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What Effect does the pipe pitch of drain lines in a home or commercial building have with regards to a possible backup?

Pipe pitch maybe one of the most important concepts in plumbing. It’s where the phrase “shit goes down and stink goes up” originates from. Waste water is supposed to drain away from the fixture it serves. Did you know that DWV (Drainage, Waste and Vent) fittings have pitch already built into the fittings. This means when you put pipe together with drainage fittings the piping will never be completely straight. It will always have some pitch to it.

  • Having the correct pitch is paramount when installing plumbing drain lines. Most people think that having more pitch is better for
    Picture of Pipe Pitch

    Pipe Pitch Examples

    a sewer drain line, this is not the case. Think about how water would flow with suspended solids. There is a point where too much pitch can actually be the cause of back-ups. The proper pitch for a sewer line is a ¼” per foot of pitch. As you steepen the pitch the velocity of the water increases, if the pitch is too steep the water may begin to pass up the suspended solids leaving them behind. If the solids continue to be left behind you’ll have a full blown sewer back-up. Remember this “a lazy sewer is a good sewer”. You’re better off putting less pitch on the pipe so the solids remain in suspension moving slowly along and eventually discharging into the municipal sewer.


Pitch Problems an Installer Error?

Making sure the piping and/or fittings are joined correctly and having the proper pitch are the first two things a plumber should check upon installation. But there are other factors involved. One key factor is the backfill. You must either compact the C6/C7 gravel to make sure the piping doesn’t move or backfill with a self-compacting material like pea gravel. Most municipalities specify what is allowed and most cases they also insect the installation before final backfill but I’ve experienced many occasions where plumbing contractors are left to their own discretion. You must make sure the fill supports the pitch. If you do not you run the very real risk of the ground settling creating bellies like the negative slope picture above or piping coming completely apart.

Waste Pipe Shearing

Waste pipe shearing at the foundation is one of the biggest problems we see in homes without proper backfill. Homebuilders tend to backfill with existing spoils next to the foundation. That backfill has been aerated. It’s not compacted anymore. So when you push it back in the gap around the foundation you’re prone to settling. When the ground settles around the house drain or house sewer the weight can shear off the pipe at the foundation. You now have a negative slope back to the house which creates all kinds of blockage issues. We have seen instances where the backfill expands underneath the pipe and shears it upwards which creates an instant blockage.

Can The Type of Material Lower Your Chances of Pitch Problems? 

The answer is yes with regards to the above issue above. Cast iron soil pipe is a much stronger material than PVC. We have not experienced many instances of pipe shearing when cast iron is used. For the rest of the house sewer, pitch issues remain constant regardless of the material used for pipe and fittings.



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