If you have gotten a quote for sewer lining, someone has probably told you the liner is “as thick as Schedule 35 pipe.” We have said something close to that ourselves.
It is a useful shorthand. It is also not quite right, and the reason it is not right is worth understanding before you spend $10,000 on a sewer line.
There is no such thing as Schedule 35
Schedule 40 is real. Schedule 80 is real. Schedule 35 is not.
The spec people mean when they say it is SDR 35, and the difference is not pedantic, because Schedule and SDR measure pipe in two completely different ways.
Schedule holds the outside diameter constant for a given nominal size. A 4 inch Schedule 40 pipe and a 4 inch Schedule 80 pipe have the same outside diameter. The higher the schedule number, the thicker the wall, and the smaller the opening inside.
SDR stands for Standard Dimension Ratio. It is the outside diameter divided by the wall thickness. SDR 35 means the wall is one thirty-fifth of the diameter, always, at every size. Double the pipe diameter and the wall gets proportionally thicker on its own.
One is a fixed dimension. The other is a proportion. “Schedule 35” tries to express a ratio in a system that does not use ratios, which is why it does not exist.
Which one is under your house
Both, depending on when it was installed and who installed it.
Schedule 40 PVC is covered by ASTM D1785 and is built for pressure, rated well over 100 PSI. It is the white pipe most people picture. Thick walled, rigid, and common for building drains and many residential laterals.
SDR 35 PVC is covered by ASTM D3034. It is gravity sewer pipe, rated for little to no internal pressure, often green, and standard for sewer mains and larger laterals.
Here is the part that matters: a gravity sewer line does not need pressure rating. Nothing in your sewer lateral is pressurized. It is moving water downhill. SDR 35 is not a cheaper compromise, it is the pipe engineered for that specific job, which is why municipalities specify it.
So when someone compares a liner to Schedule 40, they are comparing it against a pressure pipe standard that your gravity sewer never needed.
What a CIPP liner is actually rated for
This is where the shorthand breaks down entirely.
A cured in place liner does not have a schedule, and it does not have an SDR. It is not manufactured to a pipe dimension standard at all.
Liner thickness is engineered per job under ASTM F1216, the recognized standard for inversion lining. The design accounts for:
- Whether the host pipe is partially deteriorated (still carrying soil load) or fully deteriorated (carrying nothing, so the liner must carry everything)
- Depth of cover over the pipe
- Groundwater level above the pipe
- Soil load and any live load, such as a driveway or street above it
- Ovality, meaning how far out of round the existing pipe has gone
Two liners in the same 4 inch clay pipe on the same street can be specified at different thicknesses because one runs under a lawn at 5 feet and the other runs under a driveway at 9 feet.
That is a meaningfully better way to build a pipe than picking a schedule off a shelf. A stick of Schedule 40 is the same wall thickness whether it sits under a flower bed or a loaded truck.
So is the shorthand wrong
Not wrong. Incomplete.
When a contractor says the liner is comparable to SDR 35, what they mean is that the finished liner has structural capacity in the same class as the sewer pipe that would have been installed as a replacement. That is a fair and useful comparison, because SDR 35 is the realistic alternative. Nobody is trenching your yard to install pressure pipe in a gravity line.
The shorthand only becomes a problem when it is used to make a liner sound thinner than it is, usually by a contractor selling excavation who compares a liner against Schedule 40 rather than against the pipe that would actually go in the ground.
Questions worth asking any lining contractor
Three questions separate someone who engineers the job from someone who pulls a liner off the truck.
1. What thickness are you installing, and how did you arrive at it?
The answer should reference depth, groundwater, and whether the host pipe is treated as partially or fully deteriorated. “Whatever we normally use” is not a design.
2. Are you designing to fully deteriorated or partially deteriorated?
Fully deteriorated is the conservative assumption. It means the liner is designed to carry the load on its own, without help from the old pipe. It calls for a thicker liner and it is the right assumption on a badly degraded line.
3. What standard are you installing to?
ASTM F1216 for inversion lining, ASTM F1743 for pulled in place. If a contractor cannot name the standard they install to, that tells you something.
What we do
We run Hammerhead lining with the Blue Light UV curing system, and every job starts with the camera rather than with a default spec.
That starts with the inspection. Depth, pipe material, condition, how far out of round it has gone, what is above it. Those inputs are the design, and you cannot get them from the driveway.
We include the camera inspection free on service calls. You watch the screen with us and see the same footage we do before anyone talks about thickness or price.
Contact us or call 218-409-6356.