Plenty of repipe jobs start with a mystery stain on drywall or a drip that won’t quit. By the time the homeowner calls, the real culprit has already been at work for years: soil chemistry and water quality pushing a piping material past its limits. Choosing copper or PEX isn’t just about price per foot or a manufacturer’s warranty. It’s about the ground your home sits on, the water running through the lines, and how those two interact with metal or plastic over decades.
I’ve opened walls in coastal cottages where copper pinholed like a pepper shaker after seven years, and I’ve replaced brittle PEX in a mountain town where winters swing from thaw to freeze so often the tubing aged in dog years. Repipe Plumbing isn’t just a product choice, it’s a risk map. Let’s chart it.
The quiet enemies: soil, water, and time
Soil doesn’t attack pipes directly unless those pipes are buried or in slabs, but soils govern moisture levels, oxygen availability, and chloride concentrations. Water chemistry does the rest inside the pipe. When those inside and outside environments line up against you, problems accelerate.
Copper is a reactive metal that protects itself with a thin oxide film. Disturb that film with low pH, high dissolved oxygen, or chlorides, and you invite pitting corrosion. PEX, cross-linked polyethylene, shrugs off corrosion but doesn’t like heat, ultraviolet light, or certain hydrocarbons. It can also creep under constant stress, and it expands and contracts with temperature changes far more than copper.
So, in most homes, both materials can succeed if installed with care. The edge cases are where repipes earn or lose their keep.
What different soils actually do
Clay, sand, loam, decomposed granite, peat, and fill each create a different moisture and ion environment. Add salt spray or road de-icing, and the picture shifts again.
Clay soils hold moisture and slow oxygen movement. Around copper, that means wet conditions with limited oxygen. In the absence of oxygen, copper’s protective film can’t self-heal, which encourages under-deposit corrosion, especially where the pipe rests on soil or concrete. If the clay retains chlorides from coastal air or winter salt, pitting rates climb.
Sandy soils drain fast and bring abundant oxygen to buried pipe. That sounds good, but oxygen plus low alkalinity in the water inside the pipe can create a mildly aggressive situation for copper. Outside, sandy soils rarely stay wet long, so exterior corrosion is slower. On the other hand, sand shifts. Unsupported spans in trenches can let copper settle and rub at a single point, eventually wearing through a soft bend. PEX handles minor ground movement better, given flexibility and fewer fittings.
Loam, the gardener’s dream, is a pipe’s middle ground too. It drains moderately, holds some moisture, and doesn’t concentrate salts the way clay can. For buried plumbing, loam is usually forgiving as long as you avoid direct metal-to-soil contact and follow bedding best practices.
Decomposed granite and rocky soils don’t retain much water but can be abrasive. I’ve seen PEX sleeving worn thin where boulders were the only bed. Copper laid directly against granite edges ends up with pressure points that become dents, then blisters. The problem isn’t chemistry; it’s mechanics.
Peat and high organic soils form acidic environments. That acidity can hurt copper indirectly by mixing with groundwater that infiltrates into tiny slab cracks, changing the pH around the pipe. When copper sits in acidic condensate or wicks that acidity into fiberglass pipe insulation, pitting accelerates from the outside in. PEX is indifferent to external acidity but needs proper support because peat settles and heaves.
Fill soils are a grab bag. I’ve cut trenches through old demolition debris and hit rebar, brick, gypsum, and coal ash. Unknown salts, stray currents from nearby utilities, and sharp objects make fill unpredictable for any pipe. Sleeve aggressively, control stray currents where possible, and avoid direct metal contact with subgrade.
Copper’s strengths and failure patterns
Copper is still elegant plumbing. It handles high temperatures, resists UV better than plastics, and offers a solid fire profile. Properly installed, type L copper in a neutral water supply can last 50 years or more. The romance fades when the water or the ground plays rough.
