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Grantham Damp Proofing
Penetrating damp, Grantham

Damp patches that grow when it rains

Penetrating damp is rain getting through the fabric of the building sideways: through eroded pointing, cracked render, saturated solid brickwork, or a gutter that has been quietly overflowing down the same patch of wall for years. Unlike rising damp it can appear at any height, and it tracks the weather.

Why solid walls let rain in

A modern cavity wall gives wind-driven rain a gap to drain through before it reaches the inside. The solid brick terraces that make up so much of Grantham's older housing have no such gap: just two courses of brick and a coat of plaster between the weather and your wallpaper. That construction works while the mortar joints are sound and the wall can dry between storms. Once pointing erodes, or the wall faces the prevailing south-westerly rain with little shelter, moisture can work through, and a wet winter never gives the wall time to dry out.

Stone buildings in the town centre conservation area and in villages such as Great Gonerby and Barrowby behave differently again. Limestone and ironstone walls laid in lime mortar handle moisture by breathing it back out. They get into trouble when past repairs sealed them, with hard cement pointing, impermeable masonry paint or dense render, trapping water inside the wall where frost then spalls the stone face.

Common causes found in Grantham homes

  • Eroded or cement-repointed mortar joints on exposed elevations
  • Leaking, blocked or undersized gutters and downpipes, often visible as a green algae stripe down the wall
  • Cracked or hollow render, and blown masonry paint holding water behind it
  • Defective flashings and mortar fillets around chimneys, a frequent cause of damp patches on chimney breasts
  • Bridged wall surfaces: raised paths, planters and renders carrying moisture past the damp proof course line
  • Porous, frost-damaged brick faces on century-old brickwork

How it is diagnosed and fixed

The tell for penetrating damp is a patch that darkens during or shortly after rain and fades in dry spells, often with no salt band and no fixed height. The specialist traces the path of the water from the outside in: roofline, rainwater goods, joints, render, ground levels. Repairs are then aimed at the entry point rather than the symptom, and can include:

  • Repointing in a mortar matched to the wall, lime-based where the building needs to breathe
  • Gutter and downpipe repair or renewal
  • Render repair or removal where it is doing more harm than good
  • Chimney flashing and fillet repairs
  • Breathable water-repellent treatments for saturated, exposed brickwork where appropriate
  • Internal drying and replastering once the wall is watertight
Internal damage is often the smaller half of the bill. If a patch has been active for years, plaster, skirting and floor timbers behind it may also need attention, which is why the inspection covers timber condition as well as the wall itself.

Left alone, it gets more expensive

A saturated solid wall loses insulating value, so the room runs colder and condensation mould often arrives on top of the original leak. Frost works on wet masonry every winter, turning a repointing job into a rebuild of the wall face. The economical moment to act is when the patch first starts returning with the rain.