Weight and structural considerations of potential green roof growth: Media compositions for the Nigerian building industry

Murtala Muhammad Salihu, Muhammad Aminu Musa, Abdullahi Getso Ibrahim, Jamilu Usman, Abubakar Sadiq Salisu

Cite this article
Salihu, M.M., Musa, M.A., Ibrahim, A.G., Salisu, A.S. (2024) ‘Weight and structural considerations of potential green roof growth: Media compositions for the Nigerian building industry’, Architecture Papers of the Faculty of Architecture and Design STU, 29(2), pp. 24-29. https://www.doi.org/10.2478/alfa-2024-0009



Green roof technology is still in its incipient and exploratory stage in the Nigerian built environment industry. It is a technology that is yet to be locally embraced due to a lack of adequate awareness of its benefits, limited technical knowledge and high initial and/or maintenance costs of the system. Most importantly, the green roof system is technically associated with problems that are centred on the characteristic weight of its growing media (substrate). This is regarded as the most critical and challenging aspect of a green roof project that must be considered to avoid the ultimate failure of the primary roof system. Although the International Building Code (IBC) has stipulated that green roofs are computed as live loads calculated based on saturation of the soil and shall be within the range of 0.958kN/m2, studies have shown that values and attributes of green roofs are location-specific and each scheme must therefore be considered as a distinct case from one setting to another. This study hence becomes a necessary platform for evaluating the weight implication of the outlined potential green roof growth-media compositions in Nigeria for subsequent reference and possible adoption.

The approach adopted for the study involves laboratory procedures and experimental field observation. Guided by pertinent literature review, the most available and appropriate natural stones in the Nigerian building industry were evaluated, the stones outlined for the study include laterite stones, sandstone, granite, river gravel, pumice, and recycled masonry debris. The geometry selected for this study is a classroom block designed under the MDG (Millennium Development Goals) program for public primary school education in Nigeria. The roof design of the project is a typical reinforced-concrete flat roof which is the most suitable for use in green roof systems due to the large load-bearing capacity it can withstand. To avoid a cumbersome presentation of the load analysis for every extensive green roof model, relevant load calculations were limited to the heaviest and the lightest green roof alternatives to represent the embodiment of the best- and worst-case scenarios to guarantee adequate and efficient sampling steps required to reach any theoretical saturation. Using BS8110, the study was focused on determining the compressive strength (fcu), minimum yield strength (fy), depth (d) and the resultant design load of the primary roof structure against the density of the composite nature of the Green roof materials. The general green roof evaluation was finally tested for compliance with the IBC (2018).

The laboratory analysis revealed that the granite-based blend is the heaviest sample with 1,713.30 kg/m3 in its saturated state. River gravel blend and the laterite stones followed closely with 1,264.50 kg/m3 and 1052.20 kg/m3 respectively. The lightest in weight is the pumice blend with 869.30 kg/m3 which is a difference of 942.90 kg/m3 from the heaviest granite blend, implying that it is 50.7% lighter in weight; followed by the masonry debris blend with 1,115.90 kg/m3. A successive conversion was conducted to estimate the weight of the 50 mm, 100 mm, 150 mm, 200 mm, 250 mm and 300 mm-thick models in kg/m2. The granite blend medium recorded the heaviest values at 85.65 kg/m2 for the 50 mm and 513.90 kg/m2 for the 300 mm model, while the lightest in weight is the pumice with 43.50 kg/m2 for the 50 mm and 261.00 kg/m2 for the 300 mm. The masonry debris also recorded an encouraging figure at 55.80 kg/m2 for the 50 mm and 334.80 kg/m2 for the 300 mm model. The results show that most of the substrate blends satisfy the stipulations of both the FLL (2008) and the ASTM International (2014). Results from the structural analysis conducted on the heaviest sample (Granite substrate) and the lightest sample (Pumice substrate) showed that the saturated Granite substrate having a 0.951 kN/m2 design load falls within the stipulated range of the IBC, and can therefore be used in any extensive green roof project. On the other hand, the pumice blend, being the lightest substrate had a design load of 0.576 kN/m2. It therefore stands to offer an optimum alternative in green roof retrofitting projects for existing flat-roofed buildings.

In summary, the study concludes that all of the substrate compositions covered in the study involve materials that are readily available in the studied area and can be used with respect to their characteristic properties as presented in this study. The study therefore serves as a reference point for all stakeholders in the research and building construction industry in Nigeria and places with similar bearing in the need to develop and promote the use of green roofs as a mainstream feature of the built environment.

Keywords: lightweight construction, green roof, growth-media composition, substrate weight