Drain Specifications
Material Estimate
Complete Materials List
How Much Gravel Do I Need for a French Drain?
The gravel you need equals the trench volume minus the space taken by the pipe. For a typical residential French drain — 50 feet long, 12 inches wide, 18 inches deep — you'll need around 2.6–3.0 cubic yards of gravel (about 3.7–4.2 tons). Add 10–15% for waste and settling, and always round up when ordering.
Gravel (ft³) = Trench Length × Width × Depth − Pipe Volume
Divide by 27 to convert to cubic yards. Multiply by density (1.4 for #57 stone) to get tons.
Ordering in bulk by the ton is usually cheaper than buying bags — but for small projects under 1 cubic yard, bags may be more practical since most suppliers have minimum delivery quantities of 3–5 tons.
How Much Gravel by Trench Size — Quick Reference
| Trench Length | Width × Depth | Gravel (yd³) | Gravel (tons) |
|---|---|---|---|
| 25 ft | 12" × 18" | ~1.3 yd³ | ~1.8 tons |
| 50 ft | 12" × 18" | ~2.6 yd³ | ~3.7 tons |
| 100 ft | 12" × 18" | ~5.3 yd³ | ~7.4 tons |
| 50 ft | 12" × 24" | ~3.5 yd³ | ~4.9 tons |
| 100 ft | 12" × 24" | ~7.0 yd³ | ~9.8 tons |
These figures use #57 crushed stone (1.4 tons/yd³) with a 4-inch pipe and 10% waste factor. Actual amounts vary slightly with pipe size and exact dimensions.
How Deep Should a French Drain Be?
Depth depends on what problem you're solving. For yard drainage and redirecting surface runoff, 12–18 inches is often enough. For foundation protection or capturing groundwater, dig 18–24 inches. In cold climates, the pipe should sit below the frost line — typically 36–48 inches — to prevent freezing. Deeper installations require more material but capture more water.
| Application | Recommended Depth | Width |
|---|---|---|
| Surface runoff diversion | 12–18 inches | 8–10 inches |
| Yard / lawn drainage | 18–24 inches | 10–12 inches |
| Foundation perimeter | 18–24 inches | 10–12 inches |
| Cold climate (below frost) | 36–48+ inches | 12+ inches |
What Type of Gravel for a French Drain?
Use 3/4-inch crushed stone — known as #57 or #67 — for the best balance of drainage speed and stability. Its angular shape creates large, consistent voids that let water flow freely while resisting compaction. River rock (#67) works similarly with a smoother finish. Avoid pea gravel (#89) as a primary fill — it compacts over time and slows drainage. Never use sand, topsoil, or any fine material that will migrate and clog the pipe perforations.
| Gravel Type | Size | Density | Drainage Rate | Cost |
|---|---|---|---|---|
| #57 Crushed Stone | 3/4" | 1.4 t/yd³ | Excellent | $35–55/ton |
| #67 River Rock | 3/4" | 1.4 t/yd³ | Excellent | $40–60/ton |
| #4 Stone | 1–2" | 1.3 t/yd³ | Very fast | $30–50/ton |
| #89 Pea Gravel | 3/8" | 1.5 t/yd³ | Moderate | $45–70/ton |
What Slope Does a French Drain Need?
The minimum slope is 1/8 inch per foot (0.125 in/ft), which is roughly a 1% grade. At this slope, a 50-foot drain drops just over 6 inches from inlet to outlet. The ideal slope is 1/4 inch per foot — this doubles the flow velocity and significantly reduces clogging risk. More slope is always better as long as you have the depth to work with.
A flat or back-sloped pipe holds standing water, which accelerates sediment buildup and creates conditions for mosquitoes and root intrusion. If the terrain won't allow proper slope, consider breaking the drain into multiple sections with intermediate outlets, or using a sump pump at a low point.
