An underlayment sheet (10, 10') is applied in overlapping courses to a roof deck (36) for establishing a weather-resistant membrane for use as a final exposed surface or below a roof covering (50). The underlayment sheet (10, 10') has a bottom surface (16, 16') to which a pressure sensitive adhesive section (22, 22') is applied generally continuously along its length. A non-adhesive section (24, 24') also extends the length of the bottom surface (16, 16') adjacent its upper long edge (18, 18') for providing a contact surface with the roof deck (36) which is devoid of any adhesive material. The non-adhesive section (24, 24') is attached to the roof deck (36) using tin tags (44, 44') or other mechanical fastening components. When subsequent underlayment sheets (10, 10') are applied in overlapping courses, the adhesive sections (22, 22') bond over the preceding underlayment sheet (10, 10') to establish a water tight seal. The top surface (14) of the underlayment sheet (10) may include a supplemental bonding strip (30) which adheres directly to the adhesive section (22) of an overlying underlayment sheet (10) to establish a tenacious seal. The top surface (14') may also be impregnated with a course granular material (52) to serve as a final, exposed surface for the roof in lieu of shingles or the like. If a future re-roofing operation is required, the subject underlayment system can be removed from the roof deck (36) without damaging or delaminating the roof deck (36) material.