Getting the plumbing in the right place before the walls close up is the difference between a sink that installs cleanly and one that fights you at every turn. The sink rough in is the stage where you set the drain and water supply lines in the wall and floor, and the measurements you use here determine whether your sink, faucet, and trap all line up later. Standard heights exist for good reason, they keep the P-trap below the drain, give the supply valves room to operate, and let the fixture sit where it should. This guide walks through the standard dimensions and the details that matter.
What “Rough In” Means
Rough-in refers to running and positioning the supply and drain pipes during construction or remodeling, before the finished wall and fixture go in. At this stage you stub out the drain pipe through the wall, run the hot and cold supply lines to their valve locations, and set everything at the correct heights and spacing. Get it right and the final hookup is straightforward; get it wrong and you may be opening the wall back up. Because rough-in is hidden behind drywall, accuracy now saves major headaches later.
Standard Drain Rough-In Height
For a typical bathroom sink, the drain stub-out is generally roughed in around 18 to 20 inches above the finished floor. This range positions the drain so the P-trap sits comfortably below the sink’s tailpiece while staying above the floor. The exact height depends on the sink and vanity you choose, so always confirm against the specific fixture’s specifications. For a standard vanity around 31 to 36 inches tall, an 18 to 20 inch drain height lines up well with the bowl and trap.
Standard Supply Line Rough-In Height
The hot and cold water supply lines are typically roughed in around 20 to 22 inches above the finished floor, slightly higher than the drain. Placing the supplies a bit above the drain keeps the shutoff valves accessible and out of the way of the P-trap. The two supply lines are usually set a standard distance apart, commonly around 8 inches center to center, to match typical faucet and valve spacing, with the hot line on the left and cold on the right as you face the wall.
Quick Reference: Bathroom Sink Rough-In
- Drain stub-out: ~18 to 20 inches above finished floor.
- Supply lines: ~20 to 22 inches above finished floor.
- Supply spacing: ~8 inches apart, hot left, cold right.
- Drain centered on the sink location, with supplies flanking it evenly.
Treat these as standard starting points, not absolutes, your fixture’s spec sheet and local plumbing code always take precedence.
Kitchen Sink Differences
Kitchen sinks follow the same logic but with different numbers because the base cabinet and deeper bowl change the geometry. The drain for a kitchen sink is often roughed in around the same general range but is frequently set lower because the deep sink basin extends well down into the cabinet, the trap needs to sit below the bottom of the bowl. Supply lines for a kitchen sink are typically run inside the base cabinet to the shutoff valves under the sink. Always measure from your specific sink’s drain outlet and the cabinet dimensions rather than assuming bathroom numbers apply.
Why These Heights Matter
The standard measurements are not arbitrary. The drain has to sit high enough that the P-trap (which holds water to block sewer gases) fits beneath the tailpiece, but low enough to stay hidden behind the vanity and maintain proper drainage slope. Supply valves need clearance to turn on and off and room for braided supply lines to reach the faucet. Setting supplies slightly above the drain keeps everything organized and serviceable. Proper drain slope, typically about a quarter inch of fall per foot toward the main line, is also critical for the waste line to drain correctly.
Don’t Forget Venting
A drain cannot work properly without a vent. The vent pipe allows air into the system so water flows smoothly and the trap is not siphoned dry. During rough-in, the drain ties into the vent stack according to code, which dictates how far the trap can be from the vent (the trap arm length). This is one of the most code-sensitive parts of a sink installation, so it must follow your local plumbing code exactly.
Tips for an Accurate Rough-In
- Measure from the finished floor, not the subfloor, accounting for tile or flooring thickness you will add later.
- Have your fixture on hand or its spec sheet so you can match the drain and supply heights to the actual sink and vanity.
- Center the drain on the sink and keep supplies level and evenly spaced.
- Confirm wall-mounted vs. vanity sinks, as wall-hung and pedestal sinks demand more precise rough-in because the plumbing may be exposed.
- Check local code and permit requirements, which can specify exact heights, venting, and inspection.
When to Call a Plumber
If your rough-in involves moving the drain location, tying into the vent stack, or running new supply and waste lines, a licensed plumber should handle or inspect the work. Mistakes hidden behind a finished wall are expensive to correct, and improper venting or trap arm length can cause slow drains and sewer odors. For a straightforward fixture swap where the rough-in already exists, you can often work within the existing stub-outs.
Common Rough-In Mistakes
Several avoidable errors cause the most rework. Measuring from the subfloor instead of the finished floor throws every height off once tile or flooring goes down. Setting the drain too high leaves no room for the P-trap below the tailpiece, while setting it too low makes the trap visible or hard to connect. Spacing the supply lines unevenly or reversing hot and cold creates a fight at final hookup. Forgetting the vent, or exceeding the allowed trap arm length to the vent, leads to slow drains and gurgling. And ignoring the specific fixture’s spec sheet, assuming generic numbers, can leave a wall-hung or pedestal sink’s plumbing exposed in the wrong place. Double-check each of these before you close the wall.
The Bottom Line
A clean sink rough in comes down to standard heights, roughly 18 to 20 inches for the drain and 20 to 22 inches for the supply lines above the finished floor, with supplies spaced about 8 inches apart. Confirm against your specific fixture, measure from the finished floor, respect venting and slope requirements, and follow local code. Nail the rough-in and the final sink installation becomes the easy part.
