To plumb a double sink vanity, you connect hot and cold supply stops to each faucet, run a tailpipe from each basin, join them with a drain fitting, and send the combined flow through one P-trap to the wall drain. That is the most common layout. A second method gives each sink its own trap.
The choice between those layouts, and the way the drain is vented, decide whether the sinks drain quietly or gurgle. Local code rules on this are not something we will guess at, so questions about what your house needs go to the City of Hanahan Building and Codes department.
Start with where the wall drain and supplies are
Before buying a vanity, look at what the wall already has. A single rough-in usually has one drain stub centered in the wall and a pair of stop valves, one hot and one cold. A double vanity needs a second set of supplies, or tee fittings off the first set, to reach both faucets.
Measure the distance from the wall drain to each basin's drain hole. A long run between a basin and the wall drain affects slope and where the trap can sit. Check the height of the drain stub too, because the vanity cabinet floor and shelf may block it.
Trap arms, venting and slope are covered by the plumbing code South Carolina has adopted, the 2021 International Plumbing Code (IPC) with state modifications. This guide does not quote code numbers because local amendments can change them.
If you are altering drain or vent piping, ask the City of Hanahan Building and Codes department what applies to your project.
Sources: City of Hanahan, Building Codes and SC LLR, Building Codes in Effect for South Carolina. Checked October 2026.
Shared trap or two traps: what each layout means
Both layouts exist. Which one fits depends on the wall drain, the cabinet, and the local code.
| Layout | How it works | Trade-offs |
|---|---|---|
| Shared tailpiece and one trap | Two basin drains join at a tee or a double-sink kit, then one P-trap leads to the wall drain. | Fewer parts and easy to fit in a cabinet. A long horizontal run can hold lint and soap and slow down. |
| Two P-traps into one drain | Each basin has its own trap, and both connect to a common horizontal arm or wye ahead of the wall. | Each trap seals separately. Takes more room and more fittings. |
| Two traps into two wall drains | Rare in homes unless the rough-in was built for it. | Cleanest for flow but needs a drain stub for each basin. |
Not every layout is allowed everywhere. Ask the City of Hanahan Building and Codes department which applies to your house.
The order to assemble the drain side
A dry fit before you tighten anything saves rework.
- 1
Set the drains and tailpieces
Install the pop-up assemblies or basket drains in each basin with the supplied gasket or plumber's putty. Hand-tighten the nut underneath and check that each basin holds water without seeping.
- 2
Join the two tailpieces
Attach a tee or a double-sink kit so both tailpipes feed one outlet. Pitch the horizontal piece slightly toward the wall, not away.
- 3
Dry fit the trap
Hang the P-trap below the joined outlet, with the trap arm headed to the wall drain. The trap arm should slope gently toward the wall, never uphill.
- 4
Check the trap seal
The U-bend holds water that blocks sewer gas. Confirm the bend is intact and not twisted, and that the trap arm does not tilt back toward the sink.
- 5
Tighten and test
Snug the slip nuts by hand, then a quarter turn with pliers if the manual allows. Fill both basins, pull the plugs, and watch every joint with dry paper.
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Call (855) 575-2890Venting: why a vanity gurgles
Every trap needs air behind it. When a sink drains, the water leaving pulls air in through a vent pipe. Without that air, the trap can siphon dry or the drain can gurgle and drain slowly.
If a double vanity gurgles after install, the likely suspects are a vent that is too far from the trap, a blocked vent, or a layout that moves both basins' water past one trap at once. A plumber may recommend a different vent route or an air admittance valve where the code allows it.
Venting rules and distances are code items, and they differ by plumbing code edition. Do not copy a number from a video. Ask the City of Hanahan Building and Codes department which rule applies, and have a licensed plumber look at it if the vent is behind a finished wall.
Supply stops, lines, and faucet hookup
Water supply is the simpler half of the job:
- Shut off the water at the main valve or the nearest shutoff before disconnecting anything.
- Install a new shutoff valve (stop) for each hot and cold line, so each faucet can be isolated.
