Abstract: Roofing felt containing bitumen which is solid at ambient temperature and soft at an increased temperature is laid on roofs, the roofing felt having pieces with end edges and the method practiced by utilizing a heat emitting member in the form of a metal strip with a number of openings in it for penetration of softened bitumen. The metal strip is provided adjacent a first end edge of a first piece of roofing felt, and the first end edge of the first piece of roofing felt is overlapped with a second end edge of a second piece of roofing felt so that the metal strip engages both of them. A variable magnetic field is applied to the joint area to induce a current of sufficient strength to cause the metal strip to be heated to a temperature sufficient to soften the bitumen of both the pieces of roofing felt so that softened bitumen flows into the openings in the metal strip.