--- title: "Why a drain keeps clogging, and when it is the main line" description: "A drain that blocks every few weeks has a cause: grease, hair, a sagging pipe or a missing vent. How to find it, what you can do, and when to call." url: "https://plumbo-astro.pages.dev/blog/why-a-drain-keeps-clogging/" date: "2026-10-06" author: "Tom B." --- # Why a drain keeps clogging, and when it is the main line > A drain that blocks every few weeks has a cause: grease, hair, a sagging pipe or a missing vent. How to find it, what you can do, and when to call. A drain that blocks once is bad luck. A drain that blocks every few weeks is telling you something. Clearing it again only treats the symptom, and the symptom comes back because the cause is still in the pipe. Most repeat clogs come from one of four things. Here is how we tell which, and what you can do first. ## Where repeat clogs come from **Kitchen sinks: grease and food.** Cooking fat is liquid when it goes down the sink and solid a metre later, where the pipe is cold. It coats the inside of the pipe, and rice, coffee grounds and food scraps stick to it. Hot water and washing-up liquid push the grease further down; they do not remove it. **Bathroom drains: hair and soap.** Hair wraps around the stopper and the crossbar under the plughole, and soap binds it into a mat. Showers and basins slow down over weeks, then stop. It is the most common repeat clog we see. **Several drains at once: the main line.** When more than one fixture backs up, the problem is not in any one of them. It is further down, in the pipe they all share. Tree roots get in through old joints and cracks looking for water. A section of pipe that has settled into a dip, called a belly, holds standing water and waste. Both catch what passes and return after a quick clear. **Wrong slope or a missing vent.** A drain pipe needs a steady fall, about 2 cm per metre. Too flat and solids settle; too steep and water runs ahead of them. A drain also needs a vent to let air in behind the water. Without one, water drains slowly, glugs, and pulls the water out of traps. This shows up after renovations, when a sink or shower was moved. ## What you can do yourself ### Fit a hair catcher A simple mesh or silicone catcher over the shower and basin plughole stops most hair before it reaches the pipe. Empty it into the bin after each shower. It prevents more clogs than anything else here. ### Change what goes down the kitchen sink Wipe greasy pans with paper towel before washing them, and put fat in a jar, not the sink. Scrape plates into the bin. Rinse with warm tap water, not boiling water: boiling water does not dissolve grease for long, and in plastic waste pipes it can soften joints over time. ### Use a plastic drain snake on bathroom drains A thin plastic strip with barbs along its edges reaches under the plughole and pulls hair out. Push it in, twist, pull it up slowly. Most slow showers and basins run freely after two or three passes. ### Try cleaning the trap The U-shaped pipe under a sink is the trap. Put a bucket under it, unscrew the two slip nuts by hand, and empty it. Refit it with the washers the right way round, hand tight, then run water and look for drips. **Skip the chemical drain openers** Caustic and acid drain openers rarely clear a full blockage. They make a channel through it at best, and the clog closes again. They are hard on old metal pipes and on the seals in plastic ones, and if they do not work you are left with a sink full of corrosive water that a plumber then has to work in. Tell us if you have used one before we arrive. ## Signs it is the main line If the basics do not hold, or the symptoms involve more than one drain, look for these: - **Toilets gurgle** when a sink or bath drains. - **Water comes up in the shower or bath** when the washing machine empties. - **The lowest drain in the house backs up first**, often a floor drain or a ground-floor shower. - **A sewage smell** near drains or outside, near the cleanout. - **Clogs return within weeks** after a professional clear. Each of these points to the shared pipe. A snake through a sink will not reach it, and a machine run from a toilet may punch a hole through roots without removing them. The useful next step is a camera inspection: we run a camera through the cleanout, see what is in the line and where, and measure how far along it is. Roots, a belly and a cracked joint each need a different fix, and the camera shows which one you have before anyone digs. **What a drain visit covers** - Questions about which drains are slow and when it started - Water test on each fixture to find where the blockage sits - Trap and fixture drain cleared by hand or with a cable machine - Main line cleared from the cleanout when several drains back up - Camera inspection of the line, with a recording you keep - Slope and vent checked where the clog keeps returning - A written note of the cause and what will stop it coming back If a drain in your home keeps coming back, book [drain cleaning](https://plumbo-astro.pages.dev/services/drain-cleaning/) and tell us which fixtures are affected. If the camera finds roots, a belly or a broken pipe, our [sewer line repair](https://plumbo-astro.pages.dev/services/sewer-line-repair/) page explains the options, and the technician will go through them with you on site.