An attachment arrangement for attaching a boiler casing to a membrane tube wall utilizes an extension plate which is connected to and extends beyond an edge of the casing. The extension plate and the surface of the casing adjacent its edge define an attachment area which is attached to the tube wall. A slot extends in the casing and through part of the extension plate. The slot is covered by an expansion element having a termination covering the end of the slot. The termination is on the extension plate. The termination of the expansion element extending over the extension plate provides compensation for expansion in the attachment area which avoids deformation and tearing of the casing in this area.