How to Install a French Drain System

A French drain is a simple, reliable way to move surface and subsurface water away from a building. Done right, it protects foundations, keeps basements dry, and stops patios, driveways, and garage slabs from heaving or cracking during freeze–thaw cycles. This blog walks through a full installation from planning to final cleanup, using common components like channel drains, 4-inch pipe, filter fabric, dimpled membrane, and graded aggregates. It also covers tying in downspouts, cutting a drain across an asphalt or concrete apron, and finishing joints with polymeric sand for a professional, long-lasting result.

Why Install a French Drain

• Foundation protection: Saturated soil against a foundation pushes water into cracks and joints. Repeated cycles lead to movement and structural damage, especially on block foundations.

• Basement and garage dryness: Managing exterior water keeps moisture from finding a path into conditioned spaces.

• Freeze–thaw resilience: In four-season climates, trapped water expands when frozen, lifting slabs, opening cracks, and loosening patches.

• Slope correction: When existing patios or grading slope toward the house, a French drain collects and redirects water to a safe discharge point.

The objective is to intercept water at the building edge, carry it at a controlled slope into a lower area or a designed dispersal point, and eliminate pooling next to the foundation.

Project Overview

The system described here combines:

• A linear channel drain across a garage or patio threshold to capture sheet flow.

• Solid 4-inch pipe to carry water away from the house before any infiltration occurs.

• Perforated 4-inch pipe with a filter sock further from the building to allow safe dispersal into the soil.

• Dimpled foundation membrane and geotextile to keep sediment out and create a stable, sloped “gutter” below grade.

• Clear stone (3/4-inch angular) to surround the pipe and maintain void space for flow.

• Limestone screenings to fine-tune elevations under channel sections and edge pavers.

• Polyurethane sealant at all non-threaded joints and penetrations.

• Polymeric sand to lock pavers and fill joints at the channel drain for a clean, durable finish.

• Tie-ins from downspouts using compatible fittings, routed into the main line.

The system directs water 12–20 feet away from the foundation to a lower grade or roadside ditch.

Tools and Materials

Drainage components

• Linear channel drain (modular sections with steel or poly grates, end caps, and side outlets/knockouts)

• 4-inch pipe: solid lengths and perforated lengths with filter sock

• 4-inch fittings: tees, 90s, couplers, downspout adapters, end caps

• Dimpled foundation membrane (e.g., DMX AG or similar)

• Non-woven geotextile or stabilization fabric (for trench and stone separation)

• Polyurethane construction sealant (exterior, black recommended)

• Protection tape sized for grate (for polymeric sand and concrete work)

Aggregates and setting materials

• 3/4-inch angular clear stone

• Limestone screenings (for screeding and bedding)

• Polymeric sand for top joints

Site prep and cutting

• Chalk line, wax pencil/marker

• Circular saw with diamond blade or quick-cut saw for asphalt/concrete

• Demolition tools (sledge, pry bars)

• Shovels, trenching spade, trenching shovel

• Wheelbarrow or compact loader if available

• Tamper (hand tamper and, if available, a plate compactor)

Layout and fastening

• Tape measure, string line, line level or box level

• Laser level (optional but ideal)

• Utility knife, hole saw, oscillating multi-tool for cutouts

• Concrete nails and washers or membrane fasteners

• Powder-actuated fastener (optional) for securing channel sections temporarily

• PPE: eye/ear protection, gloves, dust mask or respirator when cutting

Planning: Slopes, Routes, and Discharge

1. Survey the site

   • Identify low spots and existing grades. Water must be routed to a location meaningfully lower than the house—ideally a swale, ditch, daylight outlet, or an infiltration area placed at least 15–20 feet away so water does not migrate back to the foundation.

2. Choose the collection points

   • Across a threshold: Add a channel drain where sheet flow crosses a garage or patio edge.

   • Along a wall: Plan a trench parallel to the foundation. The trench becomes a below-grade “gutter” that conveys water to the discharge point.

