A positive air pressure attenuation device specifically adapted for use in a drainage system of a building for draining at least draining liquid material, the device comprising: - a housing including a structure for connecting the housing to the drainage system and being associated to a pipe with an inlet, an outlet, and an inner central portion located within the housing between said inlet and outlet, said inner central portion forming a channel for conducting draining liquid material from the inlet towards the outlet, said channel having a central axis; and - an elastic flexible inflatable reservoir associated to the inner central portion of the pipe, the reservoir being operable on a positive air pressure transient within the drainage system, said flexible reservoir defining a reservoir chamber with a variable inflatable volume, said flexible reservoir being at least partly made to withstand a positive air pressure transient of at least 5,000 Pa, advantageously comprised between 10,000 Pa and 2,000,000 Pa, preferably comprised between 10,000Pa and 500,000Pa, wherein the housing forms an external casing covering at least partly the flexible reservoir, whereby the inner central portion of the pipe has a wall portion extending within the reservoir chamber, said wall portion being provided with a plurality of openings forming passages between the channel and the reservoir chamber, said wall portion defining a total closed surface within the reservoir chamber and a total open surface between the channel and the reservoir chamber, whereby said plurality of openings comprises: (a) one or more than one, preferably two or more than two lower elongated openings located in the inner central portion adjacent to the outlet, said one or more lower elongated openings having a portion with an axis substantially parallel to the central axis of the inner central portion, and (b) one or more than one, preferably two or more than two upper openings, advantageously upper elongated opening(s), located in the inner central portion adjacent to the inlet, said plurality of openings defining a total open surface such that the ratio total open surface of the openings / total closed surface of the wall is comprised from 1: 100 to 1: 4, advantageously from 1:50 to 1:8, preferably from 1:40 to 1:10, characterized in that the external casing formed by the housing has at least one air passage way adapted for ensuring at least the portion of the flexible reservoir located into the housing to be in contact with atmospheric air during operation of the device. - a housing including a structure for connecting the housing to the drainage system and being associated to a pipe with an inlet, an outlet, and an inner central portion located within the housing between said inlet and outlet, said inner central portion forming a channel for conducting draining liquid material from the inlet towards the outlet, said channel having a central axis; and - an elastic flexible inflatable reservoir associated to the inner central portion of the pipe, the reservoir being operable on a positive air pressure transient within the drainage system, said flexible reservoir defining a reservoir chamber with a variable inflatable volume, said flexible reservoir being at least partly made to withstand a positive air pressure transient of at least 5,000 Pa, advantageously comprised between 10,000 Pa and 2,000,000 Pa, preferably comprised between 10,000Pa and 500,000Pa, wherein the housing forms an external casing covering at least partly the flexible reservoir, whereby the inner central portion of the pipe has a wall portion extending within the reservoir chamber, said wall portion being provided with a plurality of openings forming passages between the channel and the reservoir chamber, said wall portion defining a total closed surface within the reservoir chamber and a total open surface between the channel and the reservoir chamber, whereby said plurality of openings comprises: (a) one or more than one, preferably two or more than two lower elongated openings located in the inner central portion adjacent to the outlet, said one or more lower elongated openings having a portion with an axis substantially parallel to the central axis of the inner central portion, and (b) one or more than one, preferably two or more than two upper openings, advantageously upper elongated opening(s), located in the inner central portion adjacent to the inlet, said plurality of openings defining a total open surface such that the ratio total open surface of the openings / total closed surface of the wall is comprised from 1: 100 to 1: 4, advantageously from 1:50 to 1:8, preferably from 1:40 to 1:10,