Plasterboard on a damp Maltese wall: why it fails

What boarding over damp actually does

A homeowner finds a stained, blistering wall, gets a quote to batten it out and board it, and the stain disappears in a day. Nothing has been fixed. The wall behind the board is the same wall it was that morning, and it is now out of sight.

Three things follow. The wall keeps carrying moisture, because nothing about a sheet of plasterboard interrupts the mechanism that brought the water there. Any salt-contaminated plaster left on the wall carries on pulling moisture out of the air. And the void between board and wall becomes a space nobody looks at, fed by a substrate that — as the Maltese evidence on porous globigerina and stone walls puts it — absorbs and releases moisture slowly, for weeks at a time.

That is the problem in one line: plasterboard is a finish, and damp is a building defect.

The three sources, and the humidity that compounds them

A 1990 University of Malta dissertation assessing a typical Maltese terraced house identified three sources of dampness — rain penetration, rising damp and condensation — together with the building defects behind each. Thirty-odd years later a Maltese consumer guide describes the same picture for flats, adding structural defects such as roof leaks and outdoor humidity as a distinct driver.

Rising damp

Rising damp is endemic in Malta's built stock, which is mostly Lower Globigerina Limestone. The stone is porous; that is the whole mechanism. This is not a quirk of old farmhouses — it is the default behaviour of the material most of the island is built from.

A 2018 Master's dissertation tested electro-osmosis as a drying method on limestone samples. It controlled moisture movement in the laboratory but concluded that further study was needed. Read that as a warning: there is no proven local cure-all, and anyone selling one is ahead of the published evidence.

Rain penetration

Water driven through external walls or in through a roof defect. The Maltese case study ties dampness to identifiable building defects rather than to bad luck, and the flat-level guidance lists roof leaks explicitly. Rain penetration is seasonal and directional, which is why it is often mistaken for rising damp on a ground-floor wall.

Condensation

Cooking, showers and laundry put water into the air; cold surfaces take it back out. Malta's outdoor relative humidity averages about 76%, and poorly ventilated flats reach 85-95% indoors between October and February — precisely the months when damp complaints peak. The healthy indoor range is 40-60%, and a reading consistently above 60% signals a problem.

Condensation matters here because it is the one source plasterboard can make worse. Board a cold external wall without addressing ventilation and you have added a surface, a void and no extra air change.

Salts are the reason the plaster has to come off

This is the part that gets skipped. UK guidance on rising damp and replastering is unambiguous: installing a damp-proof course cannot undo salt damage already in the plaster. Hygroscopic salts deposited by years of evaporation keep drawing moisture from the air on their own. The contaminated plaster must be removed and replaced.

So the sequence is not "stop the damp, then board". It is "stop the damp, strip the contaminated plaster, replace it, then board if you still want to". Fix the source and leave the salty plaster in place and you have a wall that still goes damp in October.

The same UK guidance cites BS 6576:2005 for the diagnosis of rising damp. That is a British standard, and we use it here because there is no Maltese equivalent to use instead.

What the law says, and what it leaves out

Waterproofing and damp in Malta are still governed by article 97 of the Code of Police Laws (Cap. 10) — mid-19th-century provisions, as the Kamra tal-Periti set out in its 2020 framework paper. They refer to ventilated basements and cavity walls as means of restricting damp from the ground and through external walls, and they put the onus on the perit to provide proper damp-proof courses.

Two consequences for a homeowner:

• There is a legal duty on the perit regarding damp-proof courses, so on works where a perit is engaged, damp is their province and not something to settle informally with a plasterer.
• There is no modern performance code behind that duty: no published method, no specified materials, no acceptance criteria. The Kamra's own paper is an argument that the framework needs modernising.

Gypsum works themselves sit outside this entