What's the difference between a curtain drain and a French drain?

A curtain drain is installed uphill of a structure to intercept surface and shallow groundwater before it reaches the foundation. A French drain is typically installed alongside a structure to collect water that has already arrived. The two often get used interchangeably in everyday speech, but in NH soil they do different jobs. On hillside lots, a curtain drain is almost always the right first move because it stops the water before it ever sees your foundation.

How much does a curtain drain cost in the NH Lakes Region?

Costs depend heavily on length, depth, soil type, ledge, and access. A short 50 to 100 foot run on an accessible lot in good soil typically lands in the $2,500 to $5,000 range. A medium 100 to 200 foot run with some access challenges or partial ledge runs roughly $4,500 to $9,000. A long 200 to 400 foot run with serious ledge or tight access can reach $8,000 to $18,000 or more. Every site is different. The site visit and the soil tell the story, not the phone call.

How long does a properly-built curtain drain last?

A curtain drain built with the right pipe, proper non-woven filter fabric, clean 3/4-inch crushed stone, correct slope, and a working outlet should function for 30 to 50 years before any meaningful maintenance. Drains that skip the filter fabric or use the wrong stone can clog with silt in 3 to 7 years. The pipe itself outlasts everything. The filter fabric is what actually determines lifespan.

Do I need a permit for a curtain drain in NH?

On most private property in central NH, no state permit is needed for a residential curtain drain. Two exceptions matter: if your property is within 250 feet of a public water body, RSA 483-B applies and a Shoreland Permit may be required for the disturbed area. If the drain discharges into a wetland or watercourse, NH DES wetlands rules may apply. Local building departments occasionally have rules for drainage tied to a finished basement or new construction; worth a phone call to your town before the dig.

Can I install a curtain drain myself?

On a short, easy run with rentable equipment and a buddy, yes. The hard parts are: knowing exactly where to put it (uphill of the structure on the actual hydrologic path, not just on the steepest visible slope), getting consistent slope (minimum 1 percent toward the outlet, no low spots), installing the filter fabric correctly, and finding ledge before you've committed to a route. DIY-installed drains that fail usually fail because of one of those four. Most homeowners I see who tried it end up calling me to either re-do it or extend it.

Why is my new curtain drain not working?

Four common causes: the drain is in the wrong place (downhill of where the water is actually traveling), the slope is wrong (low spots, reverse pitch, or insufficient drop to the outlet), the outlet is blocked or daylighting uphill of the destination, or the filter fabric was skipped and the pipe has silted up. A drain that worked for a year and stopped is usually outlet or silt. A drain that never worked is usually placement or slope.

Curtain Drain Installation Guide for NH Homeowners

About seven out of ten wet basements I look at across Hill, Bristol, Franklin, and the rest of the Lakes Region are not rising groundwater problems. They are surface and shallow groundwater problems. The water is sheeting downhill toward the foundation, soaking through the first few feet of soil, and finding the cracks and seams in the wall. The fix usually starts before the water ever reaches the house. That is what a curtain drain does. It catches the water uphill of the structure and redirects it somewhere harmless.

This is the working contractor's guide. What a curtain drain is, what it is not, when you need one, what gets put in the trench, what it costs in the Lakes Region in 2026, and the four reasons new curtain drains fail.

What a curtain drain actually is

A curtain drain is a perforated pipe in a gravel-filled trench, installed across the slope on the uphill side of a structure, designed to intercept surface and shallow subsurface water before it reaches the foundation. The pipe is wrapped in non-woven filter fabric to keep silt out, surrounded by clean 3/4-inch crushed stone for drainage capacity, and pitched at a minimum 1 percent toward an outlet that lets the captured water leave the property without backing up.

Three terms get used interchangeably and they do not mean the same thing:

Curtain drain. Sits uphill of the structure. Catches water before it arrives.
French drain. Sits next to or downstream of a wet area. Collects water that has already arrived and moves it elsewhere.
Foundation drain (footing drain). Sits at the base of the foundation wall, against the footing. Last line of defense. Water has reached the house and the drain captures it before it gets through the wall.

Most NH wet-basement problems get solved cleanest with a curtain drain because it stops the water before any of the harder, more expensive interventions are needed. Foundation drains are real and necessary in some situations, but they involve excavating against the foundation, which is far more invasive than a curtain drain in a yard.

When you actually need one

Signs that a curtain drain is the right fix:

The lot has any meaningful slope and your house sits below higher ground. If the topography points water at you, you eventually have a water problem.
Wet basement after spring melt or hard rain. Especially if it dries out in summer. That pattern is surface water, not the water table.
Saturated lawn that will not dry. Soggy ground for weeks after the rain has stopped means water is moving through the soil and not draining out.
Water staining on the uphill side of the foundation wall while the downhill side stays dry. The water is hitting the high side and you are watching it.
Driveway washouts on the uphill side of the drive. Same hydrology, smaller scale.
Foundation movement, settling cracks, or efflorescence (white powder) on the basement walls. Long-term saturation has been at work and the curtain drain is part of solving it.

