Abstract: Disclosed is a chimney access structure which is particularly well adapted for use in grounds subject to freezing. This structure incorporates chimney provided with an upper end on which is mounted an adaptor provided with a vertical through-hole and with a countersink hole of greater diameter that the through-hole. The countersink hole is preferably frusto-conical and extends coaxially within the through-hole at a given depth from the upper face of the adaptor. This countersink hole defines a sleeve with upper portion of the adaptor which is provided with an inwardly projecting flexible sealing ring. A floating head is slidably mounted into the sleeve. This head has a vertical through-hole coaxial with the one of the adaptor. The floating head also has a lower portion provided with an outer lateral surface whose shape and size are selected to fit into the countersink hole of the adaptor and sealingly slide within the same thanks to the sealing ring.