There are four factors that determine if your roof is a good fit for solar:
Orientation: Ideally, you should have a south-facing roof. In the northern hemisphere, south-facing roofs maximize the amount of sunlight your solar panels collect. The more sunlight they collect, the more electricity they produce, and the quicker you can pay off your system. You can still mount solar panels to your roof if it faces due east or west, but the panels will produce less energy (about 75% of what a south-facing roof would produce). If you have a flat roof, the panels can be engineered to face due south no matter how your roof is oriented.
Shading: Once you’ve determined that your roof is oriented in the right direction, the next step is to ensure that your roof is not shaded. The portions of the roof where solar will be installed should be free of shade for most of the day, as shade can significantly reduce electricity production. Trees, chimneys, dormers, and HVAC vents are factors that can cause shading on a roof. If you’re not sure if your roof is shaded, your installer can use a tool called the “solar pathfinder” to figure out if trees or other objects will cast shade during the day.
Surface: Solar arrays are most efficient when they are installed in a large, uninterrupted space. Things like dormer windows, chimneys, vents, skylights, and air conditioning units can be obstacles to installing an array.
Durability: Finally, if your roof is more than 15 years old, you may want to consider replacing it before installing solar panels. Most solar vendors recommend using roofing material that will last as long as the system (minimum of 25 years).