Underlayment and also Steel Roofing-- Metal Roofing 101 with Todd Miller
https://wiki-velo.win/index.php/Steel_Roof_and_Home_Ventilation
We have a 1890's house with a mix Mansard design roof covering and typical sloped roofing system. We are selecting a mix roof covering (Malarkey Windsor Roofing Shingles on Mansard and standing joint on slope)