Drainage + Grading Principles Reference
Why this matters
Most basement flooding, foundation cracking, and crawlspace moisture problems trace to drainage failures around the home - gutters discharging at the foundation, soil sloped toward the house, downspouts terminating too short. The tech who reads these issues during any service call (HVAC, plumbing, roofing) can flag the actual cause behind chronic moisture complaints. This is the field card.
The principle: water away from the house
Water that lands on the roof + the area around the house must flow AWAY from the foundation. Every failure of this principle leads to moisture problems inside.
Three layers of water management:
- Roof drainage: gutters + downspouts capture roof water + discharge it away.
- Surface grading: ground slope sheds rain + meltwater away from the foundation.
- Subsurface drainage: perimeter drain tile (foundation drain) intercepts groundwater + carries it to daylight OR sump.
When any layer fails, the customer sees the symptoms inside.
Grading basics
Positive slope away from the foundation: ground around the house should drop at least 6 inches vertically over the first 10 horizontal feet from the foundation. About 5% slope. Visually: the ground 10 feet from the house should be visibly lower than at the foundation.
Negative slope (sloped toward the house): causes water to pool at the foundation + saturate the soil + create pressure against the foundation wall + drive moisture inside.
Flat grade: not catastrophic, but slow drainage. Water sits, soaks in slowly, increases moisture load.
The fix: regrade the soil. Add topsoil to slope away. Avoid covering siding (maintain 6" clearance from soil to siding to prevent rot + insect access).
Common grading mistakes
- Mulch piled against the house (rises over years; effectively raises grade against the foundation)
- Landscape beds level with foundation (no slope away)
- Backfill settled around foundation (creating a dish toward the house - common 5+ years after new construction)
- New patio OR walkway poured flush with foundation, sloped wrong
- Sprinkler systems with heads positioned to spray foundation (constant wetting)
- Air conditioner condenser pad sloped toward the house (refrigerant + condensate drip back at the foundation)
Downspout discharge
Downspouts should discharge:
- At least 4 feet from the foundation (further is better)
- In the direction of grade (away from the house, not crossing the house's foundation line)
- To a hard surface OR splash block (prevents soil erosion at the discharge point)
- OR connected to a buried drainage system (4" PVC carrying water 10+ feet from house, terminating at daylight, dry well, or storm drain)
Common failures:
- Downspout terminates at foundation (3-foot extension is the minimum; many have only the 6-inch elbow)
- Buried discharge clogged (grass, leaves, debris - water backs up under foundation)
- Downspout cracked / disconnected at top (water sheets down side of house)
- Splash block missing OR pointed wrong way
Gutter capacity + sizing
5" K-style gutter: typical residential up to ~ 600 sq ft of roof per downspout.
6" K-style: handles larger roofs OR steep slopes (more water reaches gutter faster).
Slope: 1/4" per 10 ft toward downspout. Visually nearly level; functionally enough to carry water.
Common capacity failures:
- Single downspout serving too much roof (overflow during heavy rain)
- Gutters overflowing at corners (capacity exceeded at low end)
- Gutters sagging from age (water pools, doesn't flow)
In storm-prone climates, the rainfall intensity (peak inches per hour during extreme events) drives sizing. Coastal Gulf areas can see 5 - 9 in/hr peak; Northeast typical 4 - 5 in/hr; arid Southwest 1 - 3 in/hr.
Foundation perimeter drainage
Hidden underground around the foundation footing in many modern homes:
- 4" perforated drain pipe (corrugated OR rigid PVC)
- In a bed of gravel
- Wrapped in landscape fabric (filter)
- Pitched to daylight (gravity discharge somewhere downslope) OR to a sump pit (pumped out)
Common failures:
- Pipe crushed during landscaping (homeowner planting trees nearby)
- Pipe clogged with fines that bypassed the fabric
- Daylight outlet covered by mulch/landscaping (water backs up)
- Sump pit pump failed
- Original install missed sections OR fabric not installed (fines clogged pipe within years)
Diagnosing a buried perimeter drain failure usually requires excavation OR camera scope through the cleanout (if accessible).
