Every inch of rain on a typical roof sends hundreds of gallons of water down your gutters. If the downspout drops it right next to the house, that water soaks the soil against your foundation, and some of it finds its way into basements and crawlspaces.
The short answer
Most guidance says to discharge downspout water at least 4 to 6 feet from the foundation, and farther if your yard is flat or your basement has had water before. The ground should also slope away from the house. A common building-code standard is a fall of about 6 inches over the first 10 feet.
Signs your downspouts are too close
- A puddle or washed-out spot at the base of the downspout after rain
- Damp or stained basement walls on the same side as a downspout
- A musty smell in the crawlspace after storms
- Mulch or soil splashed up onto the siding
- Efflorescence (white, chalky deposits) on foundation walls
Ways to move the water
Extensions. The simplest fix. A rigid or low-profile extension attaches to the existing elbow and carries water several feet out. Low-profile styles lie flat, so they're easier to mow around and less of a tripping hazard than a big corrugated tube.
Splash blocks. Better than nothing, but they only move water a foot or two.
Buried drain lines. The cleanest look, but a bigger project, and they need to be kept clear.
Don't forget the gutters
An extension only helps if water actually reaches the downspout. Clogged gutters overflow straight down the wall, right where you don't want it. Clear them after the leaves drop. A telescoping gutter wand lets you do a single-story roof from the ground.
Downspouts are one piece of the puzzle. Grading, soil type and foundation condition all matter. Persistent basement water is worth a professional look.