A smoke or fire detector is described for mounting on a ceiling. A first part (110) is for mounting on the ceiling and a second part (110) houses a detector. The second part has a top-plate (116) with an annular recess. Electrical connections are arranged above the recess. A pair of drains (111) are provided through a surface of the recess, with conduits (113) extending downwards from the drains. The annular recess directs condensation to the drains (111) and down through the conduits (113), away from the electrical connections. In use, a differential air pressure between a ceiling void above and a lower surface of the smoke or fire detector provides a stack effect to assist drainage of condensation.