If we have to sum it up in a single rule, gutters need a downspout around every 30 to 40 feet. For most homes, this means installing spouts at either end of a gutter section, but there are many more important factors that can change this broad rule. They include:
– Roof area. The more roof surface above the gutters, the more rainwater will be caught and funneled down into those gutters. A large roof is going to move a lot of water, which means the downspout locations may need to be adjust to compensate.
– Roof slope. A high slope moves water more quickly, and if gutters have trouble dealing with a lot of water in a short period of time, an extra downspout can alleviate that issue.
– Gutter size. Gutters come in many sizes. Large gutters can hold more water and may not need as many spouts. Smaller gutters may need extra downspouts to make up for their low volume.
– Climate. Heavy rainstorms put a lot more pressure on gutters. Particularly wet climates could require better drainage systems than relatively dry climates (rainy weather also makes filters and leaf-catchers more important).
These factors are very important to help solve overflowing problems.