Grading and Drainage for a New Patio
A homeowner wants to install a 400 sq ft concrete patio (20 ft x 20 ft) adjacent to their house. The existing yard slopes toward the house at about 2% (0.25 in per ft). The soil is sandy loam with good drainage. The house has a basement, and the foundation drain is at the base of the footing. The client has noticed water pooling near the foundation during heavy rains. The patio will be at the same elevation as the house's sliding door threshold. The local code requires a minimum 2% slope away from the house for the first 10 ft. The budget is $8,000.
1. How would you regrade the site to ensure proper drainage away from the house and prevent water pooling, while staying within budget? Include calculations for slope and elevation changes.
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First, determine the required slope: 2% minimum away from house for first 10 ft. For a 20 ft patio, if it slopes away from the house, the far edge should be 2% * 20 ft = 0.4 ft (4.8 in) lower than the house edge. But the patio is at the same elevation as the sliding door threshold, so we need to ensure the patio itself slopes away. If the patio is 20 ft deep, slope it 2% away from the house: 0.02 * 20 = 0.4 ft drop. That means the far edge is 4.8 inches lower. The existing yard slopes toward the house at 2%, so we need to reverse that. We'll need to cut and fill: remove soil near the house and add fill at the far edge. Calculate volume: average depth change over 20 ft: from 0 to 4.8 in, average 2.4 in (0.2 ft). Volume = 400 sq ft * 0.2 ft = 80 cubic ft (about 3 cubic yards). But also need to consider the first 10 ft: slope 2% for 10 ft = 2.4 in drop. The remaining 10 ft can slope at 1% or less. To save budget, use a swale or French drain at the low edge to collect water and redirect it to a safe outlet. Budget: excavation and grading ~$2,000, concrete ~$4,000, drainage ~$2,000. Ensure the patio is at least 6 in below the threshold? Actually, the patio should be below the threshold to prevent water intrusion. Typically, a step down of 4-6 in is recommended. If the threshold is at the same elevation, we need to lower the patio. So the patio should be at least 4 in below the door threshold. That means we need to cut down the patio area by 4 in, then slope away. So the house edge of patio is 4 in below threshold, far edge is 4 in + 4.8 in = 8.8 in below threshold. That requires more excavation. Recalculate: cut 4 in at house edge, 8.8 in at far edge, average 6.4 in (0.533 ft). Volume = 400 * 0.533 = 213 cubic ft (7.9 cubic yards). That's more soil to remove. Budget may be tight. Consider a smaller patio or permeable pavers to reduce drainage issues. Also, ensure the swale or drain directs water away from foundation.
2. What drainage measures would you incorporate to handle the runoff from the patio and the existing yard, and how would you size them?
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Use a combination of surface and subsurface drainage. Surface: slope the patio away from house, and create a shallow swale along the low edge to convey water to a dry well or storm drain. Subsurface: install a French drain (perforated pipe in gravel) along the foundation and at the low edge of the patio. Size the French drain based on runoff: for a 400 sq ft patio plus contributing yard area, use the Rational Method. Assume C=0.9 for concrete, i=2 in/hr, A=400 sq ft=0.0092 acres. Q=0.920.0092=0.0166 cfs. For a 4-inch pipe, capacity is much higher, so a 4-inch perforated pipe is sufficient. But also consider the yard: if the yard drains to the patio area, add that area. Use a 4-inch pipe with 1% slope. Gravel trench: 12 in wide x 12 in deep, lined with filter fabric, filled with washed gravel, pipe at bottom. Direct to a dry well sized for 1-inch rainfall: volume = 400 sq ft * 1/12 ft = 33.3 cubic ft. Dry well should be at least 33 cubic ft (e.g., 4 ft diameter x 3 ft deep = 37.7 cu ft). Ensure overflow to daylight or storm sewer.