Subsurface drainage field guide: French drains and catch basins

Subsurface drainage, and the water grading cannot move

Subsurface drainage is the piped half of a drainage system. It collects the water that surface grading cannot move on its own and carries it through buried pipe to an outlet. The grading sheds water off the building and across the yard. The pipe takes over where the grade runs out, where the water has gone underground, or where there is no room left to slope. One handles the sheet flow on top. The other handles the water that ponds, soaks in, or sits against the structure.

Reach for it when the grade alone has failed or cannot win. The yard that stays soft a week after rain. The low corner that ponds because the lot drains toward it and there is nowhere to push the water. The foundation that takes on water because the soil around it stays saturated. The spot at the bottom of a driveway where surface water concentrates and has no slope left to run on. In every one of those, the surface answer is used up, and the next move is to collect the water and pipe it somewhere.

The order matters, and it is the same order as the surface work. Grade to drain first. Get every surface you can to shed on its own, because a pipe costs more to install and it is one more thing to maintain. Then add the piped system only where the surface cannot do the job. The companion grading and slope guide covers the surface side, the foundation rule, the swales, and the slopes that move sheet flow. This guide picks up where that one stops, at the collected water and the pipe that carries it.

Surface or subsurface: which do you reach for?

Surface grading wins first, every time it can. It is cheaper, it has nothing to clog, and a graded slope works the day you cut it and keeps working with no parts. So the rule is grade to drain first and pipe only what the grade cannot move. A piped system that exists because nobody graded the site right is a maintenance bill the owner did not need.

Subsurface comes in for the cases the surface cannot reach. Water that is already underground, the high water table, the spring in a slope, the saturated soil against a foundation, gets collected by a French drain, because you cannot grade groundwater. Surface water that concentrates at a low point with no slope left, the sunken patio, the bottom of a ramp, the courtyard that traps water, gets a catch basin or an area drain, because there is no room to sheet it away. And the water that has to cross a hard surface, a doorway or a driveway apron, gets a channel drain, because you cannot put a swale through a slab.

The honest version is that most working sites use both. You grade what you can, then you pick up the rest in pipe. The split to remember is simple: if you can move the water with dirt and slope, do that, and save the pipe for the water that dirt and slope cannot touch.

What is a French drain?

A French drain is a perforated pipe in a gravel-filled trench, wrapped in filter fabric, sloped to carry water to an outlet. It collects water from the soil around it. Water in the saturated ground takes the path of least resistance, which is the open gravel, drops into the perforated pipe through the holes, and runs down the slope to wherever the pipe daylights. It is a groundwater and high-water tool first. It is the wrong answer for a sheet of surface water running across a lawn, which a swale or a catch basin handles better.

The build is straightforward and the numbers matter. Dig the trench to grade, line it with non-woven geotextile, lay a few inches of washed gravel, set the perforated pipe, cover it with more gravel, and fold the fabric over the top before you backfill. Common practice is washed 1/2 in to 3/4 in stone, at least 2 in of gravel under the pipe and 6 in or so over it, with the fabric overlapped at the top. The pipe slopes to the outlet, and the whole idea is that water moves through the gravel freely and the pipe carries it away.

Perforation orientation starts arguments. The case for holes down is that the drain pulls the water table down to the bottom of the pipe, because water enters at the lowest point and the pipe sits empty above that, carrying flow. That is the common recommendation for a true subsurface drain lowering groundwater. Holes up fills the pipe before it drains, which you sometimes want for a system meant to take a slug of surface water. For drying a wet yard or pulling a water table down, holes down is the usual call. Either way, the slope to the outlet is what makes it work. Slope it at least 1 percent, about 1/8 in per foot, and 2 percent if you have the fall to give. Flat pipe does not drain, it stores, and stored water in a perforated pipe is a silt trap.

Do French drains need filter fabric?

Yes, in almost every soil a French drain needs filter fabric, because the fabric keeps the fine soil out of the gravel and the pipe so the system does not silt up and quit. Fine particles in the surrounding soil migrate toward the moving water. With no barrier they pack into the voids between the stones, the gravel loses its open structure, the perforations blind over, and a drain that worked for two seasons stops draining and nobody knows why. The fabric is a separation layer. It passes water and holds back the fines.