Sump pump systems
When perimeter drain can't gravity-discharge, water goes to a sump pit + is pumped up + out:
- Submersible pump in pit
- Float switch
- Check valve in discharge line (prevents backflow when pump stops)
- Discharge to daylight OR storm drain (NEVER to sanitary sewer; code violation most jurisdictions)
- Backup pump (battery-powered) for power outage scenarios
Annual service: clean the pit, test float, verify check valve, confirm discharge clear.
Yard drainage
Beyond the immediate foundation, larger-property drainage:
- Swales (shallow grassed depressions) that channel surface water away
- French drain (gravel-filled trench with perforated pipe, intercepting groundwater)
- Catch basin (vertical pit with grate, captures runoff, connects to drainage system)
- Dry well (gravel-filled pit collecting drainage, allowing infiltration)
- Storm drain connection (where municipal storm drains accept residential discharge - check code)
Common customer complaints traced to drainage
Wet basement only after heavy rain: surface water at foundation, OR gutter discharge issue.
Wet basement continuously: high water table; failed perimeter drain.
Settling foundation: soil erosion from drainage washout under the footing.
Soggy yard: drainage failure; either surface OR subsurface.
Mosquitoes / standing water: drainage pooling.
Cracks in concrete walkway / patio: soil settling underneath from drainage erosion.
Window-well flooding: well drain plugged OR missing (each well should have a perforated drain through the well floor to the perimeter drain).
Service-trade interactions
Plumber: sees sump pump + foundation drainage; can recommend service.
Roofer: sees gutters + downspouts; identifies overflow + capacity issues.
HVAC: sees condenser pad drainage + crawlspace moisture.
Landscape: actually executes grading work + drainage modifications.
The customer's "I have a wet basement" call may come to any of these trades; the cross-trade observation guides them to the right specialist.
When to refer
Most of what is in this article is observation and small correction: extend a downspout, clear a window-well drain, re-pitch a few feet of soil, service a sump. Past that line, hand it off. Drainage work that touches structure or moves water onto someone else's property carries liability a service trade should not absorb.
Refer to a structural engineer when:
- Foundation cracks are horizontal, stair-stepped through block, or wider than roughly the thickness of a credit card
- A wall is bowing or displaced inward
- Doors and windows across the house have gone out of square recently
- A footing has visibly settled or a slab has separated
- Anyone asks you to sign off that a foundation is "fine" after a drainage repair
Refer to a foundation or waterproofing contractor when:
- Water is entering through the wall itself rather than at a window well or slab joint
- The fix requires excavating to the footing, installing an interior drain tile loop, or applying exterior damp-proofing
- The perimeter drain is confirmed crushed or clogged and needs replacement
- A sump system needs to be added where none exists
Refer to a licensed civil or site engineer when:
- The regrade is more than shallow surface work near the house
- A retaining wall over about 4 feet is involved, or any wall carrying a surcharge (driveway, slope, structure above)
- The site is on or near a slope with any sign of movement
- Runoff would be redirected onto a neighbor's property, into a wetland, or across a shared easement
- The property is in a floodplain or under a stormwater management requirement
Refer to the municipality or utility when:
- Connecting anything to a municipal storm drain
- Public right-of-way, curb, sidewalk, or street is involved
- A sump discharge is currently tied into the sanitary sewer (a violation nearly everywhere, and the fix is permitted work)
Refer to a licensed plumber when: a buried line you find is not a drain at all. Perforated corrugated pipe and sewer lateral look similar from a shovel's-eye view, and a scope is cheaper than a repair.
What to do meanwhile: document what you saw with photos, tell the customer plainly what you can and cannot do, and give them the category of specialist to call rather than a vague "you need someone else." A homeowner told "call a structural engineer about the horizontal crack in the north wall" acts on it. One told "that looks serious" does not.
References
- IRC R401.3 (drainage adjacent to foundations)
- IRC + IPC drainage requirements
- Building Science Corporation foundation + moisture research
- IPC + UPC sump pump requirements
- Manuall internal: Gutter Components + Install, Common Window Types, Crawl Space Encapsulation