SDR 26 and SDR 35 are size classifications of PVC pipe. The SDR, or standard dimensional ratio, is the ratio between a pipe’s outer diameter and its minimum wall thickness. As the SDR rating increases, the pipe wall becomes thinner relative to the outer diameter.

That means that PVC pipes with lower SDR ratings are stiffer and can withstand higher internal pressures. Low-SDR pipes are particularly useful in applications like sewer systems where pipes are buried underground and subjected to the weight of the soil above them. High-SDR pipes are more flexible because they have thinner walls and have a much lower pressure rating.

If you are comparing SDR 26 and SDR 35 for a pipe system project, the two are similar. They are both comparable in terms of cost, size options, and types of applications. The main difference between them is the stiffness rating.

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## SDR 26 vs. SDR 35 Comparison

### Applications

SDR 26 and SDR 35 are both used in gravity flow sanitation systems and storm drains. They are only rated for non-pressure drainage applications.

### Size Options

SDR 26 is available in 4”, 6”, 8”, 10”, 12”, 15”, and 18” nominal pipe sizes. SDR 35 is available in eleven diameter options between 4” and 27″. Both pipes are manufactured in lengths of 14’ and 20’ (not including the bell end).

### Minimum Pipe Stiffness

SDR 26 is rated at a minimum pipe stiffness of 115 psi (790 kPa) in accordance with the ASTM D2412 standard. SDR 35 is rated at 46 psi (320 kPa) in accordance with ASTM D3034 and D2412 standards.

## SDR Overview

### Why Use SDR?

SDR is a useful classification if you need to evaluate the mechanical properties of a PVC pipe without taking size into account. For instance, pressure capacity and pipe stiffness are the same for all SDR 35 pipes regardless of their diameters.

### SDR Equation

SDR stands for “standard dimension ratio” and it represents the correlation between a PVC pipe’s outer diameter and minimum wall thickness. It can be calculated using the following equation:

where,

- OD = outer diameter
- t = minimum wall thickness

For example, the SDR for a 4.215” pipe with 0.120” wall thickness is equal to the SDR for a 6.275” pipe with 0.180” wall thickness. Both of these are SDR 35, meaning the pipe diameter is approximately 35 times greater than the wall thickness.

## Effect of SDR on Buried Pipe Systems

The SDR is important in several engineering applications, one example being pipe system designs for sewers. The SDR determines the pipe stiffness, which in turn determines how deep the pipe can be buried. Depending on what conditions you are working with, it could be critical to choose the correct SDR to optimize burial depth.

Pipe stiffness is one of the variables in the Modified Iowa Equation, which is used to calculate several other variables related to buried pipes. The equation looks like this:

where,

- % Defl = predicted percentage of diametric deflection
- W’ = Live Load (kPa)
- P = Prism Load (kPa)
- PS = Pipe Stiffness (kPa)
- E’ = Modulus of Soil Reaction (kPa)

According to the American Society for Testing and Materials (ASTM), the recommended maximum deflection of PVC pipe is 7.5%. However, other research has shown that PVC can deflect up to 30% before it experiences inverse curvature, at which point the Modified Iowa Equation is no longer valid.

Live load and prism load values are dependent on burial depth and can be found in a table. The modulus of soil reaction can also be found in a table; it is based on the type of soil above the pipe combined with the soil’s degree of compaction.

If all other conditions are known (external loads and percent deflection), the Modified Iowa Equation can be used to solve for one of two things:

- The pipe stiffness required to achieve a given burial depth, or
- The maximum burial depth for a given SDR

There have been cases where contractors have used SDR 35 based on Modified Iowa Equation solutions and seen failures at burial depths that should have been within specification. These failures could be a result of minor factors that are unaccounted for in theoretical calculations.

Potential causes for premature failure include PVC material that does not meet specifications, quality issues during the manufacturing process, or discrepancies between local soil and the soil used for standard testing.

Some engineers with sufficient field experience recommend using SDR 26 instead of SDR 35 for certain applications just as a precaution. They maintain that with the SDR 26 being only slightly more expensive, the upgrade and increased factor of safety is worth the extra cost.