Perforated or Solid? The Drainage Pipe Mistake That Costs the Most
Most drainage money in this county gets spent twice. Once on a system that was built wrong, and again on the system that fixes it. Almost every one of those failures traces back to a handful of decisions made before anyone put a shovel in the ground.
Here is the first and most expensive one.
A downspout never enters perforated pipe
There are two different pipes in drainage work and they do opposite jobs.
Perforated pipe — the kind with holes, packed in washed stone and wrapped in fabric — exists to collect water already moving through soil. Hillside seepage. A high water table. Saturation sitting against a footing or behind a wall. That is a French drain, and that is all it is for.
Solid pipe carries water that is already gathered — roof water, downspouts, surface flow, anything concentrated — to somewhere it can leave.
Put roof water into perforated pipe and three things happen:
The pipe discharges your roof water straight into the trench, saturating the exact soil the system was installed to dry out.
Sediment carried in from the surface clogs the perforations and fills the voids between the stone.
Fines migrate into the stone and the whole system silts shut.
The failure is slow. The system works for a season or two, then quietly stops, and by then nobody connects it back to the pipe choice. Fabric omission and sediment clogging together are the most commonly documented cause of French drain failure.
We know this one by having paid for it. It produced a warranty callback on a job of ours, and it is now a rule we do not break.
The fabric nobody checks
Non-woven, needle-punched geotextile on anything intercepting seepage. Not woven.
They look similar in a roll and cost about the same. Woven fabric is engineered for strength under load. Non-woven needle-punched is engineered to let water pass while holding soil back. Put woven fabric behind stone on a seepage application and water pressure builds behind it instead of draining through it.
We corrected this on a creek-bank job. It is invisible once backfilled, no homeowner would ever spot it, and a lot of installers do not distinguish between the two. It is worth asking which one is going in your trench — the answer tells you a great deal about who you have hired.
Bigger pipe is usually the wrong answer
Four-inch is the default. Six-inch needs a reason in writing.
It feels prudent to upsize. It usually is not. It inflates the price, and on a line with limited fall it can actually run worse, because the water spreads thin instead of carrying sediment through.
Four inches is generally sufficient where one line serves roughly one downspout — around 1,000 to 1,200 square feet of roof catchment — at adequate slope. Six inches earns its place in four situations:
Several downspouts or area drains combining into one main
Roof catchment above roughly 1,200 square feet on a single run
Available slope under 1 percent
A main running off 9-inch or 12-inch basins
If a quote upsizes the pipe without one of those conditions on the page, ask why. To be safe is not an engineering reason, and you are paying for it by the foot.
For context on what drives sizing: the Charlotte-Mecklenburg Storm Water Design Manual specifies a 4.0 inch-per-hour design rainfall intensity, and the Southeast generally sits in the 4 to 6 inch-per-hour band. Slope in the 1 to 2 percent range is the working target.
Where your water is legally allowed to go
This is the part almost nobody knows about, and it is the one that turns into a lawsuit.
The discharge point has to be legal, and it has to be on your property — not your neighbour's.
Solving your standing water by routing it twenty feet sideways onto the property next door is not a solution. It is a transfer, and North Carolina courts have addressed it. Under the reasonable use rule set out in Pendergrast v. Aiken (1977), a landowner may alter drainage on their own land, but not unreasonably, and not in a way that dumps a new burden on a neighbour.
Practically, a drainage plan needs a real answer to one question before anything is dug: where does this water end up, and is that legal? Acceptable answers are daylight on your own property, an existing storm drainage system, or a pop-up outlet sited on your land. North Carolina licensing standards require French drains to terminate to daylight or existing storm drainage, and require dry wells and cisterns to have an overflow outlet.
If a contractor cannot point at the discharge point during the walk-through, you do not yet have a plan.
Two specifications worth knowing by name
Riprap placement thickness. Where stone armours an outlet or a bank, placement thickness matters more than stone size. For a 6 to 12 inch gradation, minimum placement thickness is 12 inches. A 6-inch assumption is indefensible — it moves in the first real storm and you pay to place it twice.
Steps and stairs. If anyone proposes capping existing steps with stone or pavers, understand what that does: capping raises every riser, not just the bottom one. The IRC caps residential risers at roughly 7 and 3/4 inches. A quarter-inch cap on a stair already near the limit puts the whole flight out of compliance, and the fix is grinding or a written acceptance from you. Ask before, not after.
Test the soil before you buy anything for it
Do not lime until the results come back. Your pH may already be fine.
North Carolina residents can have soil tested through NCDA and CS, and the boxes are free outside the December to March peak window, when a $5 per-sample fee applies. Boxes come from any Cooperative Extension county center.
Lime and starter fertilizer bought before the test are frequently money spent on a problem you did not have. The test costs nothing most of the year and it tells you whether you have one.
Before you dig
Call NC811 first. Three working days' notice, filed in the state where you are digging.
Union County sits close enough to the South Carolina line that this matters — file in the state the shovel is actually in. Photograph every paint mark and every flag before work starts, time-stamped. If a line is cut later, those photographs are the only thing that establishes what was marked and what was not.
The full guide
The printable Water Where You Don't Want It: A Piedmont Drainage Guide collects all of this plus the contractor question list, the market cost ranges for drainage work in this area, and the line between what a licensed contractor may do and what requires a professional engineer.
Related
How Much Water Comes Off Your Roof? — the gallons math and gutter sizing, which is where most of this water originates
Do North Carolina's Erosion Rules Apply to My Yard? — the 21-day rule and what applies below one acre
If you would rather someone just looked at it
We do on-site property assessments in Union County and the surrounding Piedmont: a walkthrough, measurements, aerial photography, and a written scope of what is actually happening with the water on your property. Not a template. Your yard.
The assessment fee is credited toward the work if you decide to hire us. If the answer is that you do not need us, we will tell you that too. You can request a consultation here.
Alex Purdy, Carolina Terrain, Waxhaw, North Carolina. NC Landscape Contractor License CL.1872, NCSU Stormwater SCM Inspection and Maintenance Certification #5970, NDS Certified Drainage Contractor. This is an educational guide describing general conditions and recommendations. It is not a design, not an engineered or stamped plan, not a boundary survey, not utility locating, and not a site-specific recommendation for your property. No one can assess your yard from a document. I am a licensed contractor, not a professional engineer and not a registered landscape architect.
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