Is rising damp caused by rain-water or surface-water going into the wall at ground-level?

“I think i’m right in saying, rising damp must be caused by rain-water or surface-water going into the wall at ground-level?”

Wrong, but it is a reasonable assumption. It is the moisture content held below ground level that’s sandwiched between the chalk and concrete. You see, the amount of rainwater that’s in direct contact with the exterior perimeter brickwork of the property is negligible to the point of sometimes being inconsequential. However, rainfall can accumulate and remain below the surface directly saturating the soil that’s in contact with the brickwork below ground level. If the DPC is below ground level, then it’s moisture from this wet soil that’s being drawn above the DPC and via capillary action through the porous masonry and into the internal plaster. In many instances where the exterior of the property is rendered and the render extends below the ground-level surface, the moisture can be drawn both between the brickwork and render as well as the through to the internal plaster.
In cases where the ingress extends higher than 1200mm on the internal surface of the wall, the contributory issue of the external render almost always plays a part.
On a separate note, underground drainage, specifically the gully for kitchen or bathroom drainage  is so often a major contributory factor to an ingress but almost never picked up on a survey. Often, Victorian properties will still have the original gullies in-situ. Hairline cracks, gaps around outer seal or cracks in the original underground clay pipe can contribute incrementally to vast quantities of drain water filtering through the underside of the property. In many cases, the internal effects are spread out throughout the ground floor of the property and a correlation is never drawn between the two. Why ? because individuals who conduct surveys for damp generally have no groundwork experience and therefore no knowledge of such matters.
You should be aware that you now in a privileged minority in awareness of moisture content below ground level and it’s potential effects regarding damp. There are countless surveyors with qualifications pertaining to abilities in detection in causation of ingress. Unbelievably, those same individuals repeatedly site ‘lack of ventilation’ or ‘requirement for injection DPC’ as the answer to everything.
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