PURPOSE: To provide the required physical properties such as waterproofness and ventilation performance by forming an under-laid member for roof by mixingly weaving the thick fibers and extremely fine fibers, within a specific range of fineness of the single fiber made of the hydrophobic thermoplastic resin.

CONSTITUTION: As an under-laid member for roof, is used a nonwoven fabric which is formed by mixingly weaving the thick fiber having a fineness of the single fiber of hydrophobic thermoplastic resin of 10-150μm and the extremely fine fiber having a finess of 0.01-10μm. Each range of water pressure resistance of 300mmH2O, humidity transmission resistance of 0.01-1.00mmHg.Hr.m2/g is selected Further, the mixing weaving ratio between both is obtained in a SB method fiber of 10-90wt.% and an MB method fiber of 90-10wt.%, and the water pressure resistance can be maintained sufficiently high and the high humidity transmission performance are provided by setting a thickness of 0.1-3mm and a 'METSUKE' quantity of 50-300g/m2. Accordingly, moisture passes through the upper and lower surfaces on the whole surface of the under- laid member, with the extremely small humidity transmission resistance.