A few winters ago we were asked to quote for a ground-floor office in Dublin 8. Black growth along one wall, roughly two metres of it, behind a run of low storage. The office manager had already paid to have that wall cleaned three times. Our supervisor spent about ten minutes inside and then went […]
What to Do When Mould Appears on Office Walls in a Dublin Building
A few winters ago we were asked to quote for a ground-floor office in Dublin 8. Black growth along one wall, roughly two metres of it, behind a run of low storage. The office manager had already paid to have that wall cleaned three times.
Our supervisor spent about ten minutes inside and then went out to the lane at the back. The downpipe serving that gutter was packed solid with moss and a decade of leaf litter. Every heavy shower, water was running down the outside of the wall instead of into the drain.
That is the whole lesson in one job. Mould on office walls is a symptom. Cleaning a symptom does not fix a cause, and the cleaning bill just repeats. It's the same logic we apply across every commercial cleaning contract we run: diagnose before you clean.
Here is how to read what your wall is telling you, what Irish workplace law expects of you as the occupier, who normally pays under a commercial lease, and how to remove it so it stays gone.
Quick answer: Mould on office walls is a moisture problem, not a cleaning problem, so removing it without fixing the water source guarantees it returns. Read the pattern first: corners and window reveals point to condensation and cold bridging, a tide mark to about a metre suggests rising damp, and a defined patch near a downpipe suggests penetrating damp from a blocked gutter.
Do these three things today
Before anyone reaches for a cloth, spend fifteen minutes protecting your position. These three steps cost nothing and they change every conversation that follows.
- Photograph and date it. Take a wide shot first, showing the wall in the context of the room, the window, the corner, the floor level. Then take close-ups with something for scale beside the growth. A ruler or a page of A4 is fine.
Date-stamped images are the only reliable record of how fast a patch is spreading. When a landlord's surveyor arrives in six weeks and the patch has been wiped, your photographs are the evidence that it was there and how bad it was.
- Notify the landlord or managing agent in writing. Not a phone call. Email, with the photographs attached, on the day you find it.
This matters more than most tenants realise. Repairing arguments and end-of-lease dilapidations claims turn on when a defect was reported and what the other party did about it. A tenant who reported a damp patch in November is in a very different position from one who mentions it at exit.
- Record it in your risk assessment. Visible mould growth is a workplace hazard like any other, and it belongs in the safety statement review with a note of what you have assessed and what action is under way.
Do all three before you touch the wall. Cleaning first destroys the evidence and, as you will see, usually achieves nothing anyway.
Read the pattern before you touch anything
Where the mould is tells you what it is. This is the single most useful thing we can hand you, and it is the step almost everyone skips.
Mould germinates where the relative humidity at the surface stays high, generally cited at around 80%, and that happens long before there is any visible water. A cold external wall behind a filing cabinet reaches that condition while the room air sitting a metre away feels perfectly dry. That is dew point in action, and it is why the corner behind the cabinet is furred while the middle of the same wall is spotless.
Two levers control it: keep room relative humidity below roughly 60%, and raise the temperature of cold surfaces. Everything in the prevention section further down is one of those two levers wearing different clothes.
| Cause | Typical pattern and location | Seasonality | Moisture meter behaviour | Who normally fixes it | Typical cost order |
| Condensation and cold bridging | Corners, window reveals, behind furniture on external walls, the same spot on each floor, ceiling perimeters | Worst October to March, fades in summer | Surface reads damp, drops sharply as you probe deeper | Occupier plus building services, sometimes landlord for fabric insulation | Low to moderate: ventilation and heating changes |
| Rising damp | Tide mark up to roughly a metre, ground floor and basement only, salt or white crystalline deposits, plaster blown at skirting | Present year round | Damp deep in the wall, drier as you go up | Landlord or building owner, specialist contractor | High: injected course, replastering |
| Penetrating damp | Defined patch, often beside a downpipe, under a window sill, behind a blocked gutter, on an exposed elevation | Worse during and after heavy rain, dries slowly between | Damp deep and localised, matches an external defect | Landlord or maintenance contractor, sometimes the OMC | Low if it is a gutter, high if it is roof or render |
| Plumbing or roof leak | Sudden onset, sharply defined, often at a ceiling, beside a riser, or below a washroom | No seasonal pattern at all | Very wet, spreading over days | Plumber or roofing contractor, urgently | Varies, but delay multiplies it |
Dublin makes this harder than most cities. We work in a lot of Georgian and Victorian conversions where the external walls are solid brick with no cavity and no insulation, so the internal face of that wall in January is genuinely cold. We also work in sealed 1970s to 1990s blocks where the windows do not open, the mechanical ventilation was commissioned decades ago and has drifted, and nobody has balanced it since the last fit-out subdivided the floor.
