Efflorescence and Paint Failure in a Coastal School - Treating the Symptom or the Source?
A three-storey school building handed over 18 months ago in Ratnagiri, where annual rainfall is about 2,800 mm, shows white fluffy salt deposits and paint blistering on the internal face of ground-floor external walls. The band is worst between 150 mm and 900 mm above floor level, fades above that, and appears on all four elevations including the north wall. The structure is RCC framed with 230 mm brick infill using locally sourced bricks and river sand; plaster is 12 mm of 1:6 cement mortar, finished with two coats of acrylic emulsion on putty. Plinth level is 450 mm above ground and a 1.2 m wide concrete apron slopes away from the building. No crack wider than 0.3 mm is visible and there was no leakage from the terrace during the last monsoon. Repainting was done six months ago and the salts reappeared within eight weeks. The trust wants a permanent solution before the new academic year and has budgeted only for repainting.
1. Diagnose the problem. Eliminate the competing explanations and identify the mechanism and its contributing factors, naming the checks that would confirm your diagnosis.
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Eliminated alternatives:
- Condensation would appear on the coldest surfaces, higher up and on ceilings, worsen in humid winter weather, and would not concentrate in a horizontal band 150–900 mm above the floor on every elevation regardless of orientation.
- Terrace leakage is excluded by the absence of leakage through the last monsoon and by the location of the band at ground level.
- Plumbing leak would be local, not uniform around the perimeter.
Mechanism: rising damp. Ground water is drawn up through the brick and mortar by capillary suction, carrying soluble chlorides and sulphates; the water evaporates at a fairly constant height, leaving salts to crystallise there. That produces exactly the observed features: a horizontal band from floor level upwards, the same on all elevations, white fluffy efflorescence, and paint film pushed off by crystallising salt rather than by simple dampness.
Contributing factors: the plinth DPC is absent, discontinuous, or bridged by masonry, the floor slab, or the apron; coastal bricks and river sand in this region are commonly high in soluble salts; a 12 mm 1:6 plaster is porous and provides no resistance; and the apron slopes away but there is no subsoil drain or plinth-protection membrane, so water stands against the foundation through a 2,800 mm monsoon.
Confirming checks: moisture meter readings along a vertical profile (expect above 20% at 300 mm, below 12% at 1.5 m); a salt test or laboratory analysis of a plaster sample; and a physical check at plinth level for a DPC course or a bridged DPC. Record everything before any hacking starts.
2. Specify the remedial sequence with material requirements and hold points, from source control to final finish.
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The sequence must be water first, finishes last - this is the point the previous repainting missed.
- Cut off the water supply. Clean and remake the apron with correct fall; add a spoon drain or subsoil drain around the plinth. If the DPC is defective or absent, install a chemical DPC by injecting a silane/siloxane or crystallising grout through a staggered grid at 250–300 mm spacing immediately above plinth level, or saw-cut and insert a physical DPC where that is structurally feasible.
- Remove contaminated material. Hack off plaster to 1.2 m above floor, or 300 mm above the top of the salt band, back to bare brick; brush down and remove salt-laden mortar.
- Replaster. Two coats: a 12 mm base of 1:4 cement:sand with an integral waterproofing compound, and a 6 mm finishing coat of 1:6. Use salt-free sand and potable water. Aim for density and low permeability rather than richness - over-rich plaster shrinks and cracks, which restarts the cycle.
- Hold point - moisture. Do not proceed to putty or paint until the wall reads below 12–15% with a moisture meter, checked at three locations and recorded. This is the single point at which the earlier repaint failed.
- Paint. Alkali-resistant primer, then two coats of a breathable acrylic emulsion, or an anti-efflorescence/elastomeric coating system. Allow at least 28 days of drying before painting, and avoid painting in the monsoon.
- Handover notes. No watering of plants against the walls, keep the apron and drain clear, and inspect at 6 and 12 months.
3. Explain why the previous repainting failed within eight weeks, and how you would respond to a client who has budgeted only for repainting.
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Why it failed: repainting treated the symptom. Emulsion on putty is vapour-permeable, so moisture kept travelling through the wall, dissolved salts from the brick and mortar, and crystallised just under the film. Eight weeks corresponds to one wet–dry cycle in a coastal monsoon. Any coating applied while the wall is above about 15% moisture will fail; a non-breathable coating would not have solved it either, and would likely have spalled the plaster instead.
Responding to the budget: say plainly, in writing, that a repaint-only programme has a near-zero expected life - the same eight-week outcome should be expected. Then offer a staged package that fits the budget:
- Year 1: carry out DPC treatment and replastering on the two worst elevations only, chosen from the moisture readings and the prevailing wind-driven rain direction; leave these unpainted, or apply lime wash as a test patch.
- Year 2: after one monsoon, inspect the patches. If they stay dry and salt-free, paint them and treat the remaining elevations in the next cycle.
Show the annualised cost: DPC and replastering may cost three to five times the repainting rate per square metre, but with a service life above ten years against one year, the annual cost is lower. Support the argument with the moisture and salt readings so the decision is documented, and decline to guarantee a permanent solution on a weather-exposed coastal wall - offer a maintained dry wall instead.