{"id":10951,"date":"2026-11-03T11:53:55","date_gmt":"2026-11-03T11:53:55","guid":{"rendered":"https:\/\/dpcon.es\/?p=10951"},"modified":"2026-11-03T11:54:01","modified_gmt":"2026-11-03T11:54:01","slug":"drain-smells-sewage-smell-at-home","status":"publish","type":"post","link":"https:\/\/dpcon.es\/en\/drain-smells-sewage-smell-at-home\/","title":{"rendered":"Drain smells and home drainage systems: why your home smells of sewage and how to claim if it is a new build"},"content":{"rendered":"

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A sewage smell at home<\/strong> is one of the most unpleasant problems a property can have. It causes real concern because it is seen as something difficult to fix. That is not quite the case, but when it appears in the bathroom, in the kitchen or next to the washing machine, and comes and goes for no apparent reason, it is impossible not to worry about it. These drain smells permeate everything and, of course, a healthy home environment, which is generally a priority for every person and family, becomes impossible.<\/p>\n

We have to say that, in most cases, drain smells<\/strong> have a simple cause, such as a dry or dirty trap, which we can sort out ourselves in a few minutes, so there is no need to panic every time it happens. The solution may be very simple.<\/p>\n

However, if we have just bought a new-build home and have this problem, everything would suggest a design or workmanship fault in the drainage system behind it. In that case, we could claim against the building contractor or the developer.<\/p>\n

At DPcon we explain it all below.<\/p>\n

Why does my home smell of sewage?<\/strong><\/h2>\n

Every home is connected to the sewer network (or to a septic tank) through a wastewater drainage system: fixture waste outlets, traps, branch pipes, downpipes<\/strong>, collector drains and inspection chambers. Gases from the decomposition of organic matter are generated in that system, mainly hydrogen sulphide<\/strong>, which is responsible for the characteristic smell of rotten eggs, along with methane and ammonia.<\/p>\n

To stop these gases getting into the home, the installation is designed with two barriers: water seals<\/strong> (the water retained in traps and drum traps, which acts as a plug) and a ventilation system<\/strong> that balances the pressure inside the pipes and expels the gases above the roof. When either of them fails, the sewage smell reaches the home.<\/p>\n

The most common causes of drain smells<\/strong><\/h2>\n

There can be several, but they are generally well identified and we know how they can be mitigated. But of course, if our home is new, we may be looking at construction damage in the form of these bad smells<\/a><\/strong>. We will explain this further below.<\/p>\n

As for the most common causes, make a note of the following:<\/p>\n

\u2611\ufe0f Dry trap or trap with little water<\/strong><\/h3>\n

This is the number one cause. If a washbasin, shower, terrace gully or bidet is not used for weeks, the water in the trap evaporates and the pipe is left open to the system. It is typical of second homes, guest bathrooms and recently handed-over homes<\/strong> that have been closed up for some time. The solution is as simple as running the water for a few seconds.<\/p>\n

\u2611\ufe0f Dirty traps and drum traps<\/strong><\/h3>\n

Traces of soap, hair, grease and food build up on the walls of the trap and drum trap and decompose. In this case the smell tends to be localised (a washbasin, the kitchen sink, the shower tray) and improves after cleaning.<\/p>\n

\u2611\ufe0f Loss of the water seal through siphonage<\/strong><\/h3>\n

When the system is not properly ventilated, flushing the cistern or draining the washing machine creates negative pressure in the pipes that “sucks” the water out of nearby traps<\/strong> (self-siphonage or induced siphonage). If you hear gurgling in the washbasin or shower when you flush the toilet, followed by a bad smell, this is a clear clue.<\/p>\n

\u2611\ufe0f Downpipes with no primary ventilation or poor ventilation<\/strong><\/h3>\n

The downpipes<\/strong> (the vertical pipes that collect the water from all floors) must extend above the roof to ventilate the system. If the ventilation has been removed, is blocked, ends in an unsuitable place or has been replaced by air admittance valves<\/strong> that are undersized, faulty or badly positioned, the gases find their way out through the traps in the homes. This is a very common defect in new builds and usually affects several homes in the same building.<\/p>\n

\u2611\ufe0f Badly sealed joints and leaks in the system<\/strong><\/h3>\n

Badly glued pipe connections, toilet connector pipes without the correct seal, badly sealed wall-hung toilets or downpipes with open joints<\/strong> inside service shafts and false ceilings let gases escape even if the traps are working properly. The smell is usually noticeable in wardrobes, false ceilings or next to access panels.<\/p>\n

