Mould after a flood or a leaking roof: the 48-hour window and the cut line

Mould after a flood is not a cleaning job; it's a race - the first forty-eight hours belong to you, after that the wall chooses. Everything on this page is boring on purpose, because the proven wins here are dehumidifiers, plastic sheeting and judgement calls about what is porous.

The two moves that come before scrubbing

One: stop the source. A "cleaned" ceiling above an unsealed roof sheet is a cleaning subscription. Two: get the room dry with force - cross-draft fans, any dehumidifier you can borrow, nothing left damp for the weather to finish the job the flood started. Within 48 hours of wetting, most materials are recoverable; beyond it, porous ones are already colonised. Rush the drying; delay the cosmetics.

Keep, bin, or cut: the honest rule

• Bin list, no debate: anything porous that was under floodwater - mattresses, upholstered furniture that absorbed it, carpet underlay, compressed-board furniture that swelled, any ceiling tile. Floodwater is not rain; what it carried into foam is why the bin is the answer. Seal each item in its own bag on the way out of the room - mould dry-scraped off a mattress is the spore cloud that becomes your lungs.
• Keep list: solid timber (scrub, then dry slowly out of sun), metal, glass, hard plastics, and concrete block - all cleanable once the water they met was clean-ish. Salvageable with work: rugs and clothing that met rain water, washed hot within the day.
• The cut line for drywall/plasterboard: board that wicked water above 300 mm from the floor, or that sounds hollow and soft, doesn't dry "back to new" - it dries as a sandwich with a damp core. The professional rule the pros actually follow: cut out affected board up to a line about 300 mm above the highest tide mark, bag from the room, and leave the cavity open and blowing air until everything behind your cut is measurably dry. Patching paint over wet board is how a clean-up becomes a re-build.

Bleach is theatre on the wrong surface

On tile, glass and sealed metal: dilute household bleach, scrub, rinse, dry - fine. On plasterboard, wood or grout behind furniture, bleach mostly delivers water to a porous surface while the smell theatrically announces "done" - roots of mould below paint survive spraying. What actually works on surfaces is: HEPA-vacuum dry growth first, scrub with detergent solution, dry thoroughly, then and only then decide whether paint is the last step of dryness, not the first lie. Wear an N95 and gloves for all of it; seal the room off from the house with a taped plastic sheet in the doorway, and if the affected area is bigger than roughly a square metre of wall, that's the size where professional remediation pays for itself in not breathing your own repair.

The week after: keep it from returning

Furniture off walls, wardrobes angled to breathe (the corner of a built-in against an external wall is this country's favourite mould colony), bathroom door open after every bath, kitchen extraction while cooking, and the humidity gauge you can buy cheaply learning to read 60 percent in a tropical ground floor as "act now, not smell later." A flooded room that stays mould-free is won with airflow for the rest of its life, not with the one bottle of anti-mould wash. This desk's boundary, stated plainly: structural cracks, sewage water at any scale, and any room where your family includes an asthma attack or a compromised immune system - those start at the professional, not the checklist.