How Fast Does Mould Really Grow After a Leak? A Detailed Timeline
Understanding the critical 24–48 hour window and what happens if you wait too long
When water leaks into your home – whether it’s a burst pipe, a roof leak, or an overflowing bath – the race against the clock begins. Most people assume that once they mop up the water or fix the leak, the danger has passed.
But mould doesn’t wait. It doesn’t need a flood or visible damage to take hold. All it needs is moisture, a food source (like timber or plasterboard), and a bit of time. And unfortunately, it starts fast – often within 24 to 48 hours, even if the leak is now fixed.
Let’s take a look at what actually happens hour by hour, day by day, when a water leak is left untreated – or worse, cleaned only on the surface. This timeline will help you understand why immediate and thorough drying is so important, and what signs to watch for if you’ve had a leak.
The Detailed Timeline of Mould Growth After Water Damage
0 to 1 Hour After Leak Begins
Water spreads quickly beyond the visible area. It’s absorbed into porous materials like wood, plasterboard, carpet underlay, insulation, and even subflooring.
Water seeps into cracks and cavities that may not be visible – such as behind skirting boards or under floating floorboards.
The relative humidity in the area begins to climb rapidly, creating the ideal environment for mould spores already in the air.
1 to 12 Hours After Exposure
Moisture moves deeper into walls, floors, and furniture through capillary action.
Items that appear dry on the outside may be damp within.
Mould spores (which are always present in the environment) sense the moisture and begin the process of germination – they’re waking up.
12 to 24 Hours
Spores begin producing hyphae, tiny filament-like roots that penetrate materials like drywall or timber.
These hyphae start digesting and breaking down the material, using it as a food source.
This activity is still invisible to the eye, but it marks the beginning of a mould colony.
24 to 48 Hours
Mould is now actively growing. Colonies begin forming at a microscopic level.
You may begin to notice a musty smell – this comes from microbial volatile organic compounds (mVOCs) released during active mould metabolism.
At this point, the mould is firmly establishing itself. Surface drying alone may no longer be effective.
48 to 72 Hours
Visible mould may begin to appear as fuzzy patches, discolouration, or black, white, or green spots.
Materials like carpet, plasterboard, and ceiling tiles often require replacement, not just cleaning.
Mould is likely spreading beyond the original leak site via air currents and humidity.
Disturbing the area (vacuuming, scrubbing, etc.) can make it worse by sending spores airborne.
3 to 7 Days
Mould growth accelerates. Colonies grow larger and deeper into materials.
Spores are now being released into the air in higher volumes, affecting indoor air quality.
People with mould sensitivity, asthma, or underlying health conditions may begin to feel unwell – especially children or pets who are lower to the ground.
The home may begin to develop an ongoing, lingering musty or earthy odour that air fresheners can’t cover.
1 to 4 Weeks
At this stage, the mould has likely spread beyond the original site and embedded itself into building materials.
Remediation becomes more complex, requiring cutting out affected materials and possibly replacing flooring or walls.
Some types of mould (like Stachybotrys chartarum, or “black mould”) may begin to release mycotoxins, which can be harmful to human and pet health.
Soft furnishings, clothing, and HVAC systems can become contaminated, even if they were not directly exposed to the water.
1+ Months (If Left Untreated)
Mould colonies become deeply entrenched and nearly impossible to remove with DIY methods.
The indoor environment may become chronically damp or unsafe for sensitive individuals.
Mycotoxins, mould fragments, and spores can circulate throughout the home – including through heating and cooling vents.
At this point, full professional remediation and air purification are often needed.
If mould remains unchecked, it may impact the structural integrity of the home and dramatically reduce property value.
Why Just Fixing the Leak Isn’t Enough
One of the most common misconceptions is that fixing the source of the leak is enough. Unfortunately, that’s only step one. Even if the visible water is gone, moisture can remain trapped inside building materials, where it creates the perfect hidden breeding ground for mould.
Important reminder:
If the water was there for longer than 24–48 hours, it’s best to assume mould growth has already begun, especially in porous or absorbent materials. Acting quickly is key to avoiding a costly and more dangerous remediation down the track.
What You Should Do Immediately After a Leak
Fix the source of the leak – plumbing, roofing, etc.
Remove standing water quickly using mops, towels, or a wet vac
Use dehumidifiers and fans to dry the area thoroughly – this can take 24–72 hours
Remove damp materials like underlay, soaked carpet, or insulation
Use a moisture meter or thermal imaging to detect water trapped in walls
Don’t rely on surface drying alone – inside walls and under floors may still be wet
Call a professional if you suspect water has spread into inaccessible areas
Final Thoughts: Mould Grows Quietly and Quickly
Mould isn’t just a stain or a smell – it’s a living organism that begins growing faster than most people realise. Within just 24 to 48 hours, it can take hold behind your walls, under your floors, and within the air you breathe.
If you’ve had a leak, even one that’s been fixed, it’s critical to act fast, dry thoroughly, and monitor your space. Lingering fatigue, brain fog, sinus problems, or skin rashes in the weeks following water damage may be your body’s way of telling you something’s off.
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