As asphalt roofing felt made of a specially formulated organic sheet which has been fully asturated with waterproofing asphalt. This product is strong, flexible, and impervious to water penetration, but will allow water vapour to pass through at a restricted rate. Preconditioned to help it lay flat and remain stable after installation on a roof deck. Guidelines are provided every 2" across the sheet to aid in roofer in keeping asphalt shingles aligned.