- Use braided stainless or flexible supply lines sized for your faucets, and replace old lines when you change faucets.
- Tighten compression nuts snug and no further. Over-tightening splits washers.
- Open the stops slowly, then run each faucet and check each connection with a dry tissue.
- Flush the faucet aerators at first use to clear debris.
Slope and trap arm length
Drain pipe has to fall toward the wall. Too flat and solids sit. Standards give exact slopes and arm lengths, but the numbers vary by code and we will not state one for your house. Ask Building and Codes for the correct slope.
What you can check: the tailpiece goes straight down from each basin, the horizontal run falls toward the wall, and the trap is not lower than the wall drain stub. If the trap arm must run uphill to reach the wall, the vanity or the rough-in needs to change.
Slip joint plastic trap kits are the usual choice for vanities. They are easy to cut and adjust. Chrome-plated brass looks better in open vanities but is harder to cut and bend. Either works if the joints are dry fit and the washers are seated flat.
When a double vanity needs a plumber
Call a plumber if the wall drain is too low or too high, if the vent is missing or blocked, if the stub is corroded cast iron, or if the vanity floor has water damage. Opening a wall to move a drain is not a casual DIY. A plumber can also relocate supplies without leaving soft spots in old pipe.
Tell the plumber what vanity you are buying. Some double vanities have a cabinet back that blocks the wall pipes, and a plumber can give the rough-in measurements to check at purchase. Our plumbing installation and faucet and sink repair visits cover vanity hookups and leaks.
When this becomes an emergency
A supply connection that sprays or a trap that pours water into the cabinet needs the stops closed right away. If a stop will not close, shut off the main water valve, then keep the cabinet and floor dry. Our plumbers respond around the clock through burst pipe repair, and we serve Goose Creek and North Charleston too.
Final checks before the cabinet door closes
Run both sinks at once for two minutes with the plugs open. Watch for gurgling at the other drain, slow draining, and drips at every joint. Do it again the next morning, because small leaks show up after the water has had time to travel.
Check the stops turn smoothly, and mark them for the next person. Keep the box and the manual in the cabinet. If a joint seeps, back off the nut, check the washer, and re-seat it.
Hanahan local note: permits and inspection questions
Whether a vanity replacement needs a permit or inspection depends on the scope. Swapping a like-for-like vanity may be different from moving a drain or vent. We will not state a rule here. Ask the City of Hanahan Building and Codes department, which lists permit questions among its services and has a phone line with weekday office hours at (843) 576-5259.
Hanahan has many homes from the 1970s growth, so a bathroom drain stub may be roughly 45 to 55 years old and may be cast iron or galvanized. That affects how easily a new trap connects, so check the stub before the vanity goes in.
If you hire out the work, check the plumber with the SC Residential Builders Commission under LLR. Licenses cover the individual, and a Certificate of Authorization is the company license. Ask for both before work starts.
- Sources checked (City of Hanahan Building and Codes page, state licensing board (SC LLR), general trade knowledge (cast iron drain aging))
- City of Hanahan Building and Codes
- SC LLR Residential Builders Commission
Frequently asked questions
Can two bathroom sinks share one P-trap?
Many installations do this with a double-sink kit that joins the tailpipes before a single trap. Whether it is allowed for your house depends on the plumbing code. Ask the City of Hanahan Building and Codes department.
Why does my double vanity gurgle when both sinks drain?
Gurgling usually points to a vent problem or a drain that cannot carry both flows. A plumber can check the vent and the slope.
Do I need new supply stops when I replace a vanity?
Replacing old stops is cheap and avoids a leak later. A stop that is stuck or weeping should be replaced, not forced.
How much slope does a vanity drain need?
The pipe must fall toward the wall, but the exact rate is a code number that depends on pipe size and edition. Ask the City of Hanahan Building and Codes department rather than relying on a video.
Do I need a permit to install a double vanity in Hanahan?
It depends on the work. Ask the City of Hanahan Building and Codes department before you start, and verify any plumber you hire through SC LLR.