Worked Example: 50-Foot Residential French Drain
Trench: 50 ft long × 12 in wide × 18 in deep
Pipe: 4" Schedule 40 PVC, slope 1/8" per foot
Step 1 — Excavation volume: 50 × 1.0 × 1.5 = 75 ft³ = 2.78 yd³
Step 2 — Pipe volume: π × (0.1875 ft)² × 50 = 5.52 ft³ = 0.20 yd³
Step 3 — Gravel: 2.78 − 0.20 = 2.58 yd³ × 1.10 (10% waste) = 2.84 yd³
Weight: 2.84 × 1.4 = 3.98 tons of #57 stone
Pipe: 50 ft + slight slope extension = 5 pieces of 10ft pipe
Fabric: (1.5 ft depth × 2 + 1 ft width + 1 ft overlap) × 50 ft = 225 ft²
Total drop: 0.125 in/ft × 50 ft = 6.25 inches (outlet is 6.25" lower)
Material cost: ~$180 gravel + $75 pipe + $113 fabric = ~$368 materials
How to Install a French Drain — Step by Step
- Call 811 before you dig. Have underground utilities marked — gas, electric, water, and cable lines can be anywhere.
- Plan your route and outlet. Water must exit somewhere: a daylight point at a lower elevation, a dry well, or a storm drain connection (check local codes first).
- Excavate the trench. Dig to your target depth, maintaining consistent slope. Compact the trench bottom and remove roots and loose material.
- Line with geotextile fabric. Lay non-woven fabric into the trench with 12 inches of excess up each side. This prevents soil from migrating into the gravel.
- Add a gravel bed. Pour 2–3 inches of gravel on the fabric and check slope with a level or string line.
- Lay the perforated pipe — holes down. This is the most common mistake: holes should face downward, not up. Water rises through the gravel and enters the pipe from beneath; if holes face up they collect sediment.
- Fill with gravel. Cover the pipe with 6–8 inches of gravel, then continue filling to within 3–4 inches of the surface. Compact lightly in stages.
- Wrap and backfill. Fold the excess fabric over the top of the gravel. Add topsoil and reseed or sod as desired.
Common French Drain Mistakes
- Pipe holes facing up: Sediment accumulates in the perforations and blocks drainage within a few years
- No slope: Standing water in the pipe causes silting, odors, and root intrusion
- Skipping the fabric: Fine soil migrates into the gravel within 2–5 years, reducing drainage to near zero
- Wrong gravel: Fine gravel and sand clog; use angular 3/4-inch stone
- No outlet: A drain that has nowhere to go is just an underground cistern
- Too shallow in cold climates: Frozen pipe = no drainage when you need it most (spring thaw)
- Planting trees nearby: Roots will find the pipe within 5–10 years; maintain a 10-foot clearance
French Drain vs Alternatives
| Solution | Best For | Cost | Lifespan |
|---|---|---|---|
| French Drain | Persistent groundwater, foundation drainage | $20–60/LF installed | 30–40 years |
| Surface Channel Drain | Hardscape runoff, patios, driveways | $15–40/LF installed | 20–30 years |
| Dry Well | Downspout diversion, small areas | $300–1,500 total | 10–20 years |
| Regrading | Slope directs water away from foundation | $500–3,000+ | Permanent if maintained |
| Rain Garden | Ecological solution for manageable runoff | $300–2,000 | Indefinite |
Material Cost Reference (2024 National Averages)
| Material | Unit | Price Range |
|---|---|---|
| 3/4" Crushed Stone (#57) | per ton (bulk) | $35–55 |
| 3/4" Crushed Stone (bagged) | 50 lb bag | $4–7 |
| 4" Perforated PVC (Sch 40) | 10 ft length | $12–20 |
| 4" Corrugated Pipe (HDPE) | 10 ft length | $8–15 |
| Non-woven Geotextile Fabric | per ft² | $0.30–0.80 |
| Professional Installation | per linear foot | $20–60 |
DIY installation typically saves 50–70% versus hiring a contractor. The main costs are gravel (usually the largest line item), pipe, and fabric. Excavation is labor but requires only a shovel or rented trencher — a walk-behind trencher rents for $150–250/day and makes the job much faster.
Maintenance and Expected Lifespan
A properly installed French drain with filter fabric and the correct gravel should last 30–40 years. Annual maintenance is minimal: check that the outlet pipe is clear and unobstructed, and watch for any surface depressions over the trench that indicate settling. Every 2–3 years, flush the pipe from the inlet end with a garden hose to clear sediment. Avoid planting anything with aggressive root systems within 10 feet of the drain — willows, maples, and silver birch are particularly problematic.
Material quantities and costs are estimates based on standard installation practices. Local conditions, soil type, access difficulty, and permit requirements may affect your project. Always call 811 before digging and verify local codes before connecting to storm or sanitary sewer systems.
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