Signs a curtain drain is not the right fix:

Flat lot, no slope. If water is not flowing toward the house, intercepting it does nothing. The problem is something else (gutters, downspout extensions, regrading the immediate perimeter).
Rising groundwater. If the water table is genuinely above your basement floor (rare, but real), a curtain drain in the yard does not help. That is a foundation-drain or sump-pump conversation.
Plumbing leaks. Worth ruling out before you dig a trench.

What goes in the trench (materials matter)

The components of a curtain drain that lasts 30 to 50 years versus one that silts up in 5:

Perforated 4-inch pipe. Either smooth-wall PVC perforated, or corrugated double-wall (sometimes called N-12). Both work. Smooth-wall is easier to clean if it ever needs it; corrugated is cheaper and more flexible for tight curves. The perforations face down for groundwater interception. This is counterintuitive for first-timers; the rising water enters through the bottom holes as the trench fills.
3/4-inch clean crushed stone. Angular, washed, no fines. The stone gives the water somewhere to flow before it enters the pipe and provides void space so the drain works. Pea gravel is wrong (rounded, no interlock, less void space). Bank-run gravel is wrong (contains fines that clog).
Non-woven filter fabric. Wraps the entire stone-and-pipe envelope. Stops silt and fine soil from migrating into the stone and clogging the pipe. Mirafi 140N or equivalent. Skipping the fabric is the single most common reason new curtain drains fail in 3 to 7 years. Woven fabric (the green or black plastic-tape look) is the wrong product; non-woven (felt-like) is what you want.
Solid 4-inch pipe for the outlet. The last 10 to 20 feet from the trench to daylight gets unperforated solid pipe so it does not pick up additional water along the way. Capped or grated at the outlet to keep critters out.
Stone or rip-rap at the outlet. A small splash pad of larger stone where the water comes out, to prevent erosion at the discharge point.

Install process step by step

Here is how a curtain drain actually goes in:

Read the site. The single most important step. Where is the water coming from? Where can it go? The trench has to be uphill of the structure and the outlet has to be downhill of everything. If those two conditions cannot both be met, the drain will not work no matter how cleanly it is installed.
Locate utilities. Call before you dig. Electric, propane, and septic lines all live in NH yards and any of them can ruin your day.
Excavate the trench. Typical dimensions: 24 to 36 inches deep, 12 to 18 inches wide. Deeper if you are intercepting deeper groundwater or running below the local frost line; shallower is fine if the water is mostly surface flow. The bottom of the trench is sloped at minimum 1 percent (1/8 inch per foot) toward the outlet, no low spots.
Probe for ledge. Lakes Region soils hide bedrock at unpredictable depths. Hitting ledge at 18 inches when you needed to go to 30 changes the plan. Better to know early.
Lay the filter fabric. Drape it across the trench so it covers the bottom and rises up both walls with enough overlap to wrap over the top later. Do not skip this step. Do not substitute woven for non-woven.
Add 4 to 6 inches of 3/4-inch crushed stone in the bottom of the fabric-lined trench.
Lay the perforated pipe on top of the stone with perforations pointing down. Connect sections, watch the slope (a string line and a level help), make sure no low spots.
Backfill with crushed stone around and over the pipe up to within 6 to 12 inches of grade.
Fold the filter fabric over the top of the stone, sealing the envelope.
Cap with topsoil and reseed or sod. Some installers use a thin layer of native fill on top of the fabric before topsoil; either works.
Run the outlet the last 10 to 20 feet with solid pipe to daylight, a dry well, or a stormwater catch basin where allowed. Place a splash pad of stone at the outlet.

“A curtain drain is one of the highest-value drainage jobs a property can get. Done right, it solves a problem that would otherwise need foundation work to fix.”

NH-specific issues that move the cost

Things that come up here that generic guides skip:

Frost line. NH frost runs about 4 feet deep in central NH, deeper on exposed lots in Plymouth and the upper Pemi. A curtain drain does not have to go below frost (the water is mostly moving above the frost line and the drain catches it where it is), but the outlet pipe should drain freely so it does not hold standing water that freezes solid in winter. Frozen outlets back drains up.

Glacial till and ledge. Lakes Region soils are heavy on glacial till (hard, dense mix of gravel, clay, and stones) with bedrock at unpredictable depths. Till does not drain quickly, which is part of why curtain drains are needed in the first place. Ledge encountered during the dig is the single most common cost surprise. On long runs (200 feet plus), I budget for hitting some.

