The prospective tenant opened the door and left within a minute

A typical scenario based on situations we commonly see. People and details are illustrative.

Auntie Pen owns a three-storey, thirty-room dormitory in a soi near a university in Khlong Luang, Pathum Thani. For fifteen years it has been her family's main income, housing students and workers from the nearby industrial estates. This year the canal overflowed into the soi and the ground floor sat under roughly half a metre of water for nearly three weeks. Ten ground-floor rooms, the shared laundry room and the storeroom all went under.

As soon as the water dropped, she hired workers to shovel out mud, wash the floors, replace sockets and repaint the entire ground floor, all within two weeks, because the new semester was close and she knew every empty day was a day of lost rent. The rooms looked better than before. But when a first-year student and her mother came to view a room, the mother frowned the moment the door opened, sniffed twice and said quietly, "It smells musty. My daughter has allergies." Then they left to look elsewhere.

A few days later, returning tenants on the ground floor began complaining. One said the air-conditioner blew out a stale, sour smell. Another found black spots behind the built-in wardrobe, and clothes hanging inside smelled as if they had never dried. The shared laundry room gave off a drain smell every evening. Then a post appeared in the university's dorm-hunting group: "Dorms around here that flooded, watch out for mould." No names, but Auntie Pen felt it was about her.

Her situation is the one facing owners of dormitories, apartments, shophouses and rental houses in flood-prone areas across Thailand, from Pathum Thani, Nonthaburi and Ayutthaya to Hat Yai and Ubon Ratchathani. A room that looks clean is not necessarily clean, and today's tenants notice quickly, through their own noses and through online reviews. This guide is written for owners and building managers. It explains what hides in rental rooms after a flood, why a quick renovation is not enough, and how WHD's residential and building ozone disinfection service helps get rooms truly ready to let, sequenced to cost you as little time as possible.

What hides in a rental room after the water recedes

Rental buildings make post-flood recovery harder than a single house. Many rooms share drains, owners supply built-in furniture, every room has an air-conditioner, and dozens of people share common areas. Each of these is a place where germs and odours can hide.

1. Mould in walls, built-in wardrobes and ceilings

Rendered brick walls that stand in water for days wick moisture well above the flood line and release it slowly for weeks. Built-in wardrobes and counters made of plywood or particle board swell, and the back of a wardrobe pressed against a wall is a dark, damp space nobody sees. Mould can begin growing within 24 to 48 hours of materials getting wet, which is why the WHO and the US CDC stress cleaning and drying as fast as possible. Once established, mould releases airborne spores that trigger sneezing, congestion, itchy eyes and lingering coughs in tenants with allergies or asthma, and that musty smell gets into their clothes.

2. Mould and bacteria inside air-conditioners

This is the point landlords overlook most. Low-mounted units or condensers on the ground may have been flooded directly, and units that stayed dry still pulled damp, stale air through their coils for weeks. The evaporator coil and drain pan become mould reservoirs. When a tenant switches the unit on, it blows spores and odour across the room, every single time. That is why a freshly painted room can still smell. Professional air-conditioner cleaning must always happen before ozone treatment.

3. Bacteria from sewage backflow

During floods, a building's septic tank and wastewater system are often overwhelmed. Sewage pushes back up through bathroom floor drains, toilets and laundry drains, leaving faecal bacteria such as E. coli and other diarrhoea-causing germs in bathrooms and floor joints. Even after the water is gone, drains with sediment keep releasing odour. Tenants using shared bathrooms or a common laundry room are easily exposed.

4. Leptospirosis from rats

Rats escaping the water often move into buildings: ceiling voids, storerooms and service ducts. Their urine carries the bacteria that cause leptospirosis, which survive for a long time in standing water and damp areas and enter the body through cuts or mucous membranes. Cleaners and contractors doing the clean-up are at risk, as are tenants wading through water in the car park. Key symptoms are high fever, headache, muscle pain especially in the calves, and red eyes. Anyone with these symptoms after flood exposure must see a doctor immediately. Disinfecting a building reduces the source of germs; it does not treat disease.

