Abstract: A tool for use in marking large flat building material sheets, each sheet having a standardized length and a standardized width and having opposed longitudinal side edges to provide a cut line by which each sheet can be dimensionally shaped for installation on a building roof or gable the tool being an elongated bar providing at least one straight edge having a first pin affixed to the bar and extending perpendicular to it and a second perpendicular pin affixed to the bar, the spacing between the first and second pins being adjustably variable, the first and second pins being configured to slidably engage opposed longitudinal side edges of the building material sheets so that the bar straight edge extends at an angle to the longitudinal side edges, the straight edge providing an edge for marking the sheets to provide a guideline for an angle cut, the angle being selectively determined by the spacing between the pins.