Helped my neighbor rough in his kitchen sink last month. He had the diagram upside down for about twenty minutes before I noticed. We laugh about it now.
What You’re Actually Looking At
A rough-in diagram shows where pipes go before the walls close up. Water comes in, waste goes out, and everything needs to line up with your sink. Mess this up and you’re cutting drywall later.
Four things matter: supply lines, drain pipe, P-trap location, and venting. Get those right and everything else falls into place.
Water Supply Lines
Hot and cold come up through the wall or floor. Usually copper, PEX, or CPVC depending on when your house was built and who did the work.
Standard spacing is 4 to 6 inches apart, centered about 12 to 15 inches off the floor. That puts them right behind where your sink cabinet sits.
Each line gets a shutoff valve. Angle stops, they’re called. When your faucet starts dripping at 2 AM, you’ll be glad those valves are there.
The Drain Setup
Drain pipe is typically 1.5 to 2 inches diameter. Kitchen sinks need the larger size because food particles and grease go down there, whether they should or not.
The drain assembly connects your sink strainer to the wall pipe. Tailpiece drops down from the sink, P-trap makes that curved connection, then it runs into the wall.
Slope matters. Quarter inch drop per foot toward the main drain. Water flows downhill. Obvious but I’ve seen it done wrong.
P-Trap Placement
That curved pipe under your sink holds water. Not because of bad design but on purpose. The water blocks sewer gas from coming up into your kitchen.
Usually sits 18 to 24 inches below the counter. Low enough to catch water, high enough to access when something gets stuck. And things will get stuck.
Connection fittings are hand-tight plus a quarter turn with a wrench. Over-tighten and you crack plastic. Ask me how I know.
Venting
This is what most DIYers forget. Drains need air behind the water to flow properly. Without a vent, you get that gurgling sound and slow drainage.
Vent pipes run up through the roof. They’re sized and placed according to local code, which varies. Check before you assume.
Some newer installations use air admittance valves instead of traditional vents. Depends on what your inspector allows.
Measurements to Know
Before you start cutting holes, measure twice. Then measure again. Write it down.
- Supply lines: 12-15 inches from floor
- Hot and cold spacing: 4-6 inches apart
- Drain centerline: 18-24 inches from floor
These work for most standard sinks. Deep farmhouse sinks or vessel sinks need adjustments. Know your fixtures before roughing in.
When Things Go Wrong
Leaks happen at connections. Tighten compression fittings first. If that doesn’t work, take it apart and check for debris or damaged threads.
Slow drains usually mean buildup in the P-trap or past the wall. Plunger first, then snake if needed. Chemical drain cleaners are hard on pipes and rarely solve the real problem.
Clogs at the strainer just need cleaning. Pop it out, clear the gunk, put it back. Weekly if you don’t have a garbage disposal.
Code Compliance
Your local building department has rules about all this. Pipe sizes, vent requirements, fixture spacing. A permit might be required for new rough-in work.
Worth getting right. Inspectors catch problems before they become expensive. And when you sell the house, unpermitted plumbing can kill a deal.
