A portable neutron gauge is designed to detect water ingression in flat roofs and to measure with good accuracy the moisture content in the roofing materials. The gauge consists of a small 252Cf neutron source inserted in a collimator head made of borated paraffin contained in a steel vessel. Neutron detection is performed with a boron trifluoride detector and the associated electronic counting equipment. Experimental testing, calibration, and assessment are done in the laboratory using full-scale models of typical, Canadian-built flat roofs. Several experiments are conducted to determine the sensitivity of the gauge for various controlled water densities in the roofing insulation materials and for a large selection of geometries for the source and the detector with respect to the roof surface. Two different source strengths are used: 1.2 (0.5) and 4.8 MBq (2.0 µg). The results indicate that as little as 2% (volume) water can be detected and that the water content can be determined with an average accuracy of 2.5%, even with the smaller of the two sources. A small neutron gauge can indeed be designed for roofing surveys, and only a few straightforward modifications are necessary to make the gauge used in the laboratory into an apparatus that can withstand the rigors of field usage.