Semi-automatic or automatic method for detecting and flagging a leak in a roof or at least a part of roof of at least one office or apartment block, building or residence, said roof or part of roof concerned comprising, resting on a support, from the inside to the outside, at least: one insulating layer and one waterproofing layer, said method consisting in providing and implementing a system comprising: - i) a local detection and communication unit (Ui) for each roof zone (Zi) or part of roof to be monitored, each local unit comprising, on the one hand, an elongate sensor in the form of a contact sensor consisting of a ribbon incorporating two conductive lines (4"), and, on the other hand, a module for measuring the resistance between these lines (4") and for transmitting at least certain results of the measurements successively performed, after preprocessing, and - ii) a management platform configured to receive and process the messages from the modules of the different local units (Ui) managed by it, and to selectively trigger one or more predetermined actions based on the evaluation of the messages received and on the estimation of the presence or not of a leak, - each module incorporating software means for evaluating resistance measurements and estimating the rate of presence of water, these means triggering the sending of an alert message when said rate exceeds a predetermined threshold value, the lay configuration of each elongate sensor being adapted to the form of the zone monitored by the local unit considered, and the uplink communication between each local unit and the platform being performed by messages of identified origin, characterized in that - each module is autonomous, notably in terms of energy supply and of operation, - each sensor is installed under the waterproofing layer of the associated roof zone (Zi) and disposed under the insulating layer, by resting on the support or on a vapour barrier layer placed thereon, the corresponding module being housed in the thickness of said insulating layer or in a protection housing, specific or not, such as, for example, a skylight, - the local unit (Ui) considered performs repetitive measurements at regular intervals, and the messages of identified origin are transmitted unconditionally at maximum intervals, said uplink communication being achieved at least locally by a wireless transmission mode between, on the one hand, the module of the local unit (Ui) considered and, on the other hand, either a local relay terminal, or a base station or even a communication system linked to the Internet and present in the office or apartment block, the building or the residence concerned, or in a neighbouring construction. - i) a local detection and communication unit (Ui) for each roof zone (Zi) or part of roof to be monitored, each local unit comprising, on the one hand, an elongate sensor in the form of a contact sensor consisting of a ribbon incorporating two conductive lines (4"), and, on the other hand, a module for measuring the resistance between these lines (4") and for transmitting at least certain results of the measurements successively performed, after preprocessing, and - ii) a management platform configured to receive and process the messages from the modules of the different local units (Ui) managed by it, and to selectively trigger one or more predetermined actions based on the evaluation of the messages received and on the estimation of the presence or not of a leak, - each module incorporating software means for evaluating resistance measurements and estimating the rate of presence of water, these means triggering the sending of an alert message when said rate exceeds a predetermined threshold value, the lay configuration of each elongate sensor being adapted to the form of the zone monitored by the local unit considered, and the uplink communication between each local unit and the platform being performed by messages of identified origin, - each module is autonomous, notably in terms of energy supply and of operation, - each sensor is installed under the waterproofing layer of the associated roof zone (Zi) and disposed under the insulating layer, by resting on the support or on a vapour barrier layer placed thereon, the corresponding module being housed in the thickness of said insulating layer or in a protection housing, specific or not, such as, for example, a skylight, - the local unit (Ui) considered performs repetitive measurements at regular intervals, and the messages of identified origin are transmitted unconditionally at maximum intervals, said uplink communication being achieved at least locally by a wireless transmission mode between, on the one hand, the module of the local unit (Ui) considered and, on the other hand, either a local relay terminal, or a base station or even a communication system linked to the Internet and present in the office or apartment block, the building or the residence concerned, or in a neighbouring construction.