Heavy rain can cause gutters to overflow for a variety of reasons. If your downspout or any part of the gutter system is clogged with debris or pests, water cannot flow properly. Another common cause is that the gutters are too small, and if they are clean but still overflow, it should be possible to repair them. It could also be that the downspouts are clogged with piles of leaves, or that the gutters are not tilted correctly, so water flows easily through the gutters to the downspouts.
A lack of tar can also cause water to accumulate in one place, rather than flow to an outlet. Heavy rain can also send leaves and debris to gutters, clogging them. Over time, gutters can sink due to excess rainwater, and if water falls in extreme volumes, it could cause an overflow. To prevent gutters from overflowing, invest in high-quality gutters and obstruction-resistant gutters for your home.
If you have checked for debris and you have the right size gutters in the right number, and you still have gutters that overflow, call a gutter installation professional in your area to investigate the problem.