Sealed membrane comprising plates of a fluidtight material assembled in a sealed manner, the membrane comprising: - a first plate, a second plate and a third plate which are arranged on a support surface, each of the said first, second and third plates (1, 2, 3) comprising a longitudinal side (1a, 2a, 3a) and a transverse side (1b, 2b, 3b) which are orthogonal and meet in a corner zone (1c, 2c, 3c); the first plate covering a zone of the support surface which is adjacent to the transverse side of the third plate and comprising a transverse strip which extends along the transverse side of the said first plate and overlaps the transverse side of the third plate; the second plate covering a zone of the support surface which is adjacent to the longitudinal side of the third plate and comprising a longitudinal strip which extends along the longitudinal side of the said third plate and overlaps the longitudinal side of the third plate; the corner zone of the first plate and the corner zone of the second plate being positioned facing one another with a gap uncovering the corner zone of the third plate; - a first weld line extending continuously along the transverse strip of the first plate which overlaps the third plate, so as to assemble the first plate to the third plate; - a second weld line extending continuously along the longitudinal strip of the second plate which overlaps the third plate, so as to assemble the second plate to the third plate; - a fourth plate arranged on the support surface in such a way as to cover a zone of the support surface which is adjacent to the longitudinal side of the first plate and to the transverse side of the second plate, the fourth plate comprising a longitudinal side and a transverse side which are orthogonal and meet in a corner zone,, the fourth plate having a longitudinal strip which overlaps the longitudinal side of the first plate and an end portion of the first weld line and a transverse strip which overlaps the transverse side of the second plate and an end portion of the second weld line, the corner zone of the fourth plate overlapping the gap between the corner zone of the first plate and the corner zone of the second plate; the said sealed membrane being characterized in that the corner zone of the fourth plate has a convex curved portion and in that the said sealed membrane comprises a third, fillet, weld line extending continuously along the longitudinal side, along the corner zone and along the transverse side of the fourth plate so as to assemble the fourth plate with the first, second and third plates (1, 2, 3); the third weld line intersecting the first and second weld lines (6, 7). - a first plate, a second plate and a third plate which are arranged on a support surface, each of the said first, second and third plates (1, 2, 3) comprising a longitudinal side (1a, 2a, 3a) and a transverse side (1b, 2b, 3b) which are orthogonal and meet in a corner zone (1c, 2c, 3c); the first plate covering a zone of the support surface which is adjacent to the transverse side of the third plate and comprising a transverse strip which extends along the transverse side of the said first plate and overlaps the transverse side of the third plate; the second plate covering a zone of the support surface which is adjacent to the longitudinal side of the third plate and comprising a longitudinal strip which extends along the longitudinal side of the said third plate and overlaps the longitudinal side of the third plate; the corner zone of the first plate and the corner zone of the second plate being positioned facing one another with a gap uncovering the corner zone of the third plate; the first plate covering a zone of the support surface which is adjacent to the transverse side of the third plate and comprising a transverse strip which extends along the transverse side of the said first plate and overlaps the transverse side of the third plate; the second plate covering a zone of the support surface which is adjacent to the longitudinal side of the third plate and comprising a longitudinal strip which extends along the longitudinal side of the said third plate and overlaps the longitudinal side of the third plate; the corner zone of the first plate and the corner zone of the second plate being positioned facing one another with a gap uncovering the corner zone of the third plate; - a first weld line extending continuously along the transverse strip of the first plate which overlaps the third plate, so as to assemble the first plate to the third plate; - a second weld line extending continuously along the longitudinal strip of the second plate which overlaps the third plate, so as to assemble the second plate to the third plate; - a fourth plate arranged on the support surface in such a way as to cover a zone of the support surface which is adjacent to the longitudinal side of the first plate and to the transverse side of the second plate, the fourth plate comprising a longitudinal side and a transverse side which are orthogonal and meet in a corner zone,, the fourth plate having a longitudinal strip which overlaps the longitudinal side of the first plate and an end portion of the first weld line and a transverse strip which overlaps the transverse side of the second plate and an end portion of the second weld line,