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Guest House Plumbing Plan - How do find pipe sizing

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Hello everyone, thanks in advance for your assistance. I'm brand new to DIY plumbing and I'm not sure how to get started. I've made my floor plan and think I know where the plumbing will go generally, however, how do I determine pipe size? My googling last night had me attempting to learn calculus. This is a small project - One toilet, one shower, two sinks.

Does the water line size change based on how much the fixtures demand, the length of the pipe, the elevation of the pipe or all of the above? Do I need to learn specific maths or can this be overestimated?

What about my septic and graywater lines? Are these standard sizes?

Thanks again!
 

FishScreener

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The International Residential Code, or the pluming code in effect at your location will tell you what is required. The codes have tables that say what size of drain line is required for the different types of fixtures.

If you have a sketch, and a not too busy building department, they might help you out, and give you the sizes the code will require for each fixture.
 

breplum

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Buy yourself some plumbing books and read them all or hire a plumber as a consultant, designer and teacher.
The Code Check series are excellent: Redwood Kardon - Code Check
It is not advisable to try to teach yourself because so much goes into the entirety of plumbing and it takes many years to understand codes and standard practices.
Good for you for your desire, but there are countless pitfalls at each step.
 

Zanne

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You're asking about water supply lines, right?
Or do you also need to know sizes for the DWV pipes? I see Frodo gave the sizes for water supply.
Shower needs a 2" drain. Sinks usually have 1-1/4" but you can get fittings to increase to 1-1/2". Toilet gets 3" or 4" depending on main soil pipe size. The vent pipe size depends on the dfu though. You can keep the 1-1/4" or 1-1/2" vent for the sinks. I believe the shower can also have 1-1/2" vent. Toilet should probably have 2" minimum. If they merge to one I would go at least 2". But it does depend on your local codes.

Question for @frodo: Any idea why older toilet supply lines had a 3/8" fitting & shutoff? Was that just an older standard? (Not trying to hijack, was just wondering because the toilets all seem to have 1/2" but all of my old shutoffs were 3/8").
 
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Holy cow everyone, thank you for your responses. I did not expect so many so quick :)

Buy yourself some plumbing books and read them all or hire a plumber as a consultant, designer and teacher.
The Code Check series are excellent: Redwood Kardon - Code Check
It is not advisable to try to teach yourself because so much goes into the entirety of plumbing and it takes many years to understand codes and standard practices.
Good for you for your desire, but there are countless pitfalls at each step.
I hear you. I have a brain, a functional body, and very little money. I'm thinking the compromise might be to hire a plumber to rough in the leech field, septic, and water supply lines - that way all the plumbing that is buried under concrete is professional. Then, based on my floorplan, the rest of the plumbing will be exposed under cabinets or inside walls which will be easier to repair / modify should I do something wrong.

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I'm reusing the existing concrete floor so I'm trying to minimize the number of floor cuts I need to make. The idea here is to cut a roughly 3 x 6' channel in the southeast corner to run wastewater lines out of the south edge of the building. The supply line would be coming in from the east, likely right by the water heater.

Here's the top view of my 3d model. This picture is arranged for proper cardinal directions ( north is up ). It's a bit hard to see at the moment, however at the northernmost wall you can see the sink. Plumbing from that will run under the counter until it enters the false wall on the east. ( east wall is exterior so I'm building a secondary bump out so the pipes will run on the inside of the house but be covered ). Just south of center you can see a circle that represents the hot water heater, then the bathroom has the sink, shower (blue square), and toilet.



 

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Zanne

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Not sure about where you live, but most jurisdictions require leech fields/septic to be installed by a licensed professional & it requires a permit (where I live the permit was free). You can contact your local department of health/sanitation to get a list of licensed contractors & they may need to do an inspection. Look up reviews to see if you can find anything on people you select for the job bc some may have negative reviews & sometimes the government website will say if someone has received a fine for a violation.

If you look around this site, you can find what code applies to your state & sometimes the resources are available online for free to see what the required slope/pipe size/length etc are. There are minimum and maximum lengths.
 

Zanne

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Out of curiosity, does the property already have a septic tank and leech field?

I had mine replaced several years back. We went with Infiltrator Systems. I think it was the Quick4Equalizer36 ones. They are curved chambers that lock together and can go around objects. They are rather large so they require less length than using perforated PVC. It requires larger holes, but you generally don't have to put down gravel as with the perforated system. It ended up being cheaper in the long run than getting perforated. It's also a lot easier to install.
 
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The property already has a septic and leech field on the opposite end. We will likely be using the same contractor who did the main house.
 

frodo

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View attachment 25561
Thank you very much for this

To everyone who gave me measurements thank you -but how did you arrive at these numbers?
i have a chart thingy with numbers and stuff

But n reality. I have been doing this for so long that i just know what size is required
 

Zanne

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The contractor can use different materials (if the infiltrator stuff wasn't already used). Inspector will likely want a percolation test done on the area. Holes will need to be dug a specific size & you'll need a water hose or buckets full of water available for the inspector to pour in to fill the holes. Water levels will be checked later on to see how fast it drains. The inspector will use this info to decide if the soil is suitable for a leech field, size & length of the pipe, etc. There is a maximum length for field lines so if you need more than that length you need multiple lines. We would have needed 300ft of the perforated pipe but ended up needing much shorter runs of the infiltrator systems stuff-- which meant less digging.
The land will also have to be relatively flat. It can't slope too much. At least that's how it was when we had ours done in 2012.
 
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