A releasable fastening comprising: a first fastening strip comprising an elongated, flexible base carrying an array of discrete fastener elements arranged in rows and columns, the array extending across a portion of a width of the base, each of the fastener elements having a resin stem extending from an upper surface of the base, and a lip (12,14,16) disposed at a distal end of the stem and overhanging the base; the upper surface of the base and the stems of the fastener elements together forming a contiguous mass of resin; and a second fastening strip configured to releasably engage with the first fastening strip, the second fastening strip comprising a flexible base with a field of fastener elements carried on a fastening side thereof, the field of fastener elements arranged to overlap with the array of discrete fastener elements of the first fastening strip, such that when the first and second fastening strips are brought into engagement the overhanging lips (12,14,16) of the discrete fastener elements of the first fastening strip cooperate with the fastener elements of the second fastening strip to releasably hold the first and second fastening strips in an engaged state; characterized in that the first fastening strip also comprises a longitudinally continuous rib supported by the upper surface of the base of the first fastening strip, the rib extending sufficiently far from the base of the first fastening strip to engage a sealing portion of the second fastening strip, and of a bending strength sufficiently low that the rib is placed in an elastically bent state, when the first and second fastening strips are in the engaged state, thereby forming with the sealing portion of the second fastening strip a non-interlocking barrier to resist flow across the fastening with the first and second fastening strips in the engaged state. a first fastening strip comprising an elongated, flexible base carrying an array of discrete fastener elements arranged in rows and columns, the array extending across a portion of a width of the base, each of the fastener elements having a resin stem extending from an upper surface of the base, and a lip (12,14,16) disposed at a distal end of the stem and overhanging the base; the upper surface of the base and the stems of the fastener elements together forming a contiguous mass of resin; and a second fastening strip configured to releasably engage with the first fastening strip, the second fastening strip comprising a flexible base with a field of fastener elements carried on a fastening side thereof, the field of fastener elements arranged to overlap with the array of discrete fastener elements of the first fastening strip, such that when the first and second fastening strips are brought into engagement the overhanging lips (12,14,16) of the discrete fastener elements of the first fastening strip cooperate with the fastener elements of the second fastening strip to releasably hold the first and second fastening strips in an engaged state; characterized in that the first fastening strip also comprises a longitudinally continuous rib supported by the upper surface of the base of the first fastening strip, the rib extending sufficiently far from the base of the first fastening strip to engage a sealing portion of the second fastening strip, and of a bending strength sufficiently low that the rib is placed in an elastically bent state, when the first and second fastening strips are in the engaged state, thereby forming with the sealing portion of the second fastening strip a non-interlocking barrier to resist flow across the fastening with the first and second fastening strips in the engaged state.