   • From downspouts: Plan tie-ins so roof runoff enters the system instead of saturating the perimeter.

3. Design the pipe progression

   • Near the building: Use solid pipe so captured water travels clear of the foundation before any infiltration occurs.

   • Further away: Transition to perforated pipe with a sock to allow controlled leaching into the soil. If soil or slope is poor, route solid pipe all the way to daylight.

4. Confirm slopes

   • Aim for 1/8 to 1/4 inch per foot (1–2%) of consistent fall. Long runs with minor grade differences still move water if the channel and trench are set correctly.

5. Check crossings

   • Plan a safe path when crossing concrete or asphalt. The channel’s height, the finished paver height, and adjacent surfaces must all align for smooth transition and good flow.

Step 1: Demo and Access

• Remove sunken slabs, rotten patio sections, or obstructing features. Keep the area wide enough for trenching and material handling.

• Across a garage or driveway, snap chalk lines for your saw cuts. Cut wide enough to fit the channel drain and any side bedding (usually a couple of inches beyond the channel body).

• Separate asphalt or concrete carefully, pry sections free, and remove debris. Avoid damaging door stops and trim. If a door threshold or stop is loose or rotten, plan to clean, trim, or replace to achieve a flush finish.

Tip: Expect backfill settlement around foundations. Be ready to remove loose, organic, or “fluffy” material and rebuild with compacted aggregate.

Step 2: Trench and Subgrade Prep

Perimeter trench

• Excavate a trench along the foundation to the planned discharge location. Width should comfortably fit the pipe plus 4–6 inches of stone envelope on each side.

• Depth should accommodate membrane, clear stone, pipe, and top cover while preserving a consistent slope to the outlet.

Stabilization

• In loose soils or disturbed backfill, lay a stabilization fabric (non-woven geotextile) across the trench bottom. This prevents stone from punching into soft subgrade and keeps fines from migrating into the drain.

Membrane “gutter”

• Cut the dimpled foundation membrane lengthwise to match the trench width. Place it with dimples up to create a continuous airspace and keep the “gutter” open.

• Fasten the membrane to the foundation wall with approved fasteners or washers, keeping it taut. Wrap the remaining flap down into the trench to form a trough.

Add base stone

• Pour in enough 3/4-inch clear stone to support the pipe elevation. The weight of stone helps seat the membrane and makes it easy to create a precise slope with a level.

• For precision surfaces like channel drain beds, switch to limestone screenings to screed a perfectly flat, adjustable base.

Step 3: Dry-Fit the System

Channel drain

• Assemble channel drain sections on the surface. Many systems ship in 3–4 foot segments that snap together. Install end caps at the high end and plan an outlet at the low side or a side knockout.

• Dry-fit across the cut opening. Confirm the grate will sit slightly proud of surrounding surfaces so water doesn’t stall at the lip. Check with a long level for a gentle continuous slope toward the outlet.

Pipe runs

• Lay out solid 4-inch pipe from the channel outlet and from the foundation “gutter” to the discharge point.

• If the design transitions to perforated pipe, place that changeover well away from the foundation (e.g., 12–20 feet from the wall). Use socked perforated pipe in soil to resist silting.

Downspout tie-ins

• Locate downspouts and dry-fit tees or wyes so roof water enters the main line. Flexible adapters may help align to the channel or main trunk.

Test slope

• Before sealing anything, confirm falls with a level and/or laser. Adjust stone under channel and pipe to achieve the target 1–2% grade.

Step 4: Seal and Connect

Sealant at joints

• At channel connections, end caps, knockouts, and non-threaded pipe couplings, apply a continuous bead of polyurethane sealant. This reduces leakage into surrounding soils where it is not desired and prevents fines from washing into joints.

Threaded or twist-lock connections

• Many modern drain systems have twist-lock or threaded interfaces. Engage them fully until snug for a watertight fit, then back-up with sealant as needed.

Penetrations and edges

• Where membrane meets channel or foundation, close gaps and seal edges after fastening. The goal is to deter water from short-circuiting behind the drain.

