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Sch. 40 PVC vs. SDR21 PVC, how different are they?

Sch. 40 PVC vs. SDR21 PVC, how different are they?

Has anyone ever heard the argument made that ASTM D-1785 Schedule 40 PVC pipe is manufactured differently or is more brittle than ASTM D-2241 SDR-21 PVC pipe?

I design lower pressure (<125 PSI) STEP collection systems and small community wastewater treatment systems.  I was recently told by the state department of environmental quality (DEQ) that I could not specify Sch. 40 pipe because it is "not rated for burial" and that they had concern that it will not handle the soil load on the pipe (no more than 5' of cover typically).

I pointed out that both pipes are typically made with  Type I, Grade I compund, Cell classification 12454-B per ASTM-D1784 (A.K.A. 1120).  I also pointed out that for pipes <5" in diameter, Sch. 40 pipe has a thicker wall thickness than SDR21 pipe.  For pipes 2-1/2" and smaller, it is over 150% thicker.

DEQ's reply was that they think Sch. 40 pipe is manufactured for "beam strength".  And that they think it was manufactured differently to give it greater strength and less elasticity because the basis for Sch. 40 pipe is "non-pressure loading such as handling and supporting its own weight".  Therefore it is too brittle to be buried in the ground.

Is this a valid statement, or is it a load of horse feathers?  I have been told that this is their point of view, but they've not shown me anything to support it and they have basically told me to prove them wrong.

Thanks in advance for assistance!

RE: Sch. 40 PVC vs. SDR21 PVC, how different are they?

Sounds like horse feathers!  Both products are extruded, only difference is wall thickness (as you pointed out).  There is alot of schedule 40 buried pipe out there.  Many plumbing installations use sch40 with DWV fittings.  Check out  They have information on burial, and they make schedule 40 and SDR pipe.  The other one to look at would be Charlotte Pipe (

RE: Sch. 40 PVC vs. SDR21 PVC, how different are they?

See Harvel site,  They make both types of PVC pipe.  The materials and specifications are identical except for the wall thickness and diameters.  The Schedule 40 is thicker and thus stronger in all measurement methods.  The DEQ critic is ignorant and needs education in specifications of pipe.

RE: Sch. 40 PVC vs. SDR21 PVC, how different are they?

Absolutely horsefeathers!

Same material and same process but with a different way to establish the pressure rating.

PVC has been buried in the ground since it was first manufactured!  SDR 35 (very thin walls) was THE replacement for vitrified clay pipe!  SCH 40 was first manufactured for pressure applications.  Shortly thereafter drainage pattern fittings were developed for use under building slabs.  Apparently someone felt SDR 35 wasn't good enough for underslab installation.

RE: Sch. 40 PVC vs. SDR21 PVC, how different are they?

Thanks for all the replies.  The folks at Harvel have been very helpful and have basically backed up my statements.  Time will tell if the DEQ will come around.  I guess it needs to go before a committee before they can straighten it out.

On a related note...  this whole thing has piqued my curiosity.  Does anyone know the details of the history of the development of the standards?  I am assuming Sch. 40 PVC showed up first and they just carried over the dimensions from iron pipe.  But how were those dimensions developed and when?

RE: Sch. 40 PVC vs. SDR21 PVC, how different are they?

I believe you are probably essentially correct, in that I think the first version of ASTM D1785 for pvc pipes was adopted in ~1960, followed by the first version of D2241 (now mentioning SDR’s etc.) in ~1964.  I believe a system mentioning “Schedules” of steel and/or various iron etc. pipes existed long before then, as I think I remember having seen same in versions “Crane” piping books with similar thicknesses per sizes/schedules as far back as the 1940’s or ‘50’s. [However, in fairness to the DEQ folks, is it possible that even if not accurately/eloquently stated they might be aware of some experience of some sort that elicits concern, as pvc pipes with these varied designations could be more or less equally “brittle” (depending on actual formulations of fillers etc. used by the pvc manufacturer, temperature, how and with what the pipe is impacted etc.)?   – in this regard I believe DIPRA in recent years had actual impact specimens of commercially available actual pvc pipes comparatively tested at room temperatures alongside samples of ductile iron pipes, with results as now shown on Figure 8 page 6 of the document at  If impacted in very cold temperatures or with  the most unfavorable concoctions of cheapening fillers possible, perhaps the  differences could be even more “striking”.]

RE: Sch. 40 PVC vs. SDR21 PVC, how different are they?

Why is the ASTM standard for SDR 'pressure rated' wheras the ASTM standard for Sch 40 is not designated 'pressure rated'?  ie - what is the significance of 'pressure rated'?