5. Landlord-supplied mattresses, curtains and upholstered furniture

Furnished rooms usually come with mattresses, curtains and perhaps a small sofa or upholstered chair. These absorb both water and odour. A mattress that sat in floodwater should not be reused, and even curtains and furniture above the water line soak up the stale air of a room left sealed for weeks.

Which tenants are most at risk

• Families with babies and young children who crawl on floors and have immature immune systems.
• Older tenants who choose ground-floor rooms to avoid stairs and often have chronic conditions.
• Pregnant women, who should avoid contact with sewage and mould.
• People with allergies, asthma or chronic lung disease, who react strongly to spores.
• People with chronic illness or weakened immunity, such as diabetes, kidney disease or those in treatment.
• Your own cleaners and maintenance staff, who handle mud and sewage directly during recovery.

What an unclean room costs a rental business

Income: every empty day is money gone

A rental business lives on occupancy. A room that will not let because of smell or mould complaints is rent lost every day with no way to recover it. A tenant who moves out mid-contract costs you the rent plus the cost of finding someone new. The start of a semester or a factory hiring round is peak season; if rooms are not ready then, tenants sign elsewhere for the whole year.

Costs: repairs and repainting done twice

Painting over damp walls or untreated mould usually ends with bubbling, peeling paint within months. You pay contractors to scrape and repaint again, doubling labour and materials, not counting wardrobes that must be torn out because of mould behind them or mattresses replaced because they were stored in a damp room.

Reputation: one post can follow you for years

Tenants now search on websites, apps and social media groups. A single photo of black-spotted walls or a comment like "the room smells musty and I sneeze when the air-con is on" can stay online for years and be passed from one student intake to the next. Getting rooms truly clean the first time is far cheaper than repairing a reputation later.

Time and tenant relationships

Every complaint means inspecting the room, negotiating rent reductions, refunding deposits or moving tenants. Deposit disputes after floods are common, because tenants feel the damage is not their fault while owners carry the repair bill. A room restored step by step, with clear records of what was done, removes much of that friction.

Health and your duty to residents

A landlord is expected to provide a place fit to live in. If tenants fall ill repeatedly from mould or drain odour, trust erodes and disputes drag on. Building hygiene is not cosmetic; it is part of your responsibility.

Why "renovate fast and let immediately" backfires

Painting over everything. The most popular landlord fix, because it looks great in listing photos. But if the wall is still wet inside, or mould sits under the old paint, the new coat becomes a lid that traps moisture and spores. Within months it bubbles, flakes and the black spots return.

Spraying fragrance before viewings. A strong scent in an empty room makes experienced tenants wonder what is being covered up, and once they move in, the mustiness is back in days.

Bleach and open windows. Bleach disinfects tiles, but it does not penetrate render, plywood or fabric, nor reach air-con coils, ducts or the backs of wardrobes. And tenants dislike a lingering chlorine smell too.

Running the air-con to dry the room. If the unit has not been cleaned, this simply blows spores around the room and helps mould inside the unit grow.

Replacing only the mattress and curtains. New items placed in a room still full of spores and stale air absorb the old smell within weeks.

Treating only the rooms that complain. Rental buildings have connected corridors, shafts and ventilation. Treat one room and odour and germs from neighbouring rooms or common areas drift back.

All of these work only on visible, reachable surfaces, while the problem lives in the air, the air-con, behind wardrobes, in ducts and inside porous materials. Effective disinfection must reach the whole volume of each room, and be applied systematically across the building.

How WHD ozone disinfection works for rental buildings

Ozone (O₃) is a strongly oxidising gas. Released into a sealed room, it spreads into every corner, breaks down the cell structures of viruses, bacteria and mould, and breaks apart the molecules behind musty, drain and trapped odours. It then reverts to oxygen (O₂) and leaves no chemical residue, so the room smells of neither mould nor cleaning products when a tenant opens the door.

