Pipe Roughness & Relative Roughness: What Engineers Need to Know

What Is Pipe Roughness?

Pipe roughness (ε, the Greek letter epsilon) is the average height of the surface irregularities on the inside wall of a pipe. Think of it as the microscopic peaks and valleys on the pipe surface. A drawn copper tube feels smooth to the touch. A sand-cast iron pipe from a century ago feels noticeably gritty. Both have quantifiable roughness values that directly affect how much energy a fluid loses flowing through them.

Roughness is a material and manufacturing property. It depends on how the pipe was made, what it is made from, and how it has been treated or coated. The value in reference tables is always for new pipe in clean condition. What happens to roughness over time is a separate and often more important question, covered in the aging section below.

In the context of the Moody Chart, roughness is what determines which curve you follow in the turbulent flow region. Higher roughness means a higher curve, a higher friction factor, and higher pressure drop for the same flow conditions.

Absolute Roughness vs Relative Roughness

Absolute Roughness (ε)

Absolute roughness is the physical height of the surface irregularities, expressed in millimeters or meters. It is a property of the pipe material and manufacturing process. Commercial steel has an absolute roughness of about 0.046 mm. PVC has about 0.0015 mm. These values do not change based on pipe size.

Relative Roughness (ε/D)

Relative roughness is absolute roughness divided by pipe inner diameter. It is dimensionless. This is the value that actually governs friction factor in turbulent flow, not absolute roughness alone.

The reason relative roughness is what matters becomes clear when you think about it physically. A 0.046 mm roughness on a 10 mm pipe covers 0.46% of the radius. The same roughness on a 1000 mm pipe covers only 0.0046% of the radius. The large pipe barely notices the roughness. The small pipe is significantly affected. Same material, completely different hydraulic behavior.

This is why you cannot just look up a friction factor for a pipe material without also knowing the diameter. You enter relative roughness into the Moody Chart Calculator, not absolute roughness.

A Simple Analogy

Imagine walking across a field with 5 cm tall grass. For a person 1.8 m tall, that is barely noticeable. For a cat 25 cm tall, it is a serious obstacle. The grass height (absolute roughness) is the same. The grass-to-body ratio (relative roughness) is completely different. That ratio determines how much the terrain affects movement.

How to Calculate Relative Roughness

Pipe Roughness Values by Material

These values are for new pipe in clean, uncoated condition unless noted. Real-world values vary by manufacturer, age, and service conditions. Use these for design calculations. For analysis of existing systems, verify against measured pressure drop data where possible.

Sources: Moody (1944), Engineering ToolBox, and manufacturer data. Values are for new pipe in clean condition. Actual roughness increases with age and service conditions.

How to Read This Table

The ε/D range gives you a practical check on your calculation. If you compute a relative roughness outside the range for your pipe material, check your diameter units. It is the most common input error.

For materials with a roughness range rather than a single value (like concrete or riveted steel), use the midpoint for preliminary calculations and run a sensitivity check at both ends of the range to see how much friction factor varies. In rough concrete culverts, the spread from smooth to rough can change friction factor by a factor of two or more.

How Roughness Affects Friction Factor

The relationship between roughness and friction factor is not uniform across all flow conditions. It depends entirely on which zone of the Moody Chart you are operating in.

Laminar Flow: Roughness Has No Effect

When Reynolds number is below 2,300, friction factor follows f = 64/Re regardless of pipe roughness. You can have brand new smooth tubing or a heavily corroded old pipe — the friction factor is the same. In laminar flow, the viscous forces completely dominate and the flow never contacts the roughness elements directly. This is why the Moody Chart's laminar region is a single line with no roughness curves.

Turbulent Smooth Zone: Roughness Has Minimal Effect

Just above Re = 4,000, the roughness curves on the Moody Chart are close together near the smooth pipe line. In this zone, the viscous sublayer at the pipe wall is thick enough to submerge the roughness elements. Flow sees a hydraulically smooth surface regardless of the actual material. Friction factor is determined primarily by Reynolds number, not roughness.

