This work requires training and specialized equipment. The contractor you choose will first drill one or more 16mm (5/8 ″) holes as appropriate, where there is a drop. Thereafter he will install a spout on which will be attached the hose which will be used to inject the resin. Using a very powerful pump, he will then inject the polyurethane under pressure until the roof is full and the slab is at the same height as the rest of the slab. The foam used will withstand freeze / thaw cycles, mold and will not degrade over time. Once the work is finished, the holes will be sealed with mortar, making the repair almost invisible.