According to the test results the company has had certified by Thailand's Department of Medical Sciences and Intertek (UK), WHD's ozone treatment eliminates viruses, bacteria and mould by 99.99%. Owners can view the company's certification documents to help explain the building's recovery process to tenants. WHD serves homes, condos, dormitories, hotels, offices, factories, schools and hospitals, so planning work across many rooms is familiar territory.

Ozone is also the final step, after mud removal, washing, repairs and drying. It does not replace removing damaged materials, cleaning air-conditioners, cleaning water tanks or an electrician's inspection, and it never replaces medical care for anyone who is unwell.

Our process for dormitories, apartments and rental houses

1. Survey and room map. We collect the number of rooms, which floors flooded, the common areas and your target re-letting date, so our work fits around your contractors.
2. Readiness check per room. Mud removed, walls and floors reasonably dry, air-conditioners cleaned, damaged items taken out.
3. Zoning and rotation. Rooms are treated in batches by floor or wing, so contractors keep working elsewhere and downtime stays short.
4. Preparing and sealing. Everyone leaves the zone; built-in wardrobes and drawers are opened; doors and windows closed; air-con set as advised.
5. Ozone release and contact time. A typical cycle runs about 30 minutes per zone.
6. Rest and ventilation. Ozone reverts to oxygen, the zone is aired, and the team confirms when it is safe to enter.
7. Wrap-up and aftercare. Advice on humidity control in empty rooms, ventilation routines and communicating with tenants.

For a full package, combine ozone with WHD's disinfection spraying service for common-area surfaces. Ozone jobs of 15,000 baht or more receive a free medical-grade disinfectant spray your housekeeping team can use on door handles, stair rails and shared bathrooms.

10 reasons landlords should use ozone before re-letting after a flood

1. No musty hit when a viewer opens the door. First impressions are made in seconds.
2. No chemical smell either, because ozone turns back into oxygen; sensitive tenants move in comfortably.
3. It reaches behind built-ins, into ducts and ceiling corners that cleaners and contractors cannot.
4. Fewer airborne spores before move-in, meaning fewer complaints about sneezing and allergies.
5. Help with drain odour in bathrooms and shared laundry rooms, after drains and surfaces are cleaned.
6. Backed by test results. According to the company's certified results from the Department of Medical Sciences and Intertek, it eliminates viruses, bacteria and mould by 99.99%. Details on the ozone disinfection service page.
7. Fast and schedulable. About 30 minutes per zone, in batches by floor, so rooms return to the market sooner.
8. It protects your repair budget by reducing the chance that new paint, mattresses and curtains pick up odour or mould.
9. A clear story for tenants. You can state plainly that the building was repaired and disinfected step by step, protecting your online reputation.
10. A team used to multi-room buildings, from dorms to hotels, scheduling around tenants still in residence. Request a quote on our residential and building ozone page.

Before and after: a room truly ready for tenants

Note: results depend on the extent of damage, the quality of repairs and drying beforehand, and humidity control afterwards.

Example case: Auntie Pen's dormitory in Khlong Luang

An illustrative example to explain the process, not a review from an identified customer.

After losing two prospective tenants and receiving complaints from three existing rooms, Auntie Pen called WHD. The team asked for the ground-floor plan, the list of flooded rooms, her re-letting date and the status of contractor work in each room. They suggested re-ordering the work. First, have the contractor open up the backs of the built-in wardrobes in the rooms with complaints. Three wardrobes turned out to have swollen, mouldy back panels that had to be replaced. Next, bring in an air-con technician to clean all ten ground-floor units, and have housekeeping flush the laundry-room drains.

Once repairs were done and the walls were properly dry, WHD treated the ground floor on a set schedule, in batches of several rooms, with the laundry room, storeroom and ground-floor corridor as a separate zone. Tenants on the second and third floors were told in advance through the dorm's LINE group which areas would be closed and which stairway to use. Each zone ran about 30 minutes, followed by rest and ventilation.