Use a non-woven geotextile for this, not the woven weed barrier from the garden aisle. Non-woven needle-punched fabric has many small openings and a high flow rate, which is what lets water through while it captures silt. Woven fabric has fewer, larger openings and is built for separation and strength under load, not for filtering fines out of drainage. Line the trench, wrap the gravel envelope, lap the fabric over the top of the stone before backfill, and overlap the seams.

There is a real argument in the trade that fabric is a waste, that it clogs at the fabric face instead of in the gravel and chokes the drain that way. The kernel of truth is that the wrong fabric, a cheap woven sheet wrapped tight against the pipe, can blind off. The answer is the right fabric and enough gravel, not no fabric. In clean sand or pea-gravel soils some installers skip it. In silt and clay, which is most yards that have a drainage problem, skip the fabric and you will be digging it back up. The soil you actually have decides it.

What is a catch basin?

A catch basin is a box set into the ground at a low point, with a grate on top, a sump in the bottom, and an outlet pipe partway up the side. Surface water runs to the grate and drops in. The heavy sediment settles to the sump below the outlet. The cleaner water rises to the outlet and runs off through the pipe. It is a surface-water tool. You put it where the water concentrates and has to leave through a pipe: the yard low spot, the bottom of a slope, the place a downspout dumps into.

The sump is the part people skip when they buy the cheapest basin. The outlet sits up off the floor of the box on purpose, so the space below it catches the sand, grit, and leaves that would otherwise run into your pipe and pack it solid. Set the outlet too low and the sediment goes straight downstream. Set it too high and the basin holds standing water above the pipe. A deep-sump basin, with the outlet a good distance above the floor, is what keeps the line clean, and that sump is exactly what you scoop out at maintenance.

The grate stops the leaves, sticks, and trash so the pipe sees water, not debris. Size the grate and the basin to the flow, because a small basin grate at the bottom of a big drainage area will surcharge and pond in a hard rain. The catch basin is the workhorse of yard drainage, and it pairs with a piped run to an outlet. A basin with no outlet is just a hole that fills up.

Area drains, channel drains, and where each one goes

An area drain is a point inlet. It is a small grated drain set at a single low spot, a planter, a window well, a sunken entry, a courtyard corner, that takes water down into a pipe at one point. Think of it as a small catch basin or a grated inlet for a place that ponds. You set it at the low point and pitch the surface to it from every side so the water finds it.

A channel drain, also called a trench drain, is a long linear grate that catches water along a line instead of at a point. You set it where water crosses a hard surface and you cannot grade it away: the foot of a driveway, a garage apron, a pool deck, a doorway threshold, the bottom of a ramp. The water sheets toward the line, drops through the grate into the channel, and runs to the outlet. The difference from a French drain matters. A channel drain is a surface system that takes water off the top of pavement. A French drain is a subsurface system that collects water out of the soil. They look similar in a catalog and do opposite jobs.

Channel drain and trench drain get used interchangeably, with a working distinction. The pre-formed modular channel, a plastic or polymer run with a snap-in grate, handles light to medium loads and goes in fast, which suits driveways, patios, and pool decks. A formed concrete trench drain is built heavier and deeper for traffic and volume, which is what you pour at a loading dock, a wash bay, or a commercial apron. Match the load rating of the grate to the traffic, because a pedestrian grate under a delivery truck folds.

What slope and pipe does a drain line need?

Slope the pipe at least 1 percent and run smooth-wall pipe where you can. One percent is about 1/8 in per foot, and it is the practical minimum for a smooth pipe to reach the roughly 2 ft per second velocity that keeps solids moving instead of settling. Below that the line stores water and silt, and a buried silted pipe is a dig to fix. More fall is better up to a point. Steeper than about 1/2 in per foot is fine for clear storm water, though on sanitary lines too much slope outruns the solids, which is a sewer concern more than a yard one.

Two pipes do most of the work. Smooth-wall PVC, commonly SDR-35 to ASTM D3034 for gravity drainage, has a slick interior that flows well and resists clogging and roots, and it takes solvent-weld fittings that stay put. Corrugated HDPE, the black flexible pipe, comes single-wall corrugated inside and out, or dual-wall with a corrugated outside and a smooth interior. The dual-wall smooth-interior HDPE flows nearly as well as PVC and bends around obstacles. The cheap single-wall corrugated has a ribbed interior that flows slower and catches debris.