High water table on Lakes Region waterfront. Properties on Newfound, Squam, Winnipesaukee, Webster, and the Pemi corridor often have seasonally high groundwater. A curtain drain on these lots may need to discharge to a daylight outlet, a dry well, or a stormwater system, and the discharge location is governed by the Shoreland Water Quality Protection Act (RSA 483-B) within 250 feet of the water. More on shoreland-zone work in the Lakes Region →

Mud season. March and early April are the hardest stretch to install a drain. The ground is saturated, dirt roads are under posted weight limits, and the trench wants to collapse before stone goes in. I generally schedule curtain drain installs from late spring through fall when the ground is workable.

What it costs in the Lakes Region

Real 2026 ranges for residential curtain drain installs:

Short run (50 to 100 feet, accessible lot, good soil, no ledge): roughly $2,500 to $5,000. This is a typical fix for a wet basement on a small suburban or village lot.
Medium run (100 to 200 feet, some access challenges or partial ledge): roughly $4,500 to $9,000. Most Lakes Region residential jobs land here. Factors that move the number: trench depth, length to a workable outlet, whether the equipment can drive directly to the run or has to work around tight access.
Long run (200 to 400 feet, serious ledge, tight access, or complex outlet): $8,000 to $18,000 or more. Larger properties, hillside camps, or sites where the outlet has to travel a long way to reach an acceptable discharge point.

What moves these numbers up or down fastest: length (linear), trench depth (driven by where the water is and frost considerations), ledge (expensive to break through, every cubic yard of bedrock is hours of rock-hammer time), distance to a usable outlet, access for my equipment, and whether the disturbed area triggers a Shoreland Permit. More on the drainage and septic services I run →

The four reasons new curtain drains fail

If your curtain drain is not working, or you want to make sure the next one will, watch for these:

Wrong placement. The trench is downhill of the actual hydrologic path, not uphill of the structure. Water flowing into the foundation is not being intercepted. This is the single most common cause of a new drain that does not work. The fix is usually re-trenching in a different location, which is exactly the conversation no homeowner wants to have.
Wrong slope. Low spots in the trench bottom mean water pools in the pipe and never reaches the outlet. Reverse pitch (any section sloping back toward the source) means water moves the wrong way. Insufficient drop to the outlet (less than 1 percent) means flow is too slow and silt accumulates.
Outlet problems. The outlet is blocked by silt, vegetation, or ice. Or the outlet is daylighting uphill of where the water needs to go (water cannot flow uphill, even from a pipe). Or the outlet ties into a system that is itself backed up.
Filter fabric skipped or wrong fabric. Without non-woven filter fabric, fine silt migrates into the stone and pipe over years, clogging the system. Drains that worked for the first few years and then stopped are usually silt-clogged. The fix is full removal and reinstall, which is why doing it right the first time matters.

DIY vs. hire it out

Curtain drains are one of the more DIY-able drainage jobs if you have access to a rentable mini-excavator or skid steer with a trenching bucket, a helper, and a flat enough day to dedicate to it. Materials run a couple thousand dollars for a short run. Equipment rental is another few hundred. The hard parts are the four failure modes above. The easy parts are the actual trenching and pipe laying.

Where DIY runs into trouble:

Reading the site to figure out where the trench should actually go. The wrong location means starting over.
Getting consistent slope across 100+ feet of trench. Hard without a laser level or string line.
Hitting ledge. A homeowner with a rental machine usually does not have a rock hammer.
Hauling away the spoils. A 100-foot trench at 30 inches deep produces a lot of material that has to go somewhere.

If the run is short, the soil is friendly, and you have done excavation work before, it is doable. If any of those is not true, the cost difference between DIY and a contractor is often less than the cost of fixing a DIY install that did not work.

Permits in NH

For most curtain drains on private property in central NH, no state permit is needed. Two situations to watch for:

Within 250 feet of a public water body. RSA 483-B (the Shoreland Water Quality Protection Act) controls earth disturbance in the Protected Shoreland. A drain install that disturbs more than 2,500 square feet, or that sits within the 50-foot Waterfront Buffer, may need a Shoreland Permit before work starts. The licensed designer or surveyor on a complex project usually pulls this; on a straightforward residential job, NH DES Shoreland Program staff can confirm what is needed.
Discharge into a wetland or watercourse. If the drain outlet daylights into a wetland, stream, or pond, NH wetlands rules apply and the discharge needs to be designed to avoid sediment release. This is more about how the outlet is built than whether the drain itself is allowed.

Local building department rules occasionally come into play if the drainage work is part of a finished basement or new construction. A phone call to your town's building inspector before the dig is the easiest way to clear that up.

What I do on a site visit

The site visit is free. I walk the property with you, read where the water is coming from and where it can go, identify the right trench location, probe for ledge if the run is long, look at the foundation if there is an active basement issue, and check the candidate outlet locations. You get a written estimate within a few days that itemizes trench length, materials, equipment time, and the outlet work as separate line items so you can see what is driving the cost.

If a curtain drain is not the right answer for your site (sometimes the fix is gutters and a regrade), I will tell you that. I would rather not sell you a $5,000 drain than sell you one that does not solve the problem.