, and the R. H., etc. Results indicate values and location of trapped water. It is important to have competent interpretation of the measurements before planning repair and replacement of the roof. -- AATA