Bad odors from the roof drain are most often caused by a dry siphon, which allows sewer gases to enter the building. Other common causes include blocked drains, incorrect installation or lack of ventilation in the sewer system. The problem can usually be fixed by simple measures such as adding water, but sometimes professional help is needed to identify and solve underlying structural problems.
Why are there odors coming from the roof drain?
Manhole odors occur when sewer gases enter the building through the manhole itself. The most common problem is a dried-up water trap, which normally acts as a barrier between the sewer system and indoor air. When the water in this lock disappears, so does the protection against the smelly gases from the sewer.
The water trap can dry out for several reasons. If the roof drain is not used regularly, for example on a flat roof where water rarely flows, the water evaporates gradually. During hot summer months, the process is faster. Another reason may be that the drainage system is not properly ventilated, creating negative pressure that sucks the water out of the lock.
Clogged drains are another frequent cause of roof drain odors. Leaves, dirt and other particles collect in the drain and sewer line, creating an environment where bacteria thrive and produce foul-smelling gases. When rainfall cannot drain away properly, the water stays behind and the smell intensifies.
Incorrect installation can also give rise to problems with roof sewer odors. If the roof drain is installed without a proper water trap, or if the ventilation in the sewer system is not properly sized, gases can leak out even when everything else is working properly. Poorly sloped sewer lines can cause water to back up and start to smell.
How can you tell if the smell is coming from the roof drain?
To determine if the source of the odor is indeed the roof drain, you can do some simple tests. Start by locating where the odor is strongest. If it seems to be coming from upper floors or directly under the roof, the roof drain is a likely source. If safe, go out on the roof and smell near the roof drain to confirm that the odor is stronger there.
A practical test is to pour several liters of water into the roof drain. If the odor disappears or greatly diminishes within a few hours, the problem was probably a dried-out water trap. If the odor persists or even worsens, there may be other problems such as clogs or construction defects.
Also check other possible sources of odors in the building. Bathrooms, toilets and floor drains can give similar symptoms. Cover these drains temporarily with plastic sheeting and see if the smell continues. If it does, and is strongest near the ceiling, it points to the roof drain as the source.
Pay attention to when the smell is at its worst. If it gets worse after periods of no rain or on hot days, it reinforces the suspicion of a dried-out water trap in the roof drain. If the smell is worse when it rains, it may instead be due to blockages or ventilation problems.
What can you do yourself to fix the smell from the roof drain?
The simplest measure is to add water to the water trap of the roof well. Pour 5-10 liters of water directly into the well. This will restore the function of the water trap and block the sewer gases. If the roof is rarely exposed to rainfall, you should repeat this regularly, perhaps once a month during dry periods.
Clean the roof drain of leaves, dirt and other debris that has accumulated. You usually only need gloves and a bucket. Remove the grate or strainer and pick out any visible material. Then flush with plenty of water to remove any residue further down the drain. A garden hose works well for this.
For more thorough cleaning, you can use a simple drain cleaner or biological drain cleaner, which you can buy in DIY stores. Follow the instructions on the packaging. Avoid harsh chemicals if you are not sure what your sewer pipes can withstand, especially in older buildings.
Check the ventilation in the drainage system if you have access to it. Ventilation pipes on the roof should be free of obstructions. If they are blocked by bird nests or other obstructions, this can create problems with negative pressure draining water traps.
These methods are usually sufficient when the problem is simple and newly detected. If the odor returns quickly after you add water, or if you notice that the water is draining away unusually slowly, there are probably deeper problems that require professional assessment.
When do you need to contact a professional plumber for odors from the roof drain?
Contact a plumber if the odor from the roof drain persists despite refilling water and cleaning the drain. Persistent odors despite your own efforts indicate problems that require special tools or expertise to identify and resolve. These may be clogs further down the system or structural defects that are not visible.
Visible leaks around the roof drain are a clear sign that you need professional help immediately. Water entering the building can cause extensive moisture and mold damage. A plumber can assess the extent of the damage and fix both the leak and any subsequent damage.
If you notice that water is not flowing away from the roof drain at all, or only very slowly, there is probably a blockage in the drain line. Professional plumbers have equipment such as high-pressure flushing and cameras to inspect and clear drains in depth.
Construction faults such as incorrectly installed siphons, inadequate ventilation or incorrectly sloped sewer pipes often require extensive work to fix. We can assess the system, identify the problem and propose long-term solutions to prevent problems from recurring.
Prompt action is important because sewage problems rarely resolve themselves. On the contrary, they can worsen and lead to greater damage and higher costs. Waiting to contact a plumber when the warning signs are there could mean you end up needing more extensive and costly renovation.
How to prevent the roof drain from smelling again?
Regular checking of the water trap is the most important preventive maintenance. Visit the roof at least once a quarter and add water if it has not rained for a long time. During the summer months, you may need to do this more often. Put a reminder in your calendar so you don't forget.
Clean the gutters of leaves and debris at least twice a year, preferably in spring and fall. Leaves that accumulate in the fall are particularly problematic because they easily clog the drain and create an environment where bacteria thrive. Remove the grate and pick up all the material, then flush thoroughly with water.
Make sure that the ventilation in the drainage system is working properly. Check that the ventilation pipes on the roof are free of obstructions. If you have birds nesting near the ventilation pipes, it may be worth putting up guards to prevent this without blocking the air flow.
Consider installing an extension pipe or a dedicated water trap extender if the problem of drying out is recurrent. These solutions keep the water flowing longer and reduce the need for regular refills. A plumber can recommend suitable products for your particular situation.
Document your maintenance actions. Make a note of when you last refilled water, cleaned the well or noticed problems. This will help you spot patterns and understand if the problem is getting worse, which may indicate that more extensive action is needed. Good preventive care of your roof drain will save you time and money in the long run by avoiding acute problems and costly repairs.
