To obtain an expandable joint which can prevent cracking in a protective concrete layer around the joint, and has excellent appearance, by erecting a cylindrical form around a roof drain so as to be spaced away from the same, and attaching a joint material onto the peripheral surface of the form.

After a waterproof layer is formed on skeleton concrete, a cylindrical form 2 is set around a roof drain 1 so as to be equally spaced away from the same. Next, a joint material 3 is attached onto the peripheral surface of the cylindrical form 2. Placing pressure of a protective concrete layer 4 is directly applied on the joint material 3, so that the joint material 3 is attached to such an extent that the joint material 3 is not separated from the peripheral surface of the cylindrical form 2, which leads to easy work and enhanced construction accuracy. After placing the protective concrete layer 4 on the periphery of the joint material 3, the cylindrical form 2 is removed, and a gap between the periphery of the roof drain 1 and the internal periphery of the joint material 3 is finished with mortar 5. The mortar 5 is circular with no corner portions in a plan view, and therefore has excellent appearance, which prevents cracking after hardening, to thereby obtain excellent surface finish of the joint.