- Pitting from the inside: Low pH under about 7, high chloride levels above roughly 50 to 100 mg/L, or high dissolved oxygen can destabilize copper’s protective film. The risk increases when flow is sporadic and water sits. In some coastal and desert municipalities, I’ve seen pinholes in 8 to 12 years. Microbiologically influenced corrosion: In warm, low-flow lines, certain bacteria set up shop beneath deposits, creating localized acidic zones. It looks like peppered pinholes along the bottom of horizontal runs. Smart plumbing layout and periodic flushing can help, but water chemistry wins the long game. Erosion-corrosion at high velocity: Push copper beyond about 8 feet per second in cold water or 5 in hot, and you’ll scour the interior. Long sweeps and proper pump sizing keep the protective film intact. Stray current corrosion: Buried copper near DC sources or poorly bonded electrical systems can end up as the easiest path for current. The result is textbook pitting that tracks along grounding points. Proper bonding and dielectric isolation at transitions reduce the risk. External attack under slabs: If copper lies directly on concrete or rebar, moisture creates a microcell. The pipe becomes the anode in spots, and pinholes follow. Sand bedding or polyethylene sleeving is not a luxury, it’s lifespan.
When copper is the right call, the water is typically neutral to slightly alkaline, hardness moderate, and chlorides low. The soil drains well and doesn’t load the pipe with external electrolytes. Homes with existing copper that has lasted decades in the same utility district are a strong clue.
PEX’s strengths and weak spots
PEX dodges corrosion altogether. It laughs at low pH, ignores oxygen levels, and generally tolerates chloride. That makes it a hero in places where copper dissolves as fast as you can solder. Its flexibility allows long continuous runs with fewer fittings, fewer leak points, and faster repipe timelines, especially in finished homes where fishing lines through joist bays beats opening long wall runs.
- Temperature and pressure: Most PEX is rated for 180 F at 100 psi. Domestic hot water rarely holds that high, but recirculation loops can. Keep recirc temps around 120 to 130 F and control pump duty cycles. High temps over time accelerate oxidative aging of PEX, especially without an oxygen barrier. Oxidants: Chlorine and chloramines disinfect water, then nibble at polymers. Modern PEX is tested against these, but continuous high residuals in hot water lines shorten life. I pay extra attention in districts with aggressive disinfection, and I keep recirc runs to copper or stainless when temps need to stay high. UV exposure: Sunlight degrades PEX quickly. On job sites, I keep tubing out of windows and off rooftops. Even fluorescent lamps can contribute over months. Once installed in walls or underground with proper sleeving, UV becomes a non-issue. Mechanical damage and pests: PEX can kink and then weaken if you try to bend past its radius. It’s also more vulnerable to rodent chewing in crawlspaces. Sleeves, plates at studs, and disciplined routing prevent most of this. Thermal expansion: PEX expands roughly eight times more than copper per degree of temperature change. That means long runs need room to move and thoughtful anchoring. Use bend supports, leave slack, and avoid rigid traps between two immovable points.
PEX thrives where soils are damp or saline and where copper would pit, especially in slab repipes or coastal zones. It asks for good shielding and clean heat management more than chemistry control.
Water chemistry is the swing vote
I don’t spec pipe without a glance at the water quality report. Utilities publish annual Consumer Confidence Reports, and they will tell you pH, hardness, disinfectant type, chloride levels, and sometimes metals. Those numbers, plus lived experience in the neighborhood, are gold.
If pH runs 6.5 to 7.0 and chlorides sit above 60 mg/L, copper’s odds fall. If chloramines are used for disinfection and hot water recirculates at high temps, PEX needs careful product selection and controlled temperatures. When hardness is very low, copper can lose its protective carbonate scaffolding. In extremely hard water, both materials face scale issues, though copper’s internal diameter shrinks faster under heavy scale. PEX tolerates scaling a bit better because it can flex under small deposits, but flow still suffers.
Private wells add complexity. Well water can be acidic, iron-rich, or high in manganese and sulfur. I have cut out copper that looked fine except at a blue-green band every Repipe Plumbing Canby foot where fittings sat, all because a well’s low pH made contact points turn into microcells. PEX bypasses most well-water risks, but iron bacteria and biofilm can still clog fixtures. In those homes, treatment at the well head often matters more than pipe choice.
Slab, crawlspace, or attic changes the calculus
Subgrade runs under slabs live with the soil every day. Copper there needs isolation from concrete and stable bedding. Even with that, I’ve seen pinholes at soft bends pressed into sand pockets that collected chlorides from coastal fog. In slab homes where leaks have already appeared, running new lines through walls and attics with PEX is usually faster and less invasive than jackhammering floors. PEX’s flexibility lets us fish continuous lines to fixtures, using manifolds like a home’s electrical panel for water.
Crawlspaces favor either material, but rodents, humidity, and freeze risk get a say. Copper is less enticing to mice, and it resists incidental UV from open crawlspace vents. PEX wins on speed and freeze tolerance. When it freezes, it can flex, though fittings and valves still crack. Good insulation and air sealing matter more than the material itself.
Attic runs stress PEX with heat and copper with expansion noise. I’ve opened attics in summer that hit 140 F. That’s near the upper end for PEX over long durations. If the attic is the only route, keep PEX buried under insulation, away from can lights, and shielded from radiant heat. Copper up there will ping and tick with thermal changes unless you isolate hangers and give it room to move.
Case notes from the field
A coastal subdivision with sandy soil and prevailing onshore winds looked like copper country. The builder put type L copper under the slab. Seven years later, I started seeing red flags: isolated hot spots on floors where leaks were weeping into sand. Chloride levels from wind-driven sea salt were higher than the design assumed, and the sand bedding let moisture drain but constantly recharged salts. We repiped three dozen homes with home-run PEX through the attic, sleeved and buried in insulation. The failure rate dropped to zero over the next decade.
In a mountain valley with decomposed granite and hard well water, PEX split complaints popped up in winter. The culprit wasn’t the tubing itself but the routing. Lines were tight against roof decking under north eaves. Repeated freeze-thaw cycles, plus poor expansion room, stressed fittings. We rewired the layout with generous bend supports and rerouted away from the cold corners. In that same town, copper in crawlspaces lasted for decades, but we had to isolate from granite and use thicker type L, not M.
A midwestern city switched from free chlorine to chloramines to cut disinfection byproducts. Hot water recirculation loops in multi-story condos started eating PEX faster than expected. The solution was not wholesale copper but temperature control at 120 to 125 F, oxygen-barrier PEX on recirc branches, and stainless trunk lines in the boiler rooms. Mixing and matching where each material shines kept budgets sane and reliability high.
Repipe Plumbing through the lens of risk, not habit
A repipe is surgery. Know the anatomy. The ideal material selection looks at:
- The chemistry of your water source today and likely utility changes. The soil type around buried or slab runs, including chloride exposure and moisture behavior. The building’s routing options, temperatures, and movement. The history of failures in your neighborhood and the pattern of those failures.
Copper is not obsolete and PEX is not a cure-all. In a 1960s neighborhood where copper has lived quietly for 60 years, fresh type L with good practices can match that record. In a coastal tract with pinholes at year eight, PEX immediately extends lifespan, and the installation is faster with fewer holes in finished walls.
Installation makes or breaks both choices
Half of premature failures come from installation shortcuts, not bad material. The ground and water set the baseline risk, but the craft decides whether you beat the odds.
For copper, I prefer long-radius bends, cleaned and deburred ends, and water-flush soldering to keep flux out of the line. Aggressive fluxes left inside are a corrosion accelerant. Support every 6 to 8 feet horizontally, isolate from dissimilar metals with dielectrics, and keep velocity below eroding thresholds. In slabs, sleeve generously, bed in clean sand, and avoid direct concrete contact.
For PEX, the game is protection and movement. Use manufacturer-specific expansion or crimp systems correctly, calibrate expander heads, and verify ring placement. Plan for expansion with sweeping turns, bend supports, and slack. Plate every stud penetration within reach of fasteners. Keep PEX out of sunlight during staging, and don’t shove it hard against hot cans or flues in attics. Where rodents roam, sleeving and strategic copper stubs at exposed sections reduce chew risk.
Manifolds deserve a note. Home-run PEX with a central manifold simplifies shutoffs and balances flow, especially in repipes where you can’t perfectly mirror original branch trees. In larger homes, zoning manifolds by floor with accessible panels saves headaches when a fixture misbehaves.
Cost, warranties, and the parts that aren’t on the brochure
Material cost swings with markets. Copper spiked above 4 dollars per pound more than once, turning projects upside down. PEX levels that volatility but shifts cost into fittings, manifolds, and labor controls. On repipes in finished homes, labor dominates, and PEX’s speed often wins total cost even if copper’s per-foot price is comparable at the moment.
Warranties read well yet hide in the conditions. Many copper failures that look material-related trace back to water chemistry outside the plumber’s control. That limits recourse. PEX warranties typically exclude UV damage, over-temperature, and incompatible chemicals. If your district publishes high chloramine residuals, choose PEX types rated for that exposure, and document temperatures in recirc loops. When homeowners keep water heaters cranked to 140 F without mixing valves, both materials pay a price.
Insurance and code matter. Some jurisdictions still prefer copper in mechanical rooms or for certain fire-related clearances. Others require oxygen-barrier PEX for hydronic loops. Always match the local code and inspector expectations, because a perfect technical choice that fails inspection becomes an expensive reroute.
How soils steer specific choices
Clay with coastal influence: PEX gains a clear edge for slab and buried runs because chlorides stick around. If you stick with copper, move out of the slab and into interior walls, or sleeve meticulously. Expect higher monitoring.
Sandy inland soils with neutral water: Copper can serve well, especially above ground. If trenching, provide consistent bedding and avoid spans that will settle. PEX offers insurance against shifting, but not necessary if the water is gentle and the installation meticulous.
Rocky or decomposed granite: Either can succeed. Use sleeves as if they’re mandatory, not optional. For repipes where access is tight, PEX’s flexibility reduces the number of wall openings.
Peat or high-organic soils: PEX sidesteps external acidity and variable moisture. If copper is used, isolate it completely from soil and concrete with continuous sleeving and non-absorbent insulation.
Fill with unknown debris: The safest path is PEX in robust sleeves or conduit, or copper in sleeves with meticulous trench prep. Control stray currents by bonding and by separating from other utilities with code-specified clearances.
When a mixed-material strategy is smarter
I like copper stubs at water heaters and near boilers for heat and flame spacing, then PEX for distribution. In high-temp, high-chloramine zones, stainless or copper trunks with PEX branches keep stress where it belongs. In homes with whole-house filtration or neutralization on wells, copper may regain viability, but I still use PEX for long attic or crawlspace runs to reduce joints.
Blending isn’t indecision. It’s targeting strengths.
A short homeowner checklist for choosing wisely
- Pull your water report, note pH, hardness, disinfectant, chlorides. Identify soil type and whether lines run in slab, crawl, or attic. Review neighborhood history: pinholes, slab leaks, or freeze bursts. Match pipe to environment: copper where chemistry is kind, PEX where corrosion rules. Demand installation details: sleeving, supports, temperature control, and protection from UV and pests.
What success looks like 10 years later
I judge a repipe not the day after pressure testing, but after a decade. Quiet pipes, no callbacks, and the homeowner forgetting where the manifold sits. In corrosive coastal zones, that picture leans PEX with disciplined routing and insulation. In stable inland neighborhoods with balanced water, copper still earns its keep. In mixed-use buildings with hot recirculation, hybrid systems cut the peaks off the risk curve.
Repipe Plumbing is not a brand sticker or a hunch. It’s field data, local chemistry, soil behavior, and craft tied together. Copper or PEX can both be right. The soil and the water tell you which one wants to live there, and the installation decides whether it will.
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