· Michael Blair
A roof collects a very large volume of water and gutters concentrate it into a few points. Where those points discharge decides how much water reaches the clay against your foundation.
How far is far enough
Six to ten feet from the foundation is the usual guidance, further on a lot that slopes back toward the house. Splash blocks alone rarely achieve it.
The water also has to keep going once it lands. Discharging ten feet out into a low spot that drains back toward the house achieves nothing.
Buried extensions
Rigid PVC to a daylight outfall is the durable version. Flexible corrugated pipe sags, holds water, silts up and eventually blocks, and it is what most cheap installations use.
Include a cleanout and a proper pop-up or grate at the outfall, and check the fall is continuous rather than dipping.
- Six to ten feet minimum, more on adverse slope
- Rigid PVC rather than flexible corrugated
- Continuous fall to a daylight outfall
- Cleanout access at the top of the run
- Never into a footing drain or a sanitary sewer
On a flat lot
Where there is no lower ground, a dry well sized for the roof area is the option, though Middle Tennessee clay accepts water slowly and the well has to be sized for that.
Percolation is worth testing rather than assuming. A dry well in clay that cannot drain is a buried reservoir beside your house.
What it prevents
Saturated clay against a footing expands and contracts, which is the mechanism behind a great deal of Middle Tennessee foundation movement.
It is also the cheapest foundation protection available. Extending downspouts costs very little relative to any structural repair it helps avoid.