Two more recent changes have made the picture worse. Hybrid occupancy means buildings run half empty and lightly heated for days at a time, so surfaces never warm through. And a lot of Dublin tenants dialled heating back hard when energy costs jumped, which is exactly the wrong lever, because the problem is cold surfaces rather than cold air.
Condensation and thermal bridging
By a wide margin the most common cause we find in Dublin offices. Look for growth in the top corners of external walls, around window reveals and lintels, and behind anything pushed tight against an outside wall.
The giveaway is repetition. If the same corner is affected on the ground, first and second floors, you are looking at a thermal bridge in the structure, not a cleaning failure. Concrete slab edges, steel columns and window surrounds are the usual culprits.
Also check whether it seasonally disappears. Condensation mould typically fades by May and returns in October.
Rising damp
Ground floor and basement only, and far less common than people assume. The signature is a horizontal tide mark, usually somewhere under a metre, with a fuzzier edge than a leak produces.
Look for salt deposits, a white powdery bloom, and skirting or plaster that has blown and sounds hollow when tapped. It does not come and go with the seasons.
This one is not a cleaning job in any sense. It needs a building surveyor and a specialist damp contractor, and in most leases it lands on the building owner.
Penetrating damp
Defined patches with a clear edge, on external walls, worse after a run of wet weather. Go outside and look up. That is genuinely the whole diagnostic.
Blocked gutters, split or disconnected downpipes, cracked render, failed pointing and blocked gullies push water at a wall that was never designed to take it. On a solid brick Dublin building with no cavity, that water arrives inside.
We wrote at length about blocked gutters and what they do to walls, and it is worth understanding what happens when gutters are left uncleaned before you spend money on the inside of the building. If your gutters are already showing dark streaking, cleaning mildew off the gutters is a reasonable place to start.
Leaks
Sudden onset, no seasonal logic, spreading day by day. Ceiling patches, walls beside service risers, and anything directly below a washroom or a tea station are the places to look first.
Do not investigate a suspected plumbing or roof leak yourself. Report it to the landlord, agent or maintenance contractor the same day and let the right trade open it up.
What Irish workplace law expects of you
The duty sits with the employer occupying the workplace, and that surprises tenants who assume the building is entirely the landlord's problem. The building fabric may well be the landlord's responsibility to repair, but the working conditions inside it are yours to assess and manage.
Under the Safety, Health and Welfare at Work (General Application) Regulations 2007 (S.I. 299/2007), Regulation 6 requires that enclosed places of work have sufficient fresh or purified air, Regulation 7 deals with maintaining a reasonable temperature during working hours, and Regulation 18 requires the place of work, including its surfaces of floors, walls and ceilings, to be kept in an appropriate state of cleanliness.
Sitting above those, Sections 8 and 19 of the Safety, Health and Welfare at Work Act 2005 require employers to identify hazards, assess risks and record the assessment. Visible mould growth is a hazard you have identified the moment you can see it, so it belongs in the safety statement review with a dated action. The Health and Safety Authority is the enforcing body and publishes guidance on the workplace chapter of the 2007 Regulations.
Ventilation performance in newer and refurbished buildings is governed separately by Part F of the Building Regulations, published by the Department of Housing, Local Government and Heritage. If your floor was subdivided during a fit-out and the ventilation was never rebalanced, Part F is the reference point to raise with your landlord or building services contractor.
On staff concerns, answer them properly. Some people are more sensitive to damp indoor environments than others, and anyone who is worried about their own symptoms should speak to their GP or occupational health provider rather than to you. What you owe your team is a straight account of what has been assessed, what the cause appears to be, and what is happening next. Dismissing it is what turns a fabric problem into a grievance.
Who pays: reading your lease
The answer is in the repairing covenant, and it is worth reading it before the first conversation with the landlord rather than after.
Under a full repairing and insuring lease, which is common for whole-building and standalone commercial lettings in Dublin, the tenant typically carries repair of the fabric. Under many internal repairing leases, and most suite lettings inside a larger block, the structure and exterior stay with the landlord while the tenant covers the interior finishes.
In a multi-unit development, roofs, external walls, gutters and downpipes usually sit in the common areas, which fall to the owners management company under the Multi-Unit Developments Act 2011, with the managing agent as your day-to-day point of contact. Citizens Information sets out how those structures work.