Step 5: Set the Channel Drain Across the Threshold

• Screed limestone screenings as a firm, adjustable bed. This material responds well to gentle taps and allows fine height adjustments.

• Set each channel segment into the bed. Check for rocking at joints; if a section moves, lift and add or remove screenings to eliminate gaps.

• Use a hand tamper on the surrounding screenings to lock in the level. Do not hammer the grate directly; tamp the bedding beside it so the channel nestles into a tight seat.

• Confirm that from high end to low end the grate has consistent fall.

Protection before joint fill

• Apply grate protection tape sized to the channel to keep polymeric sand or mortar out of the drain during finishing.

Step 6: Install the Perimeter Pipe and Stone Envelope

• Place the solid pipe in the membrane trough near the foundation. Maintain slope to the desired outlet.

• If the route transitions to perforated socked pipe, switch after the line has carried water far enough away from the building.

• Surround pipe with clear stone on all sides. A minimum of 3–4 inches over the crown prevents crushing and ensures a free-draining envelope.

• Fold geotextile or the excess dimple membrane over the top stone where appropriate to keep fines out.

• Backfill the top with a decorative stone layer or topsoil as designed. Keep finished grade sloped away from the foundation.

Step 7: Tie In Downspouts

• Cut downspout pipes at the planned elbow height and install adapters to the 4-inch drain fittings.

• Use flexible connectors if needed to align without stressing gutters or elbows.

• Seal all joints. Support vertical drops so the drainage line is not carrying downspout weight.

Step 8: Finish the Threshold With Pavers and Polymeric Sand

• Build up shallow areas at the channel edges with limestone screenings.

• Set pavers or bricks flush with the garage slab and slightly proud of the channel to promote flow toward the grate.

• Hand tamp to lock pavers and screenings.

• Blow or sweep the area clean. With the grate taped, pour polymeric sand over the pavers and broom it diagonally into all joints and the channel edges.

• Remove excess sand and lightly mist with water per manufacturer instructions. Do not over-soak. Allow the sand to cure undisturbed.

Step 9: Test and Adjust

• Run a garden hose into the upstream end of the channel or into the tee. Verify water moves steadily through the system without pooling along the route.

• If a slow area is found, lift pavers or channel segments, add or remove screenings, and re-set. Recheck until fall is consistent.

Step 10: Backfill and Dress

• Trim membrane and fabrics to neat lines.

• Finish the trench top with decorative stone for airflow and maintenance access, or with soil and turf if specified, keeping slope away from the foundation.

• Replace door stops or trims and seal transitions as needed.

Key Design Choices and Their Reasons

• Solid pipe near the house: Carries water away before any infiltration can occur, reducing saturation against the foundation.

• Perforated pipe with sock away from the house: Allows safe dispersal into the soil once the water is far enough that capillary action won’t wick moisture back toward the foundation.

• Dimpled membrane “gutter”: Creates a controlled pathway that resists silt, keeps fines out, and maintains free-draining void space around the pipe.

• Clear stone envelope: Provides large voids for flow and prevents clogging compared with mixed gravel. Angular 3/4-inch stone locks in place.

• Limestone screenings under channel and pavers: Enables precise heights and smoothing without voids; easy to tamp and adjust.

• Polyurethane sealant: Offers durable, flexible waterproofing at joints and penetrations.

• Polymeric sand at the surface: Stabilizes paver joints, resists washout, and limits water infiltration around the channel edges.

Special Case: Cutting a Channel Across Asphalt or Concrete

• Snap lines and cut a clean trench wide enough for the channel body and bedding.

• Remove loose subgrade and rebuild with compactable aggregate.

• Bed the channel in screenings, not concrete, to allow future adjustment and reduce stress during freeze–thaw.

• Use protection tape on the grate; fill adjacent joints with polymeric sand after compacting pavers.

• Maintain a slight crown or positive fall from the garage slab to the channel and onward to the driveway to ensure quick drainage.

