The kitchen sink takes a minute to empty after the washing up. The shower tray holds water around your ankles until you’re nearly dressed. A slow draining sink is the sort of thing most households live with for weeks, because it still works, and because the alternative is finding a plumber you’ve never used. But a drain that’s slowing down is telling you something, and what you do in the first week usually decides whether this stays a ten-minute job under the cupboard or ends up as a camera down the line.
Why a slow draining sink slows down
Waste pipes don’t run under pressure. Water falls away at a grade set when the pipe was laid, carrying whatever you put down with it, and air moves through the vent behind it. When the sink empties slower than it did last year, either the inside of the pipe has narrowed or the air can’t move freely.
In a kitchen, the usual culprit is cooking fat. It goes down warm and liquid, cools against the pipe wall and sets like candle wax, and then coffee grounds and food scraps stick to it. In a bathroom it’s hair bound into soap scum, mostly caught within the first bend. Both build from the walls inwards, so nothing fails on a single day. The drain gets slightly slower each week, which is exactly why nobody notices until it stops.
What to try once on a slow draining sink
Two things are reasonable to do yourself before you call anyone, and you do each of them once. If the sink is still slow afterwards, you’ve learnt something useful: the problem isn’t where you can reach it.
A plunger, with water in the bowl. Run enough water to cover the rubber cup, because a plunger moves water, not air. On a bathroom basin, hold a wet cloth firmly over the overflow slot. On a double-bowl kitchen sink, block the second plughole with a stopper. Then work it with short, firm strokes.
The trap under the sink. The U-bend holds water to keep sewer gas out of the room, and it catches most of what falls down a basin. Photograph the cupboard before you empty it, so everything goes back where it was, then put a bucket underneath. Plastic traps undo by hand and go back hand-tight. Leave old chrome bottle traps alone, because the threads shear and you’ll be calling about a leak as well as a blockage.
Skip the chemical drain cleaner. Caustic product poured onto a blockage it can’t dissolve just sits in the pipe, and it’s still sitting there when someone opens the trap by hand. If the cupboard floor is damp while the trap is dry, you may be looking at a hidden leak somewhere behind the cabinetry, which is a different job altogether.
The signs that mean stop
Some symptoms point past the fixture and into the drainage line. When you see one of these, put the plunger down.
- More than one fixture is slow. A slow basin is a basin problem. A slow basin and a slow shower on the same side of the house is a pipe problem.
- Gurgling. The noise is air being pulled back through the water in a trap because it can’t get past a partial blockage further along.
- Water appears somewhere else. The toilet level rises when the washing machine empties, or the floor waste bubbles when you drain the bath.
- A smell. Waste sitting in a half-blocked line, or a trap that has lost its water seal, both announce themselves.
- Water at the overflow gully outside. Treat that as urgent. It means the line to the sewer isn’t taking what the house is putting into it.
Why a slow draining sink keeps coming back
We see this a lot on the North Shore: a sink that’s been cleared twice in a year, and a homeowner who assumes they’re unlucky. They’re not. A fixture that keeps slowing down has something structural behind it, because a properly graded, sound pipe clears itself every time you run the tap.
In older Ku-ring-gai and Willoughby houses, the suspects are familiar. Earthenware sewer lines with cracked or offset joints. A section that has sagged so water pools instead of running away. Roots finding their way into a joint, which is common enough here to be the usual answer. Grease that has been building for a decade in a line that was never jetted.
You can’t diagnose any of that from the plughole, which is why a drain camera matters. Send a CCTV camera down and you see the pipe’s condition, where the restriction sits and how far along it is. A high-pressure jetter then scours the pipe wall back to its full diameter, and if the pipe itself has failed, relining repairs it from the inside. The wastewater pipes on your property are your responsibility up to the boundary, and Sydney Water looks after the main they connect to, so it’s useful to know which side of that line your problem sits on before anyone starts work.
What happens when you call
You get Peter on 0403 404 307, and the person who answers the phone is the person at the door. Describe what’s slow, what else in the house is doing anything odd, and when it started, because most jobs can be quoted over the phone or from a photo. You can also send a photo through if it’s easier to show than describe.
On site, shoes come off at the door or covers go on, dust sheets go down, and the cupboard gets photographed before it’s emptied. If a repair isn’t going to last, Peter will tell you that instead of charging you for it twice.
Frequently asked questions
Because the restriction is further along the pipe than you can see. Fat, soap and hair build up on the pipe wall past the trap, so the opening looks fine while the effective diameter behind it has halved.
It can help in a kitchen, where set fat is the problem, but use very hot tap water, because boiling water can soften a plastic trap and the joints either side of it. If the sink slows again within a fortnight, the deposit is thicker than hot water will shift.
Yes, in the sense that it needs looking at. Gurgling means air is being dragged back through a trap, and that points to a blockage in the shared line well past the fixture.
Knowing which one you’ve got
The quickest test costs you nothing. Run the basin, the shower and the kitchen sink, one at a time, and watch what each one does. One slow fixture is yours to try the plunger and the trap on. Two or more, gurgling, or anything surfacing where it shouldn’t, and the drainage line is the problem, so stop before you push a blockage somewhere harder to reach.
Peter Pan Plumbing is a maintenance plumber based in Gordon, working from Wahroonga down to Mosman, with the camera and jetter to find out what the pipe is doing instead of guessing at it.
Tell Peter what’s going on. Call and talk to the plumber who’ll do the job. Most jobs can be quoted over the phone or from a photo, so you’re not waiting on a site visit just to get a price.
