To provide a sheet for reinforcing an under-roof material, having both of a high tear strength and high water repellency, and suitably usable by laminating the sheet with a roofing material.

This sheet for reinforcement, usable as a reinforcing material for the under-roof material for water-proof, impregnated or coated with asphalt is composed of a nonwoven fabric which is a filament nonwoven fabric composed of filaments having core-and-sheath structure comprising a sheath component composed of a polyolefin-based resin and a core component composed of a polyester-based resin having the melting point ≥20°C higher than that of the sheath component. The total value of tear strengths in the MD direction and the CD direction of the filament nonwoven fabric is ≥35 N, and the sheet for reinforcing the under-roof material has the water-repellency of <0.5 cm water absorption height measured by the Byreck method described in JIS L-1096.