\u2611\ufe0f Incorrect falls, excessively long branch pipes and fixtures with no trap<\/strong><\/h3>\n

If the horizontal branch pipes do not have the proper fall, water and waste stagnate and ferment. Other sources of smells are traps installed in series<\/strong>, fixtures connected without a trap and drum traps serving fixtures in another room, practices that are expressly prohibited by the Technical Building Code.<\/p>\n

\u2611\ufe0f Inspection chambers, collector drains and connection to the main sewer<\/strong><\/h3>\n

In detached houses, ground-floor properties and garages, the sewage smell may come from inspection chambers without an airtight cover, dry trapped chambers, or buried collector drains that are broken or have reverse falls<\/strong>, or from a faulty connection to the municipal sewer. It can also come from poorly ventilated septic tanks.<\/p>\n

\u2611\ufe0f Waste outlets from household appliances and air-conditioning equipment<\/strong><\/h3>\n

A washing machine, dishwasher or air-conditioning condensate tray connected directly to the system without a trap or anti-backflow bend<\/strong> provides a direct route for the gases. This is a common mistake in new installations.<\/p>\n

\n\n\n\n\n\n\n\n\n\n\n
Symptom<\/th>\nLikely cause<\/th>\nWho should fix it?<\/th>\n<\/tr>\n<\/thead>\n
Smell from a fixture that is not used<\/td>\nDry trap<\/td>\nThe owner (use and maintenance)<\/td>\n<\/tr>\n
Localised smell that improves with cleaning<\/td>\nDirty trap or drum trap<\/td>\nThe owner (maintenance)<\/td>\n<\/tr>\n
Gurgling when the cistern is flushed, followed by a smell<\/td>\nSiphonage due to poor ventilation<\/td>\nBuilding contractor\/developer if it is a new build; homeowners’ association if the downpipe is affected<\/td>\n<\/tr>\n
Smell in several rooms or on several floors<\/td>\nDownpipe with no primary ventilation or faulty valve<\/td>\nBuilding contractor\/developer (new build) or homeowners’ association<\/td>\n<\/tr>\n
Smell in wardrobes, false ceilings or service shafts<\/td>\nBadly sealed joints or connections<\/td>\nBuilding contractor\/developer (new build)<\/td>\n<\/tr>\n
Smell on the ground floor, in the garden or in the garage<\/td>\nInspection chambers, collector drain or sewer connection<\/td>\nBuilding contractor\/developer, homeowners’ association or town council, depending on the section<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n

How to get rid of drain smells step by step<\/strong><\/h2>\n

Although at DPcon we are not the ones who fix the problem of bad smells in the home, before thinking about a claim it is advisable to rule out causes related to use and maintenance.<\/p>\n

The first thing to do is refill all the traps: run the taps of washbasins, bidets, showers, baths and sinks for 10-15 seconds and pour a bucket of water into the gullies on terraces, in utility rooms and in garages. If the smell disappears within a few hours, the problem was a dry trap.<\/p>\n

We can dismantle the trap under the washbasin or sink (with a bucket underneath) and clean it. In showers, remove the drain grille and clear away hair and debris. Open the bathroom drum trap, if it is accessible, and clean it.<\/p>\n

Next, carry out maintenance cleaning. Once a week, pour very hot water (not boiling if the pipes are PVC) down the drains. The classic bicarbonate of soda and vinegar helps to loosen organic residue, but it does not resolve ventilation faults or leaks.<\/p>\n

It is wise to check the household appliances. Make sure that the hose of the washing machine and dishwasher drains into a trap and has the anti-backflow bend. Check joints and seals, and inspect the silicone at the base of the toilet and shower trays and the condition of the access panels.<\/p>\n

Finally, if the smell persists, we would advise you to contact a professional immediately. A plumber can carry out a smoke test or a camera inspection to locate leaks, reverse falls or breaks. If the home is new, at that point the recommended step is an expert report<\/a><\/strong> to determine whether there is a construction defect.<\/p>\n

How to get rid of a bad smell from downpipes<\/strong><\/h3>\n

As before, although this is not our area of work, we do know that when the smell comes from the downpipe<\/strong> (it is noticeable in several rooms, in the service shaft or in several homes in the building), home remedies do not really work. The problem is not in the trap but in the ventilation or the airtightness of the downpipe itself.<\/p>\n

What solutions are there? Check the primary ventilation on the roof.<\/strong> Make sure that the downpipe comes out above the roof, that it is not covered (nests, rubble, unsuitable cowls) and that it was not cut during the works.<\/p>\n

We would then try the following:<\/p>\n