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Why is My Basement Damp?

In London, it can be difficult to find the room or get permission to extend your property. This means that more and more people are turning to their basements for extra space.

But no one wants to spend time in a basement if it’s covered in damp and mould. Here are the most common causes of damp in the basement, and what you can do about it.

What are the causes of basement damp?

1: Ground water

Your biggest enemy if you have a basement is groundwater.

As you can probably guess, groundwater is water hidden beneath the surface of the earth, stored by soil and porous rock.

If this water seeps through cracks or is soaked up through porous building materials, the wall can become damp or even start dripping.

The water level (the level at which ground becomes saturated with water) changes from area to area, even from house to house.

If your home is built on ground with a high water level, you’re more at risk of damp and flooding in the basement.

2: Poor ventilation

For basements to be a comfortable space to live in, they need ventilation to cycle out stale, damp air and bring fresh air in.

This is especially important if you have bathrooms, pools, spas, kitchens or washing machines in the basement, otherwise all the moisture released into the air will have nowhere to go.

If moisture can’t escape, it will eventually settle onto the walls and floors as the air cools, causing damp and creating the conditions for mould to grow.

3: Cold surfaces

Even with adequate ventilation, you can still end up with damp issues if the walls or floors if they’re too cold – which is a common issue it poorly heated basements.

Cold surfaces cause moisture in the air to cool on contact, collecting on the surface as condensation. Too much condensation, and you end up with damp.

4: Faulty gutters and drains

Gutters and drains are supposed to transport rain and wastewater away from the property, but if they’re faulty they may simply be dumping it in the ground.

This water can then seep down the sides of foundations or collect beneath the basement until it seeps into the walls or floors.

How can I fix basement damp?

If the cause of your damp is poor ventilation or a faulty gutter, then fixing the problem will be relatively quick and inexpensive as all you need to do is install ventilation or fix your gutters.

Drains can be more difficult depending on the source of the fault as it may require excavating beneath the basement, while a cold basement would require the installation of a new heating system.

Groundwater seeping in from outside the basement is the most worrying cause, as it means that the tanking system has failed, or is nonexistent.

The exact cost of installing a tanking system depends on the condition of the basement walls and the surrounding water level.

A simple tanking system involves applying a waterproof coating over basement walls to block water from reaching the interior.

This is appropriate for minor damp issues where water blocked by the waterproof coating is able to drain back out of the wall into the surrounding soil.

If the soil is too saturated with groundwater, water trapped in the walls won’t be able to drain out. Pressure will build until the water forces its way through the wall.

When a simple tanking system isn’t enough to hold back the water, you should install a cavity drain membrane, where there’s a small gap between the wall and the waterproof coating for water to drain away.

This water can then be pumped out by a sump pump into the drainage system where it’s flushed away into the sewer.

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