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Water leaking back into house from gutter

Please, I do not know if I need a builder/ roofer or both. I have had this problem where water during extreme rain, back flows into the kitchen through windows and gib. I have had it looked at and someone recommended new guttering and overflows. I have also had the concrete tiles checked, paid and had new guttering installed but the issue came back last night. It looks from the picture that a seal is required under the tiles to prevent backflow. Apparently this used to be done by plumbers and should be able to be completed COMPETENTLY by a roofer. Please can someone genuinely help?