Transitional Rough Zone: Both Re and Roughness Matter

As Reynolds number increases, the viscous sublayer thins. Roughness elements start to protrude through it and disturb the flow. Friction factor now depends on both Re and relative roughness. This is the middle region of the turbulent zone where the curves splay apart. The Colebrook-White equation covers this entire region.

Fully Rough Turbulent Zone: Only Roughness Matters

At high Reynolds numbers in rough pipes, the sublayer is thin enough that roughness elements fully protrude. Friction factor becomes constant — it no longer changes as you increase flow velocity or Re. The curves on the Moody Chart run horizontally in this zone. You can calculate friction factor directly from:

This is the simplified fully rough version of the Colebrook-White equation, with the Reynolds number term dropped. Use this as a quick hand calculation when you know you are in the fully rough zone. It gives the maximum friction factor for that pipe material at any Reynolds number.

Practical Implication: Smooth vs Rough Pipe Selection

Switching from commercial steel to PVC on a 100 mm water pipe at Re = 100,000 reduces relative roughness from 0.00046 to roughly 0.000015. Running this through the Moody Chart Calculator, friction factor drops from about 0.019 to 0.016, a 16% reduction. Over a long pipe run, that difference directly reduces pump energy consumption and operating cost. The table below shows how dramatically friction factor varies across materials at the same conditions.

Rough concrete is already in the fully rough zone at Re = 100,000, so its friction factor does not change with velocity. Cast iron and galvanized steel approach fully rough behavior at high Re. PVC continues to benefit from increased Re well into the turbulent range.

The Viscous Sublayer Explained

Understanding why roughness matters in turbulent flow but not laminar flow comes down to one thin layer of fluid at the pipe wall: the viscous sublayer.

Even in fully turbulent flow, there is a very thin region right at the pipe wall where viscous forces dominate and flow is locally laminar. This sublayer is typically only a fraction of a millimeter thick, but it acts as a smooth shield between the bulk turbulent flow and the rough pipe surface.

The sublayer thickness decreases as Reynolds number increases. At low turbulent Re, the sublayer is thick relative to the roughness elements, which stay buried underneath it. The pipe behaves hydraulically smooth. As Re rises, the sublayer thins. Roughness elements start poking through. Each element creates a small wake that adds momentum loss. At very high Re, the sublayer is so thin that all roughness elements are exposed. The pipe is hydraulically rough and friction factor is controlled entirely by the surface texture.

The transition between hydraulically smooth and hydraulically rough behavior for a given pipe is described by the roughness Reynolds number: Re* = u* ε / ν, where u* is the friction velocity. When Re* is below about 5, the pipe is hydraulically smooth. Above about 70, it is fully rough. Between those values, both roughness and Re matter.

For design purposes, you do not need to calculate Re*. The Colebrook-White equation handles all three regimes automatically. But knowing the sublayer concept helps you understand why a smooth pipe at very high flow velocity can suddenly start behaving like a rough pipe, and why old pipe with increased roughness causes more problems at high flow rates than at low flow rates.

Aging, Corrosion, and Fouling

The roughness values in any reference table describe new pipe in clean condition. Real pipes in service are rarely either of those things. This section covers what actually happens to roughness over the life of a pipe.

Corrosion

Unprotected steel and iron pipes corrode when carrying water, especially water with dissolved oxygen or low pH. Corrosion products (rust, iron oxides) build up on the pipe wall and dramatically increase roughness. A commercial steel water pipe that starts with ε = 0.046 mm can reach an effective roughness of 0.5 to 3 mm after 20 to 30 years of service without corrosion protection. That is a 10 to 65 times increase in absolute roughness.

The friction factor consequence is severe. A 200 mm steel pipe in the fully rough zone at ε = 2 mm has ε/D = 0.01 and a friction factor around 0.040. The same new pipe at ε = 0.046 mm has ε/D = 0.00023 and a friction factor around 0.015. The corroded pipe needs more than twice the pump energy to maintain the same flow rate.

Tuberculation

Tuberculation is a specific form of corrosion that affects cast iron and ductile iron pipes in water service. Localized corrosion produces raised mounds (tubercles) on the pipe wall, sometimes 5 to 15 mm tall. These protrusions are far larger than the original surface roughness and dominate hydraulic behavior. Tuberculated cast iron mains can have effective roughness values of 1 to 5 mm, compared to the new pipe value of 0.26 mm. This is why old cast iron water mains often require relining or replacement to restore system capacity.

