“Three Rules of Plumbing” or “Three Laws of Plumbing”—sometimes four—is a jocular list that has been printed on many images. Authorship is unknown, but the rules/laws have been cited in print since at least 1990.
 
Some of the laws (in no particular order):
     
1. Hot’s on the left, cold’s on the right.
2. Shit runs downhill.
3. Don’t chew your fingernails.
4. Payday is on Friday.
   
 
Google Groups: comp.software-eng 
Is Programming R&D or Production?
Christopher Lott
Jun 22, 1990, 8:03:06 AM
(...)
The three rules of plumbing (heard from an ex-plumber friend)
1. Sh*t runs downhill.
2. Never chew your fingernails.
3. Payday’s on Friday.
     
Google Groups: misc.consumers.house
Moving a W.C
H. Austin Hummel
Jan 12, 1994, 11:31:13 AM
(...)
Also remember the rules of plumbing:
1) hot on the left
2) cold on the right
3) crap does not run up hill
4) vent it
 
Google Groups: aus.jokes
Three Rules of Plumbing
Robert Bissett
Oct 20, 1996, 3:00:00 AM
First things an apprentice plumber has to learn:
1. Shit flows downhill.
2. You get paid on Friday.
3. Don’t bite your fingernails.
       
Google Groups: alt.fans.heinlein
Tanstaafel???
Doug Jones
Jan 17, 1997, 3:00:00 AM
(...)       
The three laws of plumbing (illustrate with hand gestures):
1. Water seeks its own level.
2. Shit flows downhill.
3. Don’t bite your fingernails.
   
Google Groups: alt.hvac
“Seven Basic Principles Of Plumbing Design”
Brian A. Rock
Dec 2, 1998, 3:00:00 AM
(...)
I’ll go there, but I bet I know three—the “three laws of plumbing”:
1) Hot’s on left, cold’s on right.
2) Payday is Friday
3) Effluent does not flow up hill!
 
Google Groups: alt.tasteless.jokes
Physics for Amatures
Vanilla Gorilla (Monkey Boy)
May 14, 2003, 11:26:40 PM
(...)
The Three Rules of Plumbing:
1) Hot on the Left
2) Cold on the Right
3) Shit don’t flow uphill
 
Twitter
Mark Leon Watson
@MarkLeonWatson
just learned 3 golden rules of plumbing:
1. sewage flows downhill
2. payday is on friday
3. don’t chew your fingernails
4:23 PM · Oct 19, 2007·Twitter Web Client
   
Twitter
River Campbell
@absynthefaerie
Replying to @twobears29
@twobears29 Remember the two cardinal laws of plumbing.  Shit rolls downhill and payday’s Friday.
4:51 PM · Nov 25, 2009·Twitter Web Client
 
Twitter
Goozbach
@goozbach
two rules of plumbing: 1) Crap flows downhill 2) pressure makes little leaks into big leaks
6:06 PM · Apr 26, 2010·identica
     
Facebook
Pitzer’s One Hour Air Conditioning & Heating
November 10, 2010 at 1:27 PM   ·
Since we are starting plumbing soon, I think it is important that we all know the 4 Rules of Plumbing:
1. Water Runs Down Hill
2. Hot is on the Left
3. Pay Day is on Friday
4. Don’t Bite Your Nails
I think we got it!
 
Twitter
Greg Schmidt
@youenjoymybrew
Just had to use the 3 rules of plumbing:
1. Tight is to the right
2. Shit runs downhill
3. Don’t bite your fingernails.
9:27 AM · Oct 14, 2011·Twitter for iPad
   
Twitter
Smartass Dachshund
@cactibeach
The 3 Laws of Plumbing:
1. Water flows down hill.
2. Pay Day is Friday.
3. Don’t bite your nails.
11:22 PM · Mar 27, 2013 from Phoenix, AZ·Twitter for iPhone
 
Reddit—AskReddit
Posted by u/rotosound February 12, 2015
What are the “three rules” of your job?
My nephew recently started learning to be a plumber. His boss told him the three rules of plumbing are:
1. Everything washes off with soap and water.
2. Shit doesn’t run uphill.
3. Don’t bite your fingernails.
What is your job and what are the three rules of your job?
             
Twitter
Ryan Smith
@20threeto0
The three rules of plumbing
#1 Don’t chew your fingernails
#2 Shit runs downhill
#3 Paydays are Fridays
1:13 PM · Oct 8, 2015 from Odessa, TX·Twitter for Android
 
Twitter
Pipe Masters
@pipemastersoahu
THE 3 Rules of Plumbing; 1) “stuff” runs down hill, 2) Payday is Friday, & 3) Don’t eat with your fingers. Smile everyone! It’s Turkey Week!
12:54 AM · Nov 23, 2016·Twitter Web Client
 
Twitter   
Todd Keiser
@dawgpound63
Replying to @markschlereth and @YouTube
The 3 rules of plumbing every man should know:
1. Sh!t rolls down hill
2. Pay day is on Friday
3. The boss is an A- hole
2:00 PM · Aug 29, 2019·Twitter for Android
   
Twitter
On The Tools
@onthetoolstv
3 laws of plumbing:
1) Shit runs down hill
2) Pay day is Friday
3) Don’t eat with your hands
12:00 PM · Feb 15, 2020·TweetDeck
 
Twitter
Toquey
@Toquey2
Replying to @Toquey2 and @joe_hunglo
Just remember the main three rules of plumbing and you will do fine
1) Payday is on Friday
2) Shit goes downhill
3) never eat the last corner of your sandwich
11:15 AM · Nov 29, 2020·Twitter for Android
 
Twitter
Erik Hultgren
@hultgren_erik
Replying to @RadioFreeTom
Three rules of plumbing:
1. Shit runs down hill.
2. Don’t put your fingers in your mouth.
3. Friday is payday.
10:54 PM · Dec 14, 2020·Twitter for Android
 
Twitter
Walker Parks
@thewalkerparks
Remember these Three Rules of Plumbing, kids: Hot is on the left. Shit rolls downhill. Payday is on Friday.
4:57 PM · Jan 15, 2021·Twitter for iPhone
 
Reddit—Jokes
Posted by u/kimthealan101 January 20, 2021
The 4 rules of plumbing
Payday is Friday
Shit flows downhill
The boss is an ass
Don’t chew your fingernails