Roofing underlayment using a pressure sensitive adhesive and methods for making and using the same

A roofing underlayment capable of adhering to a roof deck comprising: (a) a roofing membrane having a first and a second major surface; and (b) a pressure-sensitive adhesive disposed on the first major surface of the roofing membrane and comprising: (i) at least one of butyl rubber or polyisobutylene; (ii) a first liquid plasticizer, preferably polybutene; and (iii) a tackifier, wherein the Tg of the adhesive is at most about 100 C. The adhesive provides high bond strength and excellent long-term heat aging, weathering resistance, as well as good low temperature properties while providing a moisture proof seam. A method for making the roofing underlayment includes applying the adhesive to the membrane then applying a release liner over the adhesive layer. A method for using the roofing underlayment comprises removing the release liner then adhering the underlayment to the roof deck by contacting the first to the roof deck.