Scaling and Mineral Deposits

Hard water deposits calcium carbonate scale on pipe walls, particularly in hot water systems. Scale buildup reduces pipe diameter and increases effective roughness. Both effects raise pressure drop. Scale is more of a diameter-reduction problem than a roughness problem in most cases, but the combined effect can be significant in heat exchangers and domestic hot water pipes over 10 to 20 years.

Biofilm and Fouling

Biological growth (biofilm, slime, algae) in water and wastewater pipes increases effective roughness, especially in pipes with low flow velocities where biofilm has time to establish. The effect is highly variable and difficult to predict without site-specific data.

What to Do About It

For new system design, add a roughness aging factor. A common approach in water supply engineering is to assume cast iron pipes reach an effective roughness of 0.6 to 1.2 mm over their design life, rather than using the new pipe value of 0.26 mm. For steel pipes, design with ε = 0.1 to 0.3 mm rather than 0.046 mm if long-term operation without recoating is expected.

For analysis of existing systems, do not trust new pipe roughness values. Back-calculate effective roughness from measured flow rates and pressure drops. The Darcy-Weisbach equation rearranged for roughness gives you the effective roughness directly from field measurements.

How to Find Roughness for Existing Pipes

When you are analyzing an existing system rather than designing a new one, three approaches work depending on what data you have access to.

Back-Calculate from Pressure Measurements

This is the most reliable method. Measure pressure drop (ΔP) across a known pipe length (L) at a measured flow rate (Q). Calculate mean velocity (V = Q/A) and Reynolds number. Use the Darcy-Weisbach equation to find friction factor: f = ΔP × D / (L × ρ × V² / 2). Then rearrange the Colebrook-White equation to find relative roughness from f and Re. The result is the effective roughness of the actual pipe in its current condition, accounting for all aging effects.

Use Age-Adjusted Roughness Values

Engineering references like the American Water Works Association (AWWA) manuals and Hydraulic Institute standards publish roughness data for pipes at various ages and service conditions. If you know the pipe material, age, and service fluid, these tables give more realistic values than the new pipe defaults.

Inspect or Test a Sample

For critical systems with high consequence of error, a pipe section can be removed and surface roughness measured directly using profilometry. This is rarely practical for large infrastructure, but it is done for high-value industrial systems during shutdown maintenance.

Practical Design Tips

Always Calculate ε/D, Not Just ε

Two engineers specifying the same material can get different friction factors if they are working with different pipe diameters. Always compute relative roughness explicitly before looking up or calculating friction factor. Do not assume that a rougher material always means a higher friction factor — a large rough pipe can have lower friction factor than a small smooth pipe.

Run Sensitivity Checks on Roughness

For design calculations, run your friction factor at both the low and high end of the roughness range for your material. For concrete, that spread can be an order of magnitude. If your pump selection or pressure drop budget is sensitive to roughness, that sensitivity analysis protects you from undersizing.

Account for Aging in Long-Life Systems

If you are designing water or wastewater infrastructure with a 50-year design life, do not base pump sizing on new pipe roughness. Use end-of-life roughness estimates. The AWWA Manual M11 for steel pipe and Manual M9 for concrete pipe give guidance on design roughness values that account for expected service conditions.

Consider Lining for Rough Materials

Ductile iron pipe with cement mortar lining has effective roughness of 0.025 to 0.1 mm, compared to 0.26 mm for unlined ductile iron. For large-diameter pipes where small reductions in friction factor mean significant energy savings over decades, lining is often cost-effective. The same logic applies to epoxy lining of steel pipes in corrosive service.

Check Your Result Against the Moody Chart

After calculating friction factor numerically, verify it makes sense on the interactive Moody Chart. Your operating point should sit on the correct roughness curve and in the right flow zone. If it plots far from where you expect, recheck your inputs. The visual check catches unit errors and wrong roughness values before they propagate through your calculation.

Frequently Asked Questions