With the severe thermal conditions on roofs, to secure water retention in porous concrete for use in planting is a paramount issue in plant growth. In recent years, numerous water-retentive polymers have been developed that can retain water in quantities hundreds of times heavier than their own weight. In this study, various properties needed for rooftop greenery system materials were examined and compared by using different existent absorbent polymers on thin-type porous concrete. The effects on vegetation growth of water-retentive polymers were also examined. The main results obtained are the following. 1) The water volume usable by vegetation was ranged from 62% to 87% of total water retention volume. 2) The water retention volume became constant in about eight hours. 3) The effect in vegetation growth was differed by type of water-retentive polymer. 4) The test results showed that acrylamide-acrylic acid polymer was the best water-retentive agent for use in greenery base porous concrete. And it was comfirmed that it almost had the performance similar to a lower water-retentive layer of porous concrete by the examination without sprinkling water.