How much water
About 0.62 gallons fall on each square foot of roof for every inch of rain (Texas Water Development Board). A 2,000-square-foot roof in a 40-inch year sheds roughly 50,000 gallons, and the downspouts decide where every gallon lands. Your town's page gives its normal yearly rain.
The code's 5 feet
The International Residential Code, R801.3: "In areas where expansive soils or collapsible soils are known to exist, all dwellings and townhouses shall have a controlled method of water disposal from roofs that will collect and discharge roof drainage to the ground surface not less than 5 feet (1524 mm) from foundation walls or to an approved drainage system." An interpretation by the International Code Council reads the 5 feet as measured from foundation walls; it does not apply to a slab-on-grade foundation. Where the soils are ordinary, the code does not require gutters at all, but carrying water well clear of the house is good practice anywhere.
The options
- Elbow and extension: the least costly; moves water past the beds onto ground that slopes away.
- Splash block: spreads the flow where it lands.
- Buried line: a solid pipe to a pop-up outlet, a dry well or a legal discharge point. Tidy, but it clogs out of sight; ask for a cleanout.
Local rules
Do not discharge onto a neighbor's lot. Before running a line to a street, ditch or storm drain, ask the city or county: many have rules on where roof water may go, and some cities pay part of the cost of disconnecting downspouts from sewers or of rain barrels and rain gardens.