Sub-Floor Borescope Inspection: Salt Crystals and Persistent Damp
During a recent property survey we passed a borescope through a small opening in the floor to inspect the void beneath. The images showed timber joists overhead and a sub-floor oversite covered in a heavy layer of white crystalline deposits. That is not a cosmetic issue - it is a very clear moisture signal.
What the white crystals actually are
The white bloom is efflorescence - mineral salts that have been drawn out of the masonry or oversite by evaporating water. When water repeatedly moves through brickwork, mortar or concrete and evaporates into the void, the dissolved salts are left behind on the surface. The scale of the deposits here indicates this has been happening for years, not weeks.
Why sub-floor damp matters
A damp, poorly-ventilated void puts the structural timbers at risk. Floor joists, wall plates and the underside of floorboards are all vulnerable to wet rot and, in the wrong conditions, dry rot. It also raises humidity in the rooms above, which contributes to condensation, musty smells and mould appearing on skirtings, in cupboards or against external walls.
Common causes include blocked or missing air bricks, external ground levels sitting above the internal oversite, defective drainage close to the elevation, or a bridged damp proof course allowing ground moisture into the sub-floor space.
Our recommendations
Restore proper cross-ventilation by clearing and adding air bricks where needed. Investigate and correct the external source - high ground levels, leaking gullies or defective downpipes are the usual culprits. Inspect all accessible timbers for decay and treat or replace as required. Only once the void is dry and ventilated should any internal finishes be reinstated.
If you suspect sub-floor damp
Musty smells at low level, cold floors, mould on skirtings, or springy floorboards are all reasons to inspect the void. A borescope survey is a low-disruption way to see exactly what is going on under your floor before committing to any works.