Freeze–Thaw and Four-Season Considerations

• Avoid trapped water: Continuous stone envelopes and solid-to-perforated transitions placed far from the foundation reduce heaving risks.

• Fill joints completely: Polymeric sand minimizes infiltration at the surface where tire loads and ice can pry gaps open.

• Seal edges and joints: Polyurethane at pipe and channel connections resists seepage that can freeze and expand.

• Use angular, clean aggregates: Rounded pea gravel shifts more and packs poorly; mixed gravel with fines locks moisture and can clog.

Troubleshooting

• Standing water in the channel: Recheck slope. Low spots in the screenings can be lifted and corrected. Confirm end cap locations and outlet orientation.

• Slow discharge: Inspect for crushed pipe, blocked fittings, or silted stone. Clean out access at channel if provided.

• Backflow during storms: Outlet may be submerged. Raise the discharge height if possible or lengthen the solid run to a lower point.

• Sediment in water stream: Add or repair geotextile wraps. Make sure socks are intact on perforated sections in soil.

Maintenance

• Keep grates clear of leaves and debris.

• Inspect downspout tie-ins seasonally.

• Rinse the channel in spring and fall; verify steady flow to outlet.

• Top up polymeric sand where needed after a few seasons, especially in high-traffic zones.

• After the first winter, check for settlement at paver edges or the channel bed and adjust screenings if required.

Safety Notes

• Wear PPE when cutting masonry or asphalt and when handling powdered sand.

• Use GFCI-protected tools and keep cords away from water.

• Follow local codes and call utility locating services before trenching.

• Respect slopes, trench walls, and equipment safety zones during excavation.

Cost and Efficiency Tips

• Rent by the week if using plate compactors, trenchers, or mini-loaders; daily rates add up quickly.

• Buy aggregates by the yard; delivery often costs more than the stone itself, so combine orders.

• Use modular channels that match available fittings; it simplifies assembly and future expansion.

• Pre-assemble dry to reduce rework once sealant is applied.

Example Layout Summary

1. Channel drain across garage opening, sloped slightly to the right.

2. End cap at the high end, outlet at low end feeding a 4-inch solid run.

3. Foundation “gutter”: dimpled membrane tied to the wall, dimples up, forming a trough filled with clear stone and a sloped solid pipe.

4. Downspout tie-ins at each end of the run using tees and adapters.

5. Transition to perforated, socked pipe beyond 12–20 feet from the foundation, where grade drops to a ditch.

6. Backfill with clear stone, fold fabric, and top with decorative stone or soil as specified.

7. Finish the channel edges with pavers and polymeric sand, remove tape, and test with a hose.

Frequently Asked Questions

How much slope is required?

Target 1/8 to 1/4 inch per foot. Even slight positive slope will move water if the line is continuous.

Can perforated pipe be used next to the foundation?

Use solid pipe near the house. Allow infiltration only when far enough away that moisture won’t return to the foundation.

Is filter fabric always necessary?

Use stabilization fabric in weak subgrades and to separate stone from native soils. Use a sock on perforated pipe to prevent fines entering the line.

What if there’s no lower area for discharge?

Create a dry well or infiltration basin far from the foundation. Size it to handle peak flows and surround with clear stone and fabric.

Will a channel drain survive vehicle loads?

Use a channel rated for vehicular traffic (check manufacturer class ratings). Bed it properly and support edges with compacted screenings or concrete as specified by the product.

Final Checks Before Calling It Done

• Continuous positive slope confirmed by hose test.

• All joints sealed; end caps installed where required.

• Downspouts routed into the system without leaks.

• Channel drain sits slightly proud; pavers are flush and locked with polymeric sand.

• Trench backfilled with clear stone; top dressed attractively; fabrics trimmed.

• Outlet is clear, stable, and not prone to submersion.

A well-planned French drain relocates the fight against water from your foundation to a safe part of your property. With solid pipe near the house, perforated pipe where appropriate, clean stone, sealed joints, and correct slopes, the system will keep working season after season.