The Ontario Building Code | Materials
(1) Except as required in Sentence (2), when underlay is used beneath shingles, it shall be,
(a) asphalt-saturated sheathing paper weighing not less than 0.195 kg/m², or
(b) No. 15 plain or perforated asphalt-saturated felt.
(2) Underlay used beneath wood shingles shall be breather type.