Climbing on a roof to accomplish necessary repairs is the single most dangerous job a homeowner faces. You can make the task safer. In all cases, stay focused on the task at hand and watch your footing. Make certain there are no electrical wires in the work area; wear shoes with rubber-gummed soles.
Attach a metal cleat at the top of the ladder. You can find this device in most hardware stores. The cleat has a crossbar and hooks. This goes over the roofline and secures the ladder from the top.
Position the ladder on the roof's slope.
Pre-drill holes in each end and the middle of the 1-inch x 30-inches piece of sturdy wood. Place this piece at the bottom of the ladder. Using the deck screws, secure this piece of wood through the shingles and into the wood decking. This will create a bottom cleat.
Remove the ladder carefully when finished with the repair work.
Repair the roof where the bottom cleat was positioned. To do this, remove the piece of wood and use a utility knife to break the seal of the damaged shingles. Pull up the damaged flaps. Place asphalt cement into the exposed holes using a caulking gun. Place a piece of flashing over the holes and top it with more asphalt cement. Press the shingles down over top of the flashing.