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Waste pipe sizing


chrisprickett
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A buddy of mine asked me to look at his house under construction. I found that it looks as though the builder forgot to add a dedicated waste line for one bathroom, so he tied two bathrooms into one 3" waste line.

The first bath has a toilet, sink, and bath, with a 3" waste line traveling about 30' (pitch is bad, but that's a different subject). It flows into a a 3" wye to the 3" waste stack.

The other bath is two sinks, a tub and a toilet, that ties into the same 3" wye as the first bath.

Both baths are second floor. I tried to get an answer from the IRC sizing charts (P 3004 & 3005) but it was a clear as mud. I'd appreciate some direction to the right answer and documentation.

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I think a 3" at 1/8" slope per foot can handle up to 36 drainage fixture units. I don't think 2 full baths come close to that value. The IRC should list the values per bath with an additional factor for simultaneous use. Again, it should be well below 36 DFU's.

Hope this helps until someone who knows this stuff replies.

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Originally posted by chrisprickett

A buddy of mine asked me to look at his house under construction. I found that it looks as though the builder forgot to add a dedicated waste line for one bathroom, so he tied two bathrooms into one 3" waste line.

I see this almost every day of the week. How do they normally do it in your area?

The first bath has a toilet, sink, and bath, with a 3" waste line traveling about 30' (pitch is bad, but that's a different subject). It flows into a a 3" wye to the 3" waste stack.

The other bath is two sinks, a tub and a toilet, that ties into the same 3" wye as the first bath.

Both baths are second floor. I tried to get an answer from the IRC sizing charts (P 3004 & 3005) but it was a clear as mud. I'd appreciate some direction to the right answer and documentation.

I can't help you with the IRC. Oregon doesn't use it for plumbing. But I think that the rules are the same. A toilet is 3 fixture units, a tub/shower is 2 and a sink is 1.

For the two bathrooms, you've got a total of 12 fixture units.

At 1/4 inch per foot, your 3" pipe can handle 35 fixture units in the horizontal run and 48 in the vertical run. If you've only got 1/8 inch per foot, then you multiply the horizontal quantity by .8.

Unless the pipe is at less than 1/8 inch per foot, I don't see a problem.

- Jim Katen, Oregon

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Thanks, good info.

Around here, it's common practice to run a 3" line per bathroom, and drop the waste lines vertically, and te them in under the slab. Doing a 30' horizontal run is uncommon.

The reason I had concerns was because that was not how the builder did it in other models at the same subdivision. The configuration was definitely a solution to missing a waste stub-out when they installed the soils and poured the slab

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