Abstract: An expansion joint for a grain silo is sealed by a mesh-reinforced diaphragm having its peripheral edge-portions bonded to surfaces of the silo adjoining the joint to form a continous seal around it. The intermediate portion of the diaphragm has a flexible bulging portion protecting the joint and spaced from it. The diaphragm is made by covering the joint with a separating strip and applying a thick continuous layer of a viscous sealing agent over the exposed faces of the strip and the adjoining silo surfaces. The mesh is incorporated into the layer and has stiffly-resilient strands extending in the direction of flexing of the diaphragm. The layer cures to form the diaphragm which separates spontaneously from the separation strip the first time the silo expands, to form a bulging portion in the diphragm around the joint.