Check them with a framing square or measure so the lines are parallel with the rake ends.
Roofing using half tab method.
Three tab shingles are most typically installed with a half tab offset that yields the characteristic brickwork appearance.
Mark the center of each section and use a compass to mark 30 degree angles along the sides of each.
This article discusses best roofing practices for the installation of asphalt roof shingles including installation temperature shingle sealing methods shingle course offsets asphalt shingle nailing location and specifications low slope and steep slope limits for asphalt shingles and roof flashing at building eaves and around skylights.
Use a heavy duty utility knife to separate the shingles into thirds by cutting them at each tab.
The simplest way to do this is to use the half tab offset method.
Set the first shingle of the second row back a half tab 6 inches 17 centimeters from the edge of first shingle on the first row and so that the bottom of its tabs just touches the tops of the cutout slots on the shingle below.
This 1 2 tab has to be cut off where it hangs off the left edge of the gable roof.
For a racking or midroof pyramid layout method snap two vertical lines called bond lines near the center of the roof the appropriate distance apart.