Cut slopes in rock masses start to deteriorate directly after excavation due to stress relief and weathering. The deterioration is a time dependent process that depends on the local climate and the rock mass including its history, and the environment. The amount of deterioration per time unit (‘the weathering intensity rate’) is not a constant over time, but is for most rock masses larger when the mass is less weathered and becomes smaller with further progressing weathering. A study has been carried out to establish the relationship between weathering intensity rate and exposure time for the intact rock strength (IRS) of sandstone in humid tropical areas. The data set for the study was collected in and near Kota Kinabalu, Sabah, Malaysia, which has a humid tropical climate. The geology in the area consists of thick sequences of sandstone and shale beds of the Crocker Formation. Results show that the best relationship between intact rock strength (IRS) and exposure time (t) is by a logarithmic function; IRS (t) = 105 − 34 log (1 + t). This relationship can likely be used for prediction of the intact rock strength development of similar sandstone over the engineering lifetime of man-made slopes in tropical areas.