This is precisely why diagnosis comes first. A condensation problem driven by how the space is occupied, heated and ventilated is treated very differently from penetrating damp caused by a failed roof or a blocked common-area downpipe. The first is likely to be argued as an occupier issue. The second is usually a landlord or OMC repair.
The practical sequence that works:
- Read the repairing covenant and the service charge schedule.
- Notify in writing on day one, with photographs.
- Ask for an inspection and put a date on it.
- Agree the remedial scope in writing before work starts.
- Keep every invoice and report, because dilapidations discussions happen years later.
Lease liability is never as simple as a summary makes it look. Have your own solicitor review your specific lease before you accept or refuse responsibility for a damp repair.
Removing it properly
Order matters more than product. Here is the sequence we use, and the reasoning behind each step.
Fix the water source first. Everything below is wasted money until the moisture is stopped. If that means a gutter clean, a roof repair or a ventilation rebalance, it happens before remediation, not after.
Contain the area. Screen it off, shut down local air handling serving the zone, and keep the area out of use while work is under way.
Extract, do not brush. Use HEPA-filtered vacuum extraction on the affected surface. Dry brushing or sanding live growth is the worst possible first move, because it lifts spores off the substrate and spreads them through the room and into the ventilation.
Wash with a fungicidal product matched to the substrate. Painted plaster, bare plaster, timber and masonry all behave differently. A product that is fine on a coated surface can drive moisture into bare plaster.
Understand what bleach actually does. Household bleach bleaches. On porous plaster it whitens the visible growth at the surface while the hyphae below the surface survive, and it adds water to a wall that already has too much. The wall looks fixed for about three weeks. We do not recommend bleach for mould on porous internal walls.
Dry the substrate fully. This is the step everyone rushes. A solid masonry wall in a Dublin winter can take weeks to dry, and dehumidification and background heat shorten that but do not eliminate it.
Redecorate last, with a fungicidal or mould-resistant coating. Painting over live mould is the most expensive shortcut available in this whole subject. It hides the problem for a season, then the new paint film blisters and you pay for the wall twice. If you are treating adjoining sound surfaces at the same time, the same care applies as with cleaning painted walls without damaging the finish.
Safety checklist before anyone starts
- Water source identified and stopped, not just suspected
- Area screened, out of use, local air handling isolated
- Suitable PPE issued, including respiratory protection, gloves and eye protection for anyone removing growth
- No dry brushing, no sanding, no dry sweeping of debris
- Building age checked: for pre-2000 Irish buildings, assume nothing about ceiling tiles, insulation board, textured coatings or pipe lagging until an asbestos check has been done, and stop immediately if you disturb something suspect
- Growth larger than about one square metre, or in ductwork, or recurring after treatment, handed to a specialist remediation contractor
- Waste bagged at the point of removal, not carried open through the office
- Photographs taken before and after for the file
Anything structural, anything involving drainage, gas or electrical systems, and anything at height stays with the appropriate qualified contractor. That is not caution for its own sake, it is the boundary of what a cleaning contractor should be touching. It's also why our own mould and damp cleaning service in Dublin starts with a diagnosis, not a quote.
Stopping it coming back
Prevention is unglamorous and it works. Four levers, in the order they usually pay off.
Ventilation. Check that washroom and kitchen extract fans actually run, and that they run long enough after use rather than switching off with the light. Keep trickle vents open through winter, even though someone will close them. After any fit-out that subdivided open plan into offices, get the mechanical ventilation rebalanced, because the original design assumed air could move across the floor. Washroom extract is the one people neglect most, which is a recurring theme in why staff complain about washrooms.
Heating. Steady background heat beats short intense blasts. The problem is cold surfaces, and a wall that has been cold for four days does not warm through in an hour of afternoon heating. In hybrid offices, a low continuous winter setpoint through quiet days costs less than replastering.
Humidity. Buy a hygrometer for each floor, around €30, and put it somewhere people will look at it. Target below 60% relative humidity. Kettles, dishwashers, drying coats and simply the presence of people all add moisture vapour to the air through the working day, and in a sealed building that moisture has to condense somewhere. Portable dehumidification is a sensible stopgap in a problem room while the real fix is scheduled.
Furniture and housekeeping. Leave a gap of at least 50mm between storage units and external walls so air can move behind them, and stop pushing filing cabinets into cold corners. Outside, keep gutters, downpipes and gullies clear, on a schedule rather than when someone notices water spilling over. That is where a surprising share of internal damp problems begin.
