To provide a method of mounting an under-roofing material, which employs a commercially available inexpensive under-roofing material but is free from a fear of leakage.

According to the method of mounting the under-roofing material, the under-roofing material is stuck to a sloped roof from an eaves side to a ridge side by overlapping an eaves-side end and a ridge-side end each other as overlapping portions, and staples are driven to the overlapping portions. On the other hand before non-overlapping portions are stuck to the roof, small pieces formed of the under-roofing material are superposed, and then the staples are driven to the non-overlapping portions from above the small pieces. Stapling is carried out to prevent the under-roofing material from being flapped due to wind especially at single-layered portions (non-overlapping portions), and since the single-layered portions are stapled after superposing the small pieces thereon, even if the weight of a person is loaded on the stapled portion to cause a break, the break is covered with the small piece like an umbrella, which eliminates a fear of leakage.