The created methods and the logical scheme of an alternative selection based on it, allow for the adoption of decisions that optimally satisfy the technical parameters of roof repairs and the financial possibilities of a builder in each concrete case. It is here suggested that one selects a rational version of roof repairs in a step-by-step way by assessing the alternatives with the help of multi-criteria decision methods. Each stage has pre-selected assessment criteria, decision methods and created database. The initial stages include the following steps: the assessment of the condition of the roof following the developed methods, the expected price of the work and preliminary selection of alternatives. By using the method of mathematical analysis – efficiency value analysis – the most economically and technologically efficient option of thermal renovation was determined. The reconstruction of non-usable flat roof is optimal in technical, economic and humidity exposure terms. The price of this roof construction is not the minimum price, but duration of construction work, resistance to humidity exposure are optimal.