Rising damp is moisture that wicks upward through brick, stone, or mortar from contact with wet ground, moving by capillary action through the porous material until it evaporates higher up the wall, often leaving a damp band, staining, or efflorescence above grade.
Masonry materials are porous enough that water can travel upward through them the same way a paper towel pulls water up from a dish, working against gravity through tiny capillaries in the brick, stone, and mortar. Without a damp-proof course or proper waterproofing at the base of a wall, ground moisture keeps climbing until it evaporates, and it usually carries dissolved salts from the soil and mortar with it, which show up as a white, chalky stain once the water dries.
Rising damp shows up most on older stone and brick foundations built before modern damp-proofing was standard practice, and it tends to get worse in wet seasons when the ground stays saturated longer, since the soil around the foundation has more moisture to feed into the wall. It's often mistaken for a roof leak or plumbing issue because the staining shows up partway up an interior wall, but the water is actually coming from below, not above, which changes where the fix needs to happen.
More masonry vocabulary from the glossary.
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