Some roof repairs, such as sealing leaky flashing or replacing a small area of asphalt shingles, are quite straightforward, until you add the challenge of working on the roof. If you're not comfortable with heights, call an expert.
Similarly, you'll need expert help if your roofing material is more complicated, such as tile, slate, metal or asbestos, if you don't have the ladders necessary to reach the roof, or if the roof is steeply sloped (rising more than 6 inches over 12 horizontal inches).
Always observe ladder safety measures, and use a safety line attached to a strong anchor (such as a secure chimney) when you're on the roof. Don't work alone; have someone on the ground who can call for help if you run into trouble. Most roofs aren't designed for walking, so try to step only where the rafters are.
Never venture out onto a roof when it's wet, because roofs can be incredibly slippery. Similarly, power-wash the roof, if needed, from the ground or a firmly secured ladder to keep moss at bay, which makes the roof slippery and can damage shingles. Trim any overhanging tree branches.
When power-washing roofs, use low pressure and test the spray in a small area first. If it loosens the asphalt shingle's granules, stop; it's going to create more problems than it will solve.