(57) A method of treating an underfelt comprising felted thermoplastic fibres which comprises heating at least one surface of the felt sufficiently to soften the fibres, and then passing the felt through polished callender rollers whereby to callender and quench the softened thermoplastic surface of the felt to give a glazed finish. Preferably, the rollers are polished stainless steel rollers, which conveniently may be water cooled to give the best effect. Advantageously, to give optimum heat transfer, the felt is passed around about 90° of the surface of the roller The nip of the rollers is used to give control over the initial pressure and final thickness of the felt, whereas the increased angle of wrap increases the heat transfer and quenching and thus the glazing effect on the softened thermoplastic fibre surface. The underfelts treated in accordance with the invention are particularly useful in two distinct areas. Firstly, if the surface is lightly glazed, achieved by modest application of heat before callendering, the felt, when in place on the floor, allows a carpet to move more easily when being fitted In a second application, where more heat and pressure is applied, the surface can be callendered further until it is highly glazed and is almost impervious. This enables the finished carpet underlay to be stuck to the floor and for the carpet to be stuck to the underlay.