A wall panel 100 has an elongate hollow body portion 102 having a storage cavity 103 provided therein for retaining water. The body portion 102 has a rectangular cross-section defined by an inner wall 104 and an outer wall 106 held in spaced apart relation by two side walls 108a and 108b. Each side wall 108 is adapted to be joined to the side wall 108 of an adjacent wall panel 100' so as to form a wall tank 110 for a building. Each wall panel 100 thus forms a structural component of the wall 110, which is sufficiently strong to support a roof structure 112 thereon. The side walls 108 are provided with an opening 120 therein, adapted to align with a matching opening 120' in the side wall 108' of an adjacent wall panel 100', to permit the communication of water from the cavity 103 in one wall panel to the cavity 103' in the adjacent wall panel. The storage cavity 103 is bounded at the top by a lid 132 and at the bottom by a base tray 130.