--- title: "Blocked Outside Drain vs Inside Drain: What Is the Difference?" canonical: "https://tavistockplumber.co.uk/blog/blocked-outside-vs-inside-drain-devon/" pubDate: 2026-07-23 author: James Bron description: "An outside drain blocking is different from an inside drain blocking — in cause, in responsibility, and in how it is fixed. Devon homeowners often confuse the two, which leads to calling the wrong person or attempting the wrong fix." tags: [blocked drain, outside drain, inside drain, devon, drainage, cctv drain survey] categories: [Drainage Advice] --- When a drain blocks, the first question is: where? An outside drain blocking and an inside drain blocking look similar — water not draining, backing up, or overflowing — but the causes are typically different, the responsible parties may be different, and the correct fix is different. This guide explains the distinction and tells Devon homeowners what to do for each. ## What Is the Difference Between an Outside Drain and an Inside Drain? **Inside drain:** the drainage system within the property — sink waste pipes, bath and shower traps, toilet waste pipes, and the underground pipework that runs from the house to the point where it connects to the external drainage. Blockages here are almost always the homeowner's responsibility. **Outside drain:** the drainage outside the building line. This includes: - The inspection chambers (manholes) in your garden or driveway - The connection from your private drain to the public sewer at the property boundary - The public sewer beyond the boundary Responsibility for an outside drain blockage depends on where exactly the block is — see the responsibility section below. ## Signs of a Blocked Inside Drain - Water slow to drain from a single sink, bath, or shower - Single toilet flushing slowly or gurgling - Bad smell from one fixture - Gurgling noises from one outlet when another is used A blockage affecting only one fixture is almost always in the trap or the short waste pipe between that fixture and the main drain stack. These are easily accessible and often cleared without specialist equipment. A blockage affecting all fixtures simultaneously — all sinks, all toilets, the bath — suggests the blockage is in the main underground drain run, below the point where individual outlets combine. This requires a drain engineer. ## Signs of a Blocked Outside Drain - Water appearing on the surface near an inspection chamber - Multiple inside fixtures backing up simultaneously (toilets are the first to be affected when the underground drain is blocked) - Sewage smell or sewage backup in the garden - Water backing up through a ground-floor toilet or shower tray ## What Causes Outside Drain Blockages? Common causes in Devon properties: **Tree root intrusion.** Devon's rural properties have mature gardens with significant root systems. Oak trees, mature Devon hedgerows, and large garden trees produce roots that seek water and penetrate clay pipe joints. This is the most common cause of persistent outdoor drain blockages in older Devon properties. **Fat, grease, and wipes buildup.** Fatbergs — accumulated fat and non-flushable wipes — form in any drain that receives kitchen waste or flushed wipes, building up in the section of drain just beyond the property boundary. **Collapsed pipe sections.** Ground movement, vehicle loading on driveways, or simply age causes clay pipe sections to crack and sag. A sagging section collects solids and eventually blocks. This is common in older Devon properties with Victorian clay-pipe drainage. **Leaf accumulation in shallow outside drains.** Surface water channels and shallow outside drains without covers accumulate leaves, particularly in autumn. A drain full of decomposed leaf matter will back up and overflow in heavy rain. ## Who Is Responsible for Each Type? See our dedicated guide on [drain responsibility in Devon](/blog/who-is-responsible-for-blocked-drains-in-devon/) for the full legal position. In summary: **Inside drain (within the property):** always the homeowner's responsibility. **Private sewer run (between the property and the public sewer boundary):** the homeowner's responsibility for the section within their land boundary. **Public sewer:** South West Water's responsibility. If the blockage is in the public sewer — often confirmed by the blockage affecting multiple neighbouring properties — South West Water will clear it at no cost. Call South West Water before calling a private drain engineer if neighbours are also affected. ## How to Clear a Blocked Outside Drain **For surface water drains (rainwater only):** 1. Lift the drain cover with a drain key (available from hardware shops) or carefully with a flat blade. 2. Clear visible debris by hand or with a garden trowel. 3. Flush with a bucket of water — if water runs through, the blockage was surface. 4. If water still backs up, the blockage is deeper — drain rodding or jetting is required. **For foul drains (toilet and sink waste):** Do not attempt to clear a foul drainage blockage by hand without appropriate PPE (gloves, eye protection, disposable clothing). Foul drains carry sewage and pathogens. For most foul drain blockages, a drainage engineer with [drain jetting](/services/drain-jetting/) equipment is the appropriate solution. Jetting clears blockages including fat deposits, partial root intrusion, and accumulated solids that rodding alone cannot shift. ## When to Use a CCTV Drain Survey Drain jetting clears the blockage. A [CCTV drain survey](/services/cctv-drain-survey/) tells you what caused it. For a Devon property with a recurring outside drain blockage — cleared this year, blocked again next year — the underlying cause is almost certainly root intrusion, a pipe defect, or a collapsed section. Clearing without knowing the cause means the blockage returns. The correct sequence is: 1. Jet the drain to clear the blockage 2. Survey with CCTV camera to identify the cause 3. Address the cause — relining, root treatment, or section replacement as required This is especially relevant for properties with large trees near the drain run, older clay pipes, or pitch fibre drainage. See our full [CCTV drain survey cost guide](/blog/cctv-drain-survey-cost-devon-2026/). ## Outside Drain Blocked: Devon-Specific Causes **Clay soils in the Tamar Valley.** Properties in the river valley soils around Tavistock, Bere Alston, and the Tamar Valley sit on heavy clay. Clay ground heaves in wet conditions and shrinks in drought, causing pipe movement and joint displacement over time. **Mature Devon garden trees.** Sycamore, ash, and oak roots routinely penetrate clay pipe joints within 10–15 years of planting. Victorian-era properties with century-old trees have almost certainly experienced root penetration in the drain run. **Victorian combined drainage systems.** Many older Devon properties have combined systems — surface water and foul drainage sharing the same pipe. Heavy rain events can overwhelm these pipes, causing backflow into property drains. This is a system design issue, not a clearance problem. ## Frequently Asked Questions **Why does my outside drain keep blocking?** The most common cause of a recurring outside drain blockage is root intrusion into the pipe or a structural defect (collapsed section, offset joint) causing partial obstruction. A CCTV survey will identify which. **Is it my responsibility or South West Water's?** If the blockage is within your land boundary, it is your responsibility. If it affects multiple properties and is in the public sewer, it is South West Water's. Call South West Water first if multiple neighbours are affected. **Can I clear a blocked outside drain myself?** For a surface water drain, yes — with appropriate tools and PPE. For a foul drain, the recommendation is to use a professional drainage engineer with jetting equipment. **How much does outside drain clearance cost in Devon?** Typically £80–£200 for a standard jet clearance, depending on access and blockage severity. See our [blocked drains](/services/blocked-drains/) service page for current pricing across the Tavistock service area. Our drainage team covers Tavistock, Okehampton, Launceston, Yelverton, and the wider Devon service area for [drain clearance](/services/blocked-drains/), [drain jetting](/services/drain-jetting/), and [CCTV drain surveys](/services/cctv-drain-survey/).