A construction material in sheet form having at least two water barriers including a water-impermeable foamed intermediate layer, a mineral-based interior layer, and a prefinished, weatherable, water- impermeable non-asphaltic outer layer, with adjoining layers firmly adhered. Sheets of the construction material having a foamed intermediate layer and mitered edges exposing the foamed layers are joined by the steps of positioning the sheets with the mitered edges abutting opposite sides of the stem of a bulb-T rafter, the sheets being supported by the cross-member of the bulb-T rafter, forming a foam filler in the cavity created by the abutted mitered edges, and sealing the cavity with weatherable tape. A roof deck for use with bulb-T rafters, the roof deck constructed of the aforementioned sheets joined by the aforementioned method is also shown.
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