Pipe Slope: How To Slope a Short Trap Arm

Have you ever wondered how to slope a trap arm (aka the fixture drain)?

The trap arm is the short piece of pipe between the plumbing trap and the vent.

It’s a valid question…

After all, the plumbing code wants horizontal drains sloped at ¼” per foot…

But how does code expect you to slope a pipe that’s a mere few inches long?

Brute force isn’t going to move a pipe that short.

So what do you do?

The answer is easier than you think…

Let’s first clear up a common misconception:

Most folks think a sanitary tee makes a 90-degree angle…

People also believe quarter bends make 90-degree angles.

But do these fittings create a true 90 degree angle?


No, those fittings ARE NOT 90-degrees.

The angle they create is found in ASTM D3311.

According to this standard, all 90° fittings must have “built-in pitch” in the sockets.

And here’s the beauty of it all…

This built-in pitch provides horizontal pipes with ¼” per foot slope.

That means a sanitary tee is actually 91.2 degrees…

And a quarter bend is actually 91.2 degrees.


Pretty cool, right?

Keep in mind, DWV fittings that don’t make a 90-degree pattern don’t have built-in pitch.

For example, an eighth-bend (45) does not have built in pitch, it makes a true 45 degree angle.

Also, this pitch does not occur in vent fittings…

The vent 90 (and the vent tee) make true 90-degree angles.


So what’s the secret to sloping a short trap arm?

Install your vertical pipe “plumb” and that short trap arm will have proper slope thanks to the fitting’s built-in pitch.


See, it’s easier than you thought!

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