The thing the corrugated debate misses is the interior. Water does not care about the outside of the pipe. It cares whether the inside is smooth. Single-wall corrugated is fine for a short downspout extension to daylight, where it runs clear and you can flush it. For a buried collector carrying any sediment, run smooth-wall PVC or smooth-interior dual-wall HDPE, because the ribs on cheap pipe hold the grit that eventually blocks it. And whatever you run, hold the slope, because no pipe drains uphill.

Where does the water go? The outlet decides everything

Every drainage system has to end somewhere legal and lower, and that somewhere is the outlet. The options, in rough order of preference, are daylight to a lower spot on grade, a connection to a storm system where the authority allows it, a dry well that lets the water soak into the ground, and a sump pump where gravity cannot get there. Settle the outlet first, not last, because the whole system slopes back from it. No outlet means no drainage. It means a pipe that fills and a problem you moved underground.

Daylight is the best outlet there is. The pipe runs downhill to a point where the ground falls away and the water comes out at the surface, by gravity, with no parts and nothing to fail. It works in every rain, it has the full capacity of the pipe, and you can see it running, which tells you the system works. If the site has the fall to daylight a line, take it. Protect the end with a pop-up emitter or a rodent grate and some rip-rap so it does not erode or get blocked.

A storm connection ties the line into a public or site storm system, and that is allowed only where the authority permits it, so confirm before you tie in. A dry well is for sites that cannot daylight and have soil that drains, which the bioretention and infiltration guide covers in depth. A sump pump is the last resort, for when the water has to go up to reach the outlet, a basement drain below the sewer, a low courtyard with nowhere downhill. Gravity never quits and never loses power. A pump does both, so you use it only when there is no gravity path.

The foundation drain at the footing

A foundation drain, or footing drain, is a perforated pipe run around the base of the foundation footing, in gravel and fabric, to collect the water in the soil before it reaches the wall. It is a French drain doing one specific job: keeping the water table and the saturated soil away from the structure so it does not push through the wall or up under the slab. On a basement or a below-grade wall it pairs with the wall waterproofing and a gravel backfill, and it drains to daylight or to a sump.

The detail that decides whether it works is where the pipe sits and where it goes. The pipe belongs at or below the bottom of the footing, in clean gravel wrapped in fabric, so it collects water at the level you are trying to protect, not above it. Run it to an outlet with fall, daylight if you have it. The waterproofing and the drainage work together. The membrane keeps water off the wall. The drain takes away the water in the soil so there is less head pushing on the membrane. Do one without the other and you are relying on half a system. The below-grade waterproofing detailing is its own topic. Here the point is the drain has to be at footing level and it has to have an outlet.

Cleanouts and maintenance access

A drain line you cannot get a rod or a jet into is a drain line you cannot clean, and any buried pipe that takes sediment will eventually need cleaning. Put cleanouts in. A cleanout is a capped vertical riser tied into the line and brought to grade, that lets you run a snake or a hose down the pipe to clear a blockage. Put one at the upstream end of a run, at sharp bends, and at intervals on a long line so you can reach the whole pipe from accessible points.

Catch basins double as cleanouts, which is one of their quiet advantages, because the sump gives you a place to reach the pipe and the basin itself collects the silt you would otherwise be rodding out. On a French drain, the maintenance access is the harder problem, since the pipe is buried in gravel with no easy entry, which is exactly why the fabric and the slope and the washed stone matter so much up front. A French drain you cannot easily clean has to be built so it does not silt up in the first place. Plan the access on the day you plan the system, because adding a cleanout to a buried line later means digging the line up to do it.

Dry wells and infiltration

A dry well is a buried chamber, a pit of stone or a perforated plastic tank, that takes collected water and lets it soak into the ground instead of carrying it to a surface outlet. It is the answer when a site has no place to daylight a line and no storm connection, but it only works if the soil under it will actually take the water. Dig it into clay and it fills and stays full, a buried pond with a lid. Dig it into sand and gravel and it does its job.

Test the soil before you commit to a dry well, because the whole thing rides on the infiltration rate. A percolation test, or a soils report on a bigger job, tells you whether the ground drains fast enough to empty the well between storms. Generally the deeper you go, the worse the soil drains, so a deep well is not automatically a better one. Size it to the runoff it has to take and give it an overflow to somewhere safe for the storm that exceeds it, because a dry well with no overflow surcharges back up the pipe. Infiltration as a stormwater strategy, the engineered media and the drawdown, is the subject of the bioretention and rain garden guide. For a piped system, the dry well is the infiltration outlet at the end of the line.