If the air in the building has also started to smell close or musty, that is often the same root cause showing up a different way, which we cover in why an office smells stale despite daily cleaning. And if you are seeing water pooling on frames and sills each morning, read the companion piece on condensation damaging window frames in winter.
Frequently asked questions
What causes mould on office walls? Moisture, in one of four forms. Condensation on cold surfaces is by far the most common in Dublin offices, followed by penetrating damp from a defect such as a blocked gutter, then plumbing or roof leaks, then rising damp at ground and basement level. The mould itself is only the visible result. The pattern and location tell you which cause you are dealing with.
Is mould in the workplace a health and safety issue in Ireland? Yes, it is treated as a workplace hazard. Regulation 18 of the General Application Regulations 2007 requires workplace surfaces to be kept in an appropriate state of cleanliness, Regulation 6 requires adequate ventilation, and the 2005 Act requires employers to identify hazards and record the assessment. Visible growth should appear in your safety statement review with a dated remedial action.
Who is responsible for damp in a rented commercial premises in Ireland? It depends on the repairing covenant in your lease. Full repairing and insuring leases typically place fabric repair with the tenant, while many internal repairing leases leave the structure and exterior with the landlord. In multi-unit buildings, common areas including roofs and rainwater goods usually sit with the owners management company. Have your solicitor read your actual lease.
Does bleach kill black mould on walls? Not reliably on porous surfaces. Bleach removes the colour from surface growth, so the wall looks clean, but it does not penetrate plaster where the growth is rooted, and it adds water to an already damp wall. The patch typically reappears within weeks. A fungicidal wash suited to the substrate, applied after the moisture source is fixed, is the correct approach.
How long does a wall take to dry out after damp is fixed? Longer than most people plan for. A plastered stud wall may take one to two weeks with heat and dehumidification, while a solid masonry wall in a Dublin winter can take several weeks or more depending on thickness and how saturated it became. Redecorating before the substrate is dry is what causes blistering paint and returning growth.
Can blocked gutters cause mould inside a building? Yes, and it is one of the most common causes we trace. An overflowing gutter or split downpipe soaks the outside of a wall repeatedly. On solid brick construction with no cavity, which is typical of older Dublin buildings, that water reaches the internal face and mould follows on the inside. The fix is outside, not inside.
Talk to us before you pay to clean that wall again
We survey moisture, ventilation and the external fabric before quoting any mould work in Dublin, including the gutters and downpipes, which is where a surprising number of internal damp problems begin. If the fix is a builder rather than a cleaner, we will say so.
Premier Contract Cleaning is a family-run commercial cleaning company based at Mount Argus Mill in Dublin 6W, working across Dublin 8, the Liberties, Ranelagh and Rathmines, Blackrock and Dún Laoghaire, Santry, Swords and out into the north Kildare commuter belt, delivering both office cleaning and wider commercial cleaning contracts. Catalin Fatul, our founder, has spent over a decade in the Irish cleaning industry, and clients deal with him directly rather than a call centre, backed by over 100 five-star Google reviews.
Every engagement starts with a free site survey and a written, room-by-room scope before any quote. Where remediation is genuinely a cleaning job, our re-clean guarantee applies: if a scheduled clean is not to standard, we redo it at no charge. Where it is not, you get an honest answer and a note of who you should be calling instead.
Call Catalin on 086 083 6141 or email enquiries@premiercontractcleaning.ie. We clean with pride, and part of that is telling you when cleaning is not the answer.
Catalin Fatul is the founder and expert behind Premier Contract Cleaning, dedicated to providing top-notch cleaning solutions and tips. With a passion for cleanliness and a commitment to quality, Catalin brings years of experience in the cleaning industry to help readers maintain pristine spaces. Whether it's offering the latest cleaning hacks or recommending the best products, Catalin's mission is to make cleaning efficient, effective, and enjoyable.
If your cleaning contractor cannot cover annual leave, the problem is not the holiday. It is the pricing. Every cleaner in Ireland earns paid annual leave and public holidays, so the absence was predictable to the day. Ask for a named, site-inducted relief operative, and get a written reduced specification for any period where full […]
A TUPE dispute on a cleaning contract handover in Ireland is not settled by the client. It is settled by the facts of the specific handover, and if the parties cannot agree, by the Workplace Relations Commission. Ireland has no automatic rule that changing contractor triggers a transfer. So your job as the client is […]
Ask their broker instead of asking them. In Ireland, public liability and employers' liability cover is not compulsory by statute, so a cleaning company that will not provide an insurance certificate is not breaking the law. It is telling you something else. A contractor with current, correctly worded cover can have a regulated broker email […]
