Attic and crawlspace mold: why London’s older housing stock is prone to it

If you own a home in London, a flooded basement is one of the most common — and most stressful — property problems you can face. A finished basement can go from dry to damaged in a matter of minutes, and the water often brings the added worry of contamination, ruined belongings, and the risk of mould.

The good news is that most basement flooding has a clear cause, and many of those causes can be reduced with the right preparation. Below, we explain why basements flood across London and Southwest Ontario, and the practical steps you can take to lower your risk before the next heavy storm or spring melt.

Why basements flood in London

London’s climate and housing stock create a specific set of conditions that put basements at risk throughout the year.

Heavy rainfall and overwhelmed drainage. Intense summer thunderstorms can drop more water than the ground and municipal drainage systems can absorb at once. When stormwater has nowhere to go, it finds its way into basements through foundation cracks, window wells, and floor drains.

Spring melt and saturated ground. As winter snow melts — often while the ground is still partly frozen — large volumes of water move toward foundations that can’t drain it away quickly. This is one of the most common causes of seasonal basement flooding in the region.

Freeze-thaw cycles. London’s repeated freezing and thawing stresses foundations and plumbing. Hairline foundation cracks widen over time, and pipes can freeze and burst, sending water into the basement from the inside.

Sewer and sanitary backups. During major storms, municipal sewers can become overwhelmed and surcharge, pushing water — and sometimes sewage — back up through floor drains and basement fixtures. This is both a flooding problem and a health hazard.

Sump pump failure. Many London homes rely on a sump pump to keep groundwater out. Pumps fail when they’re undersized, poorly maintained, or — most commonly — when a storm knocks out the power at exactly the moment the pump is needed most.

Older homes and aging systems. London has a large stock of older homes, and with age comes older weeping tile, dated foundations, and plumbing that’s more prone to failure. Homes in established neighbourhoods like Old East Village, Byron, and parts of central London can be especially exposed.

Poor grading and downspouts. When the ground slopes toward the house, or downspouts release water right at the foundation, rainwater is directed exactly where you don’t want it.

How to reduce your risk

You can’t control the weather, but you can make your basement far more resilient. These steps make the biggest difference.

Install or upgrade a sump pump — and add a battery backup. A properly sized sump pump is your first line of defence against groundwater. Because storms often cause power outages, a battery backup system keeps the pump running when you need it most.

Add a backwater valve. A backwater valve closes automatically during a sewer surcharge, preventing water and sewage from backing up into your basement. It’s one of the most effective protections against storm-related backups.

Improve grading and extend downspouts. Make sure the ground slopes away from your foundation, and extend downspouts so they discharge well away from the house — generally at least two metres from the foundation wall.

Seal foundation cracks. Even small cracks let water in under pressure. Sealing them early prevents bigger problems later.

Maintain eavestroughs and window wells. Keep gutters clear so water is carried away from the roofline, and consider covers for window wells that tend to collect water.

Protect your plumbing in winter. Insulate pipes in unheated areas and keep your home adequately heated during cold snaps to reduce the risk of frozen, burst pipes.

Look into the City of London Basement Flooding Grant Program. The City offers financial assistance to qualifying homeowners for eligible flood-protection measures, including backwater valves and disconnecting weeping tiles with redirection to a new sump pump. Funding levels and eligibility are set by the City and can change, so confirm the current details and whether you qualify directly with the City of London’s Sewer Engineering team before starting any work.

What to do if your basement floods

Even with good prevention, flooding can still happen. If it does:

1. Put safety first. Don’t enter standing water if there’s any chance it’s in contact with electrical outlets, panels, or wiring. If sewage is involved, avoid contact entirely — it’s a health hazard.
2. Stop the source if it’s safe to do so. Shut off the water supply for a burst pipe, or the power to the affected area if you can reach the panel safely.
3. Call for professional help quickly. The faster water is removed and the area is dried, the less damage you’ll face — and the lower the risk of mould, which can begin to develop within 24 to 48 hours.
4. Document everything for insurance. Photos and a record of what was damaged will support your claim.

This is where Restoration Mate serving London and the surrounding Southwest Ontario region comes in. Our certified technicians respond 24/7 with professional water extraction and industrial drying equipment, handle the cleanup and any required reconstruction, and work directly with your insurance company to document the damage and help you get the coverage you’re entitled to.

Frequently asked questions

Does home insurance cover basement flooding? It depends on your policy and the cause. Many policies require separate endorsements for overland water and sewer backup, while a burst pipe is often covered under standard coverage. Review your policy with your provider, and ask about adding coverage if you’re unsure. Our team can also help document damage to support your claim.

How fast does mould grow after a basement floods? Mould can begin to develop within 24 to 48 hours of water exposure. Prompt water removal and professional drying are the best ways to prevent it.

Is a sump pump or a backwater valve more important? They protect against different problems — a sump pump manages groundwater, while a backwater valve prevents sewer backups. Homes at risk of both often benefit from having each one.

Can a flooded basement be fully restored? In most cases, yes. With fast water extraction, thorough structural drying, proper sanitizing, and any necessary repairs or reconstruction, a flooded basement can be returned to a safe, dry, and comfortable condition.

How quickly can Restoration Mate respond in London? In most cases we can dispatch a team within hours of your call, depending on conditions and location. Fast response is critical to limiting damage and preventing secondary issues like mould.

Need help now?

If your basement is flooding, don’t wait — water damage gets worse and more expensive the longer it sits. Contact us today to learn about our mold remediation and inspection services.

Restoration Mate provides 24/7 emergency restoration across London and Southwest Ontario. Call us anytime, including nights, weekends, and holidays: (226) 721-1161.