A modular assembly for fully enclosing a site during construction of a structure, the modular assembly comprising: a plurality of portal frames, each portal frame comprising two columns and a roof beam, each column and roof beam comprising a plurality of pre-assembled units; a jack-up frame for each column; at least two longitudinal first rails, each of the first rails being mounted to at least one of the roof beams; at least one transverse second rail slidably mounted to the at least two first rails; and a plurality of panels of a flexible material configured to provide a full enclosure of the site and mounted above the roof beam; wherein, the or each second rail is configured to slidably receive at least one lifting device; wherein, the pre-assembled units are configured to be connected on site, and lifted using said jack-up frames, to form the portal frames, and wherein each first rail, and the or each second rail are configured to be mounted on the portal frames on site; the modular assembly characterized in that it further comprises at least one edge rail system mounted adjacent an edge of the assembly, the or each edge rail system comprising: at least two longitudinal edge third rails (702, 704) each of the third rails (702, 704) being mounted to at least one of the roof beams; at least one transverse edge fourth rail slidably mounted to the at least two third rails (702, 704) to form an edge rail system and configured to slidably receive said at least one lifting device, wherein said at least one fourth rail is configured to be aligned with said at least one second rail to enable transfer of said at least one lifting device between the at least one second rail and the at least one fourth rail. a plurality of portal frames, each portal frame comprising two columns and a roof beam, each column and roof beam comprising a plurality of pre-assembled units; a jack-up frame for each column; at least two longitudinal first rails, each of the first rails being mounted to at least one of the roof beams; at least one transverse second rail slidably mounted to the at least two first rails; and a plurality of panels of a flexible material configured to provide a full enclosure of the site and mounted above the roof beam; wherein, the or each second rail is configured to slidably receive at least one lifting device; wherein, the pre-assembled units are configured to be connected on site, and lifted using said jack-up frames, to form the portal frames, and wherein each first rail, and the or each second rail are configured to be mounted on the portal frames on site; the modular assembly characterized in that it further comprises at least one edge rail system mounted adjacent an edge of the assembly, the or each edge rail system comprising: at least two longitudinal edge third rails (702, 704) each of the third rails (702, 704) being mounted to at least one of the roof beams; at least one transverse edge fourth rail slidably mounted to the at least two third rails (702, 704) to form an edge rail system and configured to slidably receive said at least one lifting device, wherein said at least one fourth rail is configured to be aligned with said at least one second rail to enable transfer of said at least one lifting device between the at least one second rail and the at least one fourth rail. at least two longitudinal edge third rails (702, 704) each of the third rails (702, 704) being mounted to at least one of the roof beams; at least one transverse edge fourth rail slidably mounted to the at least two third rails (702, 704) to form an edge rail system and configured to slidably receive said at least one lifting device, wherein said at least one fourth rail is configured to be aligned with said at least one second rail to enable transfer of said at least one lifting device between the at least one second rail and the at least one fourth rail.