Afterwards, Auntie Pen photographed every room on a bright day and wrote honest listings: the ground floor had flooded, been repaired, had damaged materials replaced, air-conditioners cleaned and been ozone-disinfected, with dates. Several viewers asked about it, and she could explain each step. The ground-floor rooms filled up before the semester began, and the tenants who had complained reported that the air-con no longer smelled.

Her takeaway: rushing to repaint cost her more time, not less. Getting the order right, remove, repair, clean, dry, then ozone as the final step, made the rooms genuinely ready and nothing had to be redone.

Recovery checklist for owners and building managers

First week after the water drops

• Have an electrician check wiring, breakers and sockets before restoring power.
• Tell every tenant the recovery plan, when rooms are unavailable and how to reach you.
• Photograph damage in every room before removing anything; keep receipts for insurance or assistance claims.
• Make sure staff wear rubber gloves, boots and masks and cover any cuts while removing mud.

Remove, repair, clean

• Discard flooded mattresses, cushions and carpets.
• Open up wardrobe backs and under-counter spaces; replace swollen panels.
• Clean every air-conditioner in flooded or long-sealed zones.
• Flush drains, inspect the septic system and clean water tanks if floodwater got in.
• Put rat control in place.

Dry before you paint

• Ventilate, push damp air outside with fans, and use dehumidifiers in stuffy rooms.
• Paint only fully dry walls, and always remove mould before painting.

Treatment day

• Notify tenants on other floors about closed areas and routes.
• Confirm no people, pets or plants are in the zone.
• Open wardrobes, drawers and en-suite doors so ozone reaches everywhere.
• Keep everyone out until the team confirms rest and ventilation are complete.

After treatment, before move-in

• Air empty rooms regularly rather than sealing them for long periods.
• Take new photos, record the date of each step and communicate honestly.
• Ask tenants to ventilate, avoid drying wet laundry in closed rooms and report damp spots quickly.
• Staff or tenants with fever, calf pain, diarrhoea or a persistent cough after the flood should see a doctor promptly.

Frequently asked questions from landlords

Does the whole building need to be vacated?

No. Only the room or zone being treated must be empty and sealed. The team plans the zones and gives the building manager instructions to share with tenants elsewhere in advance.

How many rooms can be done in a day?

It depends on room size, equipment and zoning. A typical cycle is about 30 minutes per zone plus rest and ventilation. The team will schedule it after the survey.

Can tenants move in straight after treatment?

Yes, once the rest and ventilation period is complete, provided the room has also been repaired and properly dried. If walls are still damp, mould can return even after treatment.

Should we treat before or after repainting?

We recommend ozone as the final step, once repairs are done and paint has dried, so all odours from the works are handled at once. Mould on walls must always be removed before painting.

Does ozone fix smelly air-conditioners?

Ozone treats germs and odour in the room air, but grime and mould on the coil and drain pan must be cleaned out by a technician first.

Can tenants' belongings stay in the room?

Furniture and ordinary belongings can stay, but no people, pets or plants. Tenants with special concerns, such as important documents or collectibles, can discuss them with the team beforehand.

Get your rooms ready to let: clean in what tenants see and what they don't

Free consultation, building survey and an ozone schedule aligned with your contractors and re-letting date.

Get a Quote for Rental Room Ozone Treatment

Call 065-556-6294 · LINE @whd268 · worldhealthdisinfection@gmail.com

Promotion: ozone jobs from 15,000 baht receive a free medical-grade disinfectant spray.

Related services

• Residential and building ozone disinfection
• Ozone cleaning service
• Disinfection spraying for common areas
• Certificates and test results
• Contact us

References

• CDC: Floods – cleaning buildings and dealing with mould after a flood
• Department of Health, Thailand: sanitation and housing after floods