We have delivered material to hundreds of French drain projects across DFW and Denver. The most common mistake we see is homeowners or inexperienced contractors using fill dirt where gravel belongs. Put fill dirt around a perforated pipe and you have buried a useless tube of plastic in the ground. Every layer in a French drain serves a specific purpose, and the material you choose for each layer determines whether the drain works for 20 years or clogs in 20 months.
Anatomy of a French Drain: What Goes Where
A French drain is a simple system with five components stacked in a specific order inside a trench. From bottom to top:
Layer 1 — Gravel bed (6 inches). Washed #57 stone lines the bottom of the trench beneath the pipe. This creates a reservoir that collects water even below the pipe's invert.
Layer 2 — Perforated pipe (4-inch diameter). Standard 4-inch perforated corrugated or rigid PVC sits on top of the gravel bed. Holes face down so water rises into the pipe from the gravel reservoir below.
Layer 3 — Gravel surround (to within 6-12 inches of grade). More #57 stone fills the trench around and above the pipe. This is the primary drainage zone. Water from the surrounding soil migrates through this gravel layer and enters the pipe.
Layer 4 — Filter fabric. Non-woven geotextile fabric wraps over the top of the gravel layer. This is the barrier that prevents the topsoil cap from migrating down into the gravel and clogging the system over time.
Layer 5 — Fill dirt or topsoil cap (6-12 inches). Clean fill dirt or topsoil covers the filter fabric and brings the trench back to grade. This is the only layer where fill dirt belongs in a French drain.
Why You Do NOT Use Fill Dirt Around the Pipe
Fill dirt contains clay particles, silt, and fine sediment. Those fines migrate into the perforations of the drain pipe and pack together over time. In DFW, where the native soil is 40-60% expansive clay, this happens even faster. The clay swells when wet, seals around the pipe, and your French drain becomes a buried clay plug.
Gravel is the only material that belongs within 6 inches of the pipe in any direction. The voids between gravel stones allow water to flow freely to the perforations. Fill dirt eliminates those voids. It is that simple.
Material Selection Guide
Around the Pipe: Washed #57 Stone
#57 stone is crushed granite or limestone screened to 3/4-inch to 1-inch diameter. The "washed" specification matters because unwashed stone carries fines that defeat the purpose of using gravel. Do not substitute pea gravel — it is too small and packs tighter than #57, reducing flow rate. Do not use river rock — the rounded shape creates fewer contact points and the pipe can shift over time.
Cap Layer: Fill Dirt or Topsoil
For the top 6-12 inches, you have two options. If you are covering with sod or seed, use topsoil. It holds moisture and nutrients that grass needs to establish. If the area will be covered with mulch, pavers, or a non-vegetated surface, clean fill dirt works fine and costs less. In DFW, fill dirt runs $10 per yard delivered and topsoil runs $17 per yard. In Denver, fill dirt is $15 per yard and topsoil is $22 per yard. All our prices include delivery with a 10-yard minimum.
Trench Dimensions and Slope
Standard French drain trenches are 12 inches wide and 18-24 inches deep. For severe drainage problems or areas with heavy clay saturation, go 18 inches wide and 24 inches deep. Wider trenches hold more gravel, which means more water storage capacity before the pipe even gets involved.
Slope is non-negotiable. The trench must drop at least 1% grade from inlet to outlet — that is 1 inch of fall for every 8 feet of length. For a 100-foot French drain, the outlet end must be at least 12 inches lower than the starting end. In flat DFW yards, this often means the outlet discharges to a pop-up emitter or dry well rather than daylighting to the surface. In Denver, natural lot grades toward the street usually provide enough fall.
How Much Material You Need Per Linear Foot
For a standard 12-inch wide by 24-inch deep trench with a 4-inch pipe:
Gravel: A 12-inch wide trench filled with gravel from the bottom to within 8 inches of grade uses roughly 0.035 cubic yards of #57 stone per linear foot. For a 100-foot French drain, that is approximately 3.5 cubic yards of gravel.
Fill dirt or topsoil cap: The top 8 inches of a 12-inch wide trench uses about 0.02 cubic yards of fill per linear foot. A 100-foot drain needs roughly 2 cubic yards of fill dirt or topsoil for the cap.
For an 18-inch wide trench at the same depth, multiply both numbers by 1.5. That puts you at about 5.25 yards of gravel and 3 yards of fill for 100 linear feet. Use our material calculator to dial in exact quantities for your trench dimensions.
DFW Clay Soil: Why French Drains Are Critical
The Dallas-Fort Worth Metroplex sits on expansive clay soil that swells when wet and shrinks when dry. This seasonal movement cracks foundations, buckles driveways, and turns flat yards into standing ponds after every rain. French drains intercept subsurface water before it reaches your foundation and redirect it away from the structure.
In DFW clay, the filter fabric layer is especially important. Without it, the surrounding clay will infiltrate the gravel zone within two to three years. We recommend a minimum 4 oz/yd non-woven geotextile rated for soil separation. Wrap it over the gravel with at least 6 inches of overlap at the seam — do not just lay it flat across the top.
Denver Freeze-Thaw Considerations
In the Denver metro, French drains must be installed below the frost line or designed to tolerate seasonal freeze cycles. The frost depth in the Front Range averages 36 inches. A standard 24-inch French drain will freeze in winter. This is acceptable for most residential applications because peak drainage demand is spring snowmelt and summer storms, not mid-winter. However, if you need year-round function, trench depth should be 36 inches minimum, which increases gravel volume by roughly 50%.
Cost Breakdown for a 100-Foot French Drain
Here is a realistic material cost estimate for a 100-foot residential French drain with a 12-inch wide, 24-inch deep trench in DFW:
Gravel (#57 stone): 4 cubic yards — sourced locally, typically $35-45/yard picked up or $50-65/yard delivered.
Fill dirt cap (delivered): 2 cubic yards — but our minimum delivery is 10 yards at $10/yard in DFW. If you are also grading around the drain outlet, backfilling a utility trench, or leveling low spots in the yard, 10 yards goes fast. Topsoil at $17/yard if you are sodding over the drain.
4-inch perforated pipe: $0.50-0.80 per linear foot for corrugated, $1.50-2.50/ft for rigid PVC. Budget $50-80 for corrugated or $150-250 for PVC on a 100-foot run.
Filter fabric: $30-50 for a 6-foot by 100-foot roll.
Total material cost for a 100-foot DIY French drain in DFW runs $250-500 depending on pipe type and whether you use fill dirt or topsoil for the cap. Labor for professional installation typically adds $25-50 per linear foot.
Order Fill Dirt or Topsoil for Your French Drain Cap
We deliver fill dirt and topsoil across 80+ cities in DFW and 14+ cities in the Denver metro. Tandem trucks carry 10 yards, tri-axles carry 16 yards, and end dumps carry 18 yards. If you place your order before 10 AM, same-day delivery is available. Text us at (469) 523-6420 with your address and how many yards you need. Payment is through Zelle or Venmo. No hidden fees — the delivered price is the price you pay.