Why drying takes so long
Water leaves masonry by evaporating from the surface, and the water held deeper in the wall has to migrate outwards to reach that surface. Brick, stone, block and plaster are all porous but not especially permeable, so this is a slow process governed by the thickness of the wall, the materials, the temperature, the humidity of the air around it and how freely that air is replaced. A widely used rule of thumb is around 25 to 30mm of drying per month in reasonable conditions. A 225mm solid wall wetted through its full thickness therefore represents many months, not weeks, and a thicker stone wall or a wall that has been saturated for years can take a year or more.
Realistic timescales
For a plaster surface wetted by a short-lived leak that has been repaired, expect a few weeks. For a 100mm internal block or brick wall, a couple of months. For a 225mm solid external wall wetted through, six to nine months in favourable conditions. For a solid stone wall, a thick rubble wall or a wall that has been repeatedly saturated, twelve months or more. For a concrete floor slab, drying is slower still, and a screed that has been wet can take several months per 25mm. Winter, cold rooms and closed-up properties all extend these figures considerably; warm, moving air shortens them.
Why the surface misleads you
A wall surface will feel dry, look dry and take paint long before the material behind it has dried. Surface evaporation is fastest, so the outer few millimetres reach equilibrium with the room air while the core remains wet. This is why a wall can be redecorated successfully in appearance and then blister two months later: moisture continues to arrive at the surface from behind. It is also why surface meter readings are a poor guide to drying progress. Readings taken at depth, or carbide tests on drilled samples at intervals, are what actually tell you whether the wall is drying and how far it has to go.
What speeds it up, and what does not
Three things genuinely help: gentle background warmth, air movement across the surface, and removing the moist air from the room so the drying can continue. A dehumidifier in a closed room is effective because it lowers the humidity of the air the wall is drying into; opening every window in January is less effective than it feels, because cold air holds little moisture and the room cools. Fan heaters aimed at a wall dry the surface faster than the core and can cause salting and cracking. Sealing the surface with an impermeable coating to "hold it back" does not dry anything; it traps the moisture and usually moves the problem to the adjoining area.
Salts appearing during drying
As a wall dries, dissolved salts travel to the surface and are left there as a white bloom or fluffy crystals. This is a normal part of drying rather than a new problem, and it will continue while the wall gives up water. Brush the deposits off dry rather than washing them in, and do not decorate while they are still forming. Where the salts are hygroscopic and heavily concentrated in the plaster, they will keep the surface feeling damp after the wall behind is dry, and at that point replastering becomes the sensible option rather than more waiting.
Knowing when it is safe to redecorate
The reliable test is measurement rather than appearance or time elapsed. Readings should be taken at depth, at several points, on more than one occasion, and should show a consistent downward trend to a stable level rather than a single acceptable figure. Salting should have stopped. Where a flood or a long-standing leak is involved, we recommend monitoring over a period and only reinstating when the trend is clear. Redecorating too early is the most common reason a repaired wall appears to fail: the repair worked, the finish went on too soon.
Where monitoring is worth commissioning
For an insurance claim, a flood, a long-running leak or a property being prepared for sale or letting, it is worth having the drying monitored and documented. That gives you a record of readings over time, an evidenced decision point for reinstatement, and protection against paying twice for finishes. We can measure at depth, advise on drying arrangements and tell you when the background is genuinely ready, which is usually later than everyone hopes but earlier than doing the work twice.
Drying after a flood or an escape of water
Flood and escape-of-water claims deserve particular care, because the pressure to reinstate quickly is strongest exactly where drying takes longest. Solid walls, party walls, timber floors and any insulation that has been wetted all hold water long after the surface reads dry, and material such as mineral wool in a floor or wall will not dry in place at all in some situations. A proper drying programme means removing what cannot dry, setting up controlled drying with monitoring rather than simply leaving machines running, recording readings at depth over time, and agreeing a documented decision point for reinstatement. Where an insurer's contractor is working to a fixed timetable, a second, documented set of readings is a useful protection: it establishes whether the structure was genuinely dry when the finishes went back on.
Limitations of general guidance
This article is general guidance. It cannot confirm the cause of dampness in a specific property. Recommendations should follow an inspection of the building, its external environment and its internal conditions.
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