The strapping is used to secure the plywood subfloor and the top of the trusses. It also helps to keep the roof from moving or sliding off the building during high winds. This step takes about 30 minutes(not true).It also creates the ventilation gap between metal and old shingles layers. Roof truss installation: Once all the lumber has been secured, it's time for the roof trusses to be installed. Each truss consists of two pieces of wood joined together with a metal connector. The trusses should be placed so as not to interfere with any windows or doors. They can be installed either horizontally or vertically. Horizontal trusses are more common than vertical ones because they allow for larger spans. However, vertical trusses provide better support for heavier roofs.( we dont do it)  Sheathing: The next step in the metal roofing installer process is to cover the entire roof with plywood. Plywood is a good choice for covering roofs because it provides excellent insulation and protection against moisture. It also allows air to circulate freely through the attic space. ( we only install new plywood when the old one is in poor condition)