Evaluating the physical properties of potential green roof growth-media compositions for use in the Nigerian built environment

Abstract

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The primary objective of this paper is to examine the physical properties of potential green roof growth-media compositions practicable for use in the Nigerian built environment. The study carried out an essential selection of material constituents of growth media blends mixed in a 3:1:1 ratio of natural stone-based gravels, soil and compost. Six substrate blends based on laterite stones, sandstone, granite, river gravel, pumice and recycled masonry debris were studied using relevant laboratory and empirical field evaluation methods. The granite-based blend is the heaviest sample with 1,713.30 Kg/m3 in its saturated state, while the lightest in weight is the pumice blend with 869.30Kg/m3 which is 50.7% less than the granite blend. The result revealed that up to 50-150mm green roof thicknesses can be obtained using the pumice blend on a light-weight construction, while 50-100mm thicknesses can be attained using the debris-based blend. In the case of other alternative blends, however, only the 50mm-thick extensive green roofs can be obtained without special structural considerations. After a one-year physical observation of all the sampled models, the plants remained in good condition with no form of deformation, clogging and leaching of the substrate. Therefore, the study submits that all the selected stone-based growth media blends are suitable for use locally. It also established that the pumice and debris-based substrates are the lightest in weight, and are hence more suitable for retrofitting and other remodelling exercises. The study creates an avenue for further research on ways to optimise the studied blends in a bid to enhance their performance and improve on the benefits the green roof system stands to offer locally.