Penetrating Damp
/ ˈpenɪtreɪtɪŋ damp /
Also known as: lateral damp, rain penetration, horizontal damp
Definition
Penetrating damp is moisture that enters a building horizontally through defects in the external fabric, as opposed to rising from the ground. It can appear at any height on a wall and is directly linked to rainfall - damp patches typically worsen in wet weather and partially dry out during dry spells. Common entry points include eroded pointing, cracked render, failed flashings, bridged cavities, and porous masonry.
In practice
Penetrating damp is one of the most frequently misdiagnosed forms of dampness in buildings - often incorrectly labelled as rising damp by contractors recommending expensive injection treatments. The key diagnostic difference is that penetrating damp patches correlate with specific external defects and worsen with driven rain, while rising damp shows a characteristic tide mark at low level and is more consistent regardless of weather.
Common causes include: eroded mortar joints that allow rainwater to soak into solid brick walls; cracked, hollow, or delaminating render; missing or failed flashings at chimneys, dormers, and roof-wall junctions; cavity wall ties carrying mortar droppings that bridge the cavity and conduct water to the inner leaf; and poorly sealed window and door frames. In solid-wall properties, driving rain on exposed elevations can saturate the wall and penetrate straight through.
Treatment is always remediation of the source - repointing with an appropriate mortar, repairing render, renewing flashings, clearing cavity ties, or applying a breathable masonry waterproofing cream to porous brickwork. Treating internal symptoms without fixing the external defect will always fail within a short time.
Building Regulations
Approved Document C requires walls to resist the penetration of moisture to the inside of the building. For existing buildings, remediation of penetrating damp is generally not notifiable under Building Regulations unless it forms part of a larger project. The BRE Good Repair Guides provide detailed guidance on diagnosing and treating different forms of dampness.
Full Building Regulations guidanceRelated Calculators
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