The Ontario Building Code | Installation
(1) When used with shingles, underlay shall be installed parallel to the eaves with head and end lap of not less than 50 mm.
(2) The top edge of each strip referred to in Sentence (1) shall be fastened with sufficient roofing nails to hold it in place until the shingles are applied.
(3) The underlay referred to in Sentence (1) shall overlap the eave protection by not less than 100 mm.
9.26.7. asphalt Shingles on Slopes of 1 in 3 or Greater