Thermoplastic polyolefin, or TPO, is a fabric-reinforced plastic membrane that’s reportedly more resistant to punctures than EPDM. It can be glued down (fully adhered), mechanically attached or ballasted. All seams must be sealed with a heat gun specially made for this purpose.
TPO is increasingly common as a roofing under roof-decks, but care needs to be taken particularly with inside corners at roof-wall intersections.
A prefabricated corner piece is heat-welded to all three surfaces.
Peel-and-stick membrane flashing adhered to the sheathing serves as a counterflashing. It will later be covered with housewrap.
For more about detailing TPO, as well as EPDM, built-up roofing and other low-slope roofing options, visit the JLC Field Guide.