When gravity will not reach: the sump pump

When the water has to go up to reach an outlet, gravity cannot do it and a sump pump takes over. The collected water drains by gravity to a low basin, the sump, and a pump with a float switch lifts it up and out through a discharge line when the basin fills. You see it where the drain is below the outlet, a basement perimeter drain below the sewer, a low elevator pit, a courtyard with no downhill path. It is mechanical, so it is the outlet you maintain and the outlet that fails in a power outage.

The pump is a last resort for a reason. Gravity never loses power and never wears out. A pump does both, and it tends to fail in the storm that fills the basin fastest, which is when you need it most. Where a sump pump is the only option, the moves that save you are a check valve so the discharge does not run back into the basin, a discharge that carries the water well away from the building instead of dumping it next to the foundation, and a battery backup or a second pump on jobs where a wet basement is a real cost. Test it before you hand it over, because the float that sticks is the callback.

Tying in the downspouts

Roof water is the biggest single source of water at most buildings, and where the downspout dumps decides whether the foundation stays dry. Tie the downspout into the drainage system and carry that water away, to daylight or into the piped run, instead of letting it pour out at the base of the wall. A downspout that discharges against the foundation is feeding the exact problem the rest of the system is trying to fix.

The clean way is a solid pipe from the downspout to an outlet that gets the roof water well away from the building. Keep the roof water in its own solid line where you can, rather than dumping it into a perforated French drain, because a downspout in a heavy rain delivers a slug of water that surcharges a perforated pipe and pushes that water right back into the soil you were trying to drain. A pop-up emitter at the end lets the water out at grade and reseats when the flow stops. The gutter and downspout sizing and detailing is its own topic. The drainage point is simple: get the roof water into a pipe and get it away from the wall, never against it.

Stormwater, detention, and the permit

On anything bigger than a single yard, where the water is allowed to go is a regulated question, not a contractor's choice. A site that adds impervious area, roofs and pavement, sheds more water faster than the ground it replaced, and the local authority usually requires that the site detain that extra water and release it at a controlled rate, or treat it, or both. You cannot just pipe a commercial site's runoff to the neighbor or the street and walk away. The civil drawings carry the stormwater design, and the permit sets what is allowed.

Detention holds the storm and lets it out slowly, through a basin or an underground chamber with a controlled outlet, so the downstream system is not overwhelmed. Discharge to a storm system or a watercourse is allowed only where the authority permits it and at the rate they set. The water-quality side, treating the runoff before it leaves, is where bioretention and the green-infrastructure practices come in, and that companion guide covers the BMP design. For the piped system, the rule is to build to the approved civil plan and confirm the discharge point and rate with the authority. An inspector signs off on where the water goes, and tying into the wrong outlet is a violation, not a detail.

Data center and campus site drainage

On a data center or a large campus, site drainage stops being a yard problem and becomes a reliability one. These sites are mostly roof and pavement, so they generate a lot of runoff fast, and the equipment they protect does not tolerate water near it. The drainage has to keep the water away from the building pads, the generator yards, the electrical rooms, and the underground vaults, and it has to keep working in the storm the site was sized for, because a flooded vault or a wet equipment room is an outage, not a cleanup.

The work is the same parts at a larger scale and a tighter tolerance. Catch basins and area drains at every low point, smooth-wall piped collectors sized for the design storm, detention to hold the runoff the impervious site sheds, and redundancy where a single failure is not acceptable. The difference from a residential job is the consequence and the documentation. The system is engineered, the civil drawings govern every pipe and slope, and the as-built record matters because the people who operate the site for the next twenty years need to know where every line, basin, and outlet is. Build it to the plan, and record what you built.

The upkeep after handoff

A drainage system is not a set-and-forget install. The day it is commissioned, the owner inherits a maintenance job, and the systems that fail early are almost always the ones nobody told the owner to maintain. Three things need doing on a schedule: clean the catch basin sumps, rod or jet the pipe runs, and service the pumps. Skip them and the system silts up, backs up, and floods the thing it was built to protect, usually in the first big storm after it stopped working.

Tell the owner what they own. Catch basin sumps fill with sediment and need scooping out, once or twice a year on a dirty site, before the silt reaches the outlet and runs into the line. Pipe runs need rodding or jetting when flow slows, which is why the cleanouts went in. Pumps need testing before the wet season, the float checked, the check valve checked, the discharge confirmed clear. The French drain is the one with no easy service, which is the whole argument for building it right with fabric and washed stone, because the maintenance plan for a silted French drain is excavation. Hand over a simple schedule with the as-built and the system lasts. Hand over nothing and it fails on the owner's clock and comes back as your callback.

What to document

The drainage you can find on a drawing is the drainage the next person can maintain. Most of this system is buried, so the record is the only way anyone knows where the pipes, basins, and outlet are without a shovel. Capture the as-built before the trench is backfilled, while you can still see and measure what you laid.

Record each element and its pipe size, the slope you held, where the line outlets, and whether the gravel was wrapped in fabric. Note the invert depths at the basins and the outlet, because the slope is the difference between them and a future tie-in needs those numbers. Photograph the open trench with a tape in the shot. The day someone adds onto the site or chases a wet spot, this record is what saves them from digging blind.

Common mistakes

• No outlet, or an outlet with no fall, so the water has nowhere to go, the pipe fills, and the problem moves underground instead of away.
• No filter fabric in silt or clay soil, so fines pack the gravel and blind the perforations and the drain silts up and quits within a season or two.
• Slope too flat, below about 1 percent, so the line stores water and sediment instead of carrying it, and a buried silted pipe is a dig to fix.
• Downspout discharged against the foundation, or roof water dumped into a perforated drain that surcharges and pushes it back into the soil.
• No cleanouts and no basin access, so a blocked line cannot be rodded or jetted without excavating it.
• Perforations oriented wrong for the job, holes up on a drain meant to pull a water table down, so the pipe fills before it drains.
• Cheap single-wall corrugated pipe on a collector run, where the ribbed interior catches sediment and slows the flow.
• A catch basin with no sump, or the outlet set too low, so sediment runs straight into the pipe and packs it solid.

Field checklist

Standards and references

Subsurface drainage is more craft and civil engineering than a single code book, so the references are layered. On anything engineered, the civil and geotechnical drawings govern the design. The geotechnical report carries the soil and the infiltration rate that decide whether a dry well or an infiltration outlet will work. The civil plan carries the pipe sizes, slopes, inverts, and the approved outlet. Build to those numbers, not to a rule of thumb, where they exist.

The pipe is where ASTM shows up. Smooth-wall PVC gravity sewer and drain pipe is commonly specified to ASTM D3034, the SDR-35 standard, and corrugated polyethylene drainage pipe has its own ASTM specifications for single and dual wall. Cite the spec the drawing or the manufacturer calls out, and match the pipe to it rather than naming a standard from memory. The exact designation and edition belong to the project documents.

Where the water is allowed to discharge is set by the local stormwater authority and the AHJ, and on a regulated site by the stormwater permit. Connecting to a storm system, discharging to a watercourse, and the detention and treatment a site has to provide are all decided there, not in the field. Confirm the discharge point and rate before you tie in, and let the project specification and the permit override any general practice in this guide when they are stricter.

Units, terms, and conversions

Subsurface drainage borrows terms from grading, plumbing, and civil work, and the same part goes by different names across a drawing set and a supply house.

Slope shows up as a percent, as a ratio, and as a drop per foot, and they describe the same fall: 1 percent is about 1/8 in per foot. Pipe is sized by inside diameter in inches, 3 in and 4 in for most yard work and larger on collectors. Invert is the inside bottom of the pipe, and the difference in invert from one end to the other, over the length, is the slope. Perc, or percolation, is how fast the soil takes water, which decides whether infiltration works.

French drain

Perforated pipe in a gravel trench wrapped in filter fabric, sloped to an outlet, that collects water from the soil

Catch basin

A grated box with a sediment sump and an outlet pipe, set at a low point to take surface water into a pipe

Area drain

A point inlet, a small grated drain at a single low spot that takes water down into a pipe

Channel / trench drain

A linear grate that catches surface water along a line across pavement or a doorway

Invert

The inside bottom of a pipe; the difference in invert over the run sets the slope

Daylight

An outlet where the pipe discharges at the surface by gravity at a lower point on grade

Geotextile

Non-woven filter fabric that passes water and holds back soil fines so the drain does not silt up

Perc rate

How fast soil absorbs water, the measure that decides whether a dry well or infiltration outlet works

Related tools

Calculators and readiness checks for this work

Compare your options

FAQ

People also ask

• How much should the ground slope away from a house?
• What is bioretention, and is it the same as a rain garden?
• What is a pavement edge drain?
• When do you need a SWPPP?
• What is a detention pond?