The Problem With Splash Blocks
A 1,500 sq ft roof sheds roughly 935 gallons of water during a 1-inch rain event. Most homes have 4–6 downspouts, each handling a portion of that load. A splash block discharges that water 2–3 feet from the foundation. Even with a 6" per 10 ft positive grade — the building code standard — water discharged 2–3 feet from the foundation saturates the first 2–3 feet of soil before it moves away from the house.
In Chester County's clay soils, “moves away” is optimistic. Clay holds water in place for hours or days. During a multi-day rain event, the soil within 5 feet of your foundation is perpetually saturated — exactly the condition that causes basement moisture and drives hydrostatic pressure against foundation walls.
What Underground Downspout Drainage Does
Underground downspout drainage carries roof water away from the foundation entirely — not just a few feet away, but to a pop-up emitter at least 10 feet from the house, or further if site conditions allow.
The system consists of:
- A downspout adapter that connects the above-grade downspout to underground pipe
- Solid (not perforated) pipe — typically 4" Schedule 40 or SDR-35 — running underground to the discharge point
- Cleanout access points for future maintenance
- A pop-up emitter at the discharge point, which opens under flow and closes when dry
The result: roof water that was saturating soil at the foundation is now discharging safely away from the house.
How Far Is Far Enough?
The minimum standard is 10 feet from the foundation. In Chester County's clay soils, further is better. JHL Drainage Solutions designs discharge points based on:
- Available run length on the property
- Grade between the downspout and the discharge point (slope is required for gravity flow)
- Location of underground utilities and hardscape
- Whether discharge to a dry well, swale, or daylight outlet is appropriate
For properties where 10 feet isn't achievable (small lots, existing hardscape), dry well infiltration chambers can absorb roof water volume underground rather than discharging at the surface.