Sedum species, which utilize crassulacean acid metabolism (CAM) photosynthetic pathway, have been the most commonly used plants in extensive green roofs. Previous investigations on Sedum green roofs have focused on their establishment and survival under drought conditions. Several Sedum species have been described as “inducible type of CAM” plants, suggesting that their photosynthetic and transpiration rates under wet conditions might greatly affect the carbon sequestration and the cooling effect of a Sedum green roof. Therefore, we investigated the photosynthetic and transpiration rates of three Sedum species (Sedum japonicum Siebold ex Miq., Sedum lineare Thunb., and Sedum sarmentosum Bunge) under wet and dry treatments. All three Sedum species exhibited CO₂ uptake only during the light period under wet conditions as well as under dry treatment for several days after irrigation. Moreover, in the wet treatment, their photosynthetic and transpiration rates were found to be similar to those of the other green roof plants, Zoysia matrella (L.) Merr. and Ophiopogon japonicus (Thunb.) Ker Gawl. Thus, we suggest that the carbon sequestration and the cooling effect of the Sedum green roofs are equivalent to those of the green roofs utilizing the other plant under wet conditions from the standpoint of physiological traits.