Abstract: A cavity floor is formed by placing on a structural floor bottom a flexible floor mold which has leg forming portions arranged in an array with generally planar portions extending between and surrounding the leg forming portions. A flowable substance is applied over the mold, which subsequently hardens to form an upper floor having a plurality of legs defined by the leg forming portions, with a cavity region therebetween. The planar portions of the flexible floor mold permit adjacent leg forming portions each separately to contact the floor bottom under the weight of the applied flowable substance, even if the floor bottom is uneven.