Additionally, schedule pipe allows for up to 20% non PVC content, and SDR pipe doesn't.

I would appreciate any comment you may have on these items.

RE: Sch. 40 PVC vs. SDR21 PVC, how different are they?



Why is the ASTM standard for SDR 'pressure rated' wheras the ASTM standard for Sch 40 is not designated 'pressure rated'?  ie - what is the significance of 'pressure rated'?
RYANTD:  My understanding of the significance of "pressure rated" is this.  "Pressure rated", in regards to SDR21 pipe, means just that.  Since the ratio of wall thickness to pipe diameter is constant, all SDR21 pipe regardless of diameter will have the same pressure rating.  That is all "pressure rated" means.  SDR21 pipe has a pressure rating of 200 PSI.
SDR26 pipe has a pressure rating of 160 PSI.  Every pipe diameter in a particular SDR is rated at the same pressure.  This can be very useful if you are designing a system with a range of pipe sizes operating at the same pressure throughout.

Schedule 40 pipe on the other hand has a varying ratio of wall thickness to pipe diameter.  Therefore, there is no way one can make the general statement that Schedule 40 pipe is rated at XX psi.  Because the ratio varies, every pipe diameter is capable of withstanding a different pressure.  However, due to the physical characteristics of the pipe, the "pressure rating" of Sch. 40 pipe tends to follow the trend of varying inversely proportional to the diameter.  That is, the smaller the diameter of the pipe, the larger the pressure it is capable of withstanding.

The systems that I design generally do not exceed 40,000 GPD.   These systems have never required any pipe greater than 4" in diameter.  I have no use in specifying a whole range of pipe diameters that are "pressure rated" at a constant pressure.  Especially when I can specify a material that is far more robust than any readily available SDR product.


Additionally, schedule pipe allows for up to 20% non PVC content, and SDR pipe doesn't.

This might have some bearing on the argument they are making.  Where does that statement come from?  If I specify that the pipe be made of PVC 1120, Type I, Grade I Cell Classification 12454 per ASTM D1784 (as is typical of both types of pipe) are they not then made of the same material?

For anyone else following this thread that might find this information useful:

Here is what Harvel had to say on the matter:


Thank you for contacting Harvel with your inquiry.  Please note that Harvel manufactures both Sch 40 and SDR series PVC pipe from our own blend of NSF Listed PVC material.  There is no difference in the PVC material that we utilize to manufacture these products (PVC 1120, Type I, Grade I Cell Classification 12454 per ASTM D1784) nor any differences manufacturing processes.  As you are aware, the difference is in the physical dimensions of the products (wall thickness/pressure bearing capability).  Perhaps they may have Sch 40 PVC solid wall pipe confused with Sch 40 foam core PVC pipe?.

Harvel  solid wall Schedule 40 PVC pipe has been used for direct burial applications since its inception over 45 years ago (as have the other schedules and SDR's that we produce). I have attached a copy of our product specification sheet applicable to our Sch 40 piping for your review and submission to MDEQ.  It should be noted that there are basically two separate ASTM standards pertaining to Sch 40 PVC; ASTM D2665 for DWV (includes deflection load requirements for buried lines) and ASTM D1785 for pressure (includes hydrostatic pressure requirements for long-term pressure bearing).  Harvel Sch 40 PVC pipe is Listed by NSF as meeting both of these requirements, and is dual-marked as such.   This information is referenced in the attached product specification.  Please also refer to the information below from our on-line Technical Support Center pertaining to Critical Collapse Pressure of the various piping products we produce.  As can be seen, small diameter Sch 40 piping (4" & smaller) can handle a higher load than the same size SDR 21 piping due to the difference in wall thickness of the products.  I have also attached information below pertaining to the installation of buried pipe which may prove beneficial for future reference.

Of particular interest was the attached engineering data pertaining to burial pressures.  Here is a link to that information:

It shows a comparison of PVC duct, SDR 41, SDR 26, SDR 21, SCH 40, SCH 80, and SCH 120 pipe.

Thanks to everyone for all of the information and comments.

RE: Sch. 40 PVC vs. SDR21 PVC, how different are they?

You might, too, want to check out AASHTO Specification M-278-04 which is for PS46 PVC Pipe used in french drains for comparisons of other PVC specifications.  While wall thicknesses may be more than schedule 40 (wall thicknesses depend on the size of the pipe - 4 inch minimum), the specification of the pipe does give the various test values required of the pipe which might relate to your schedule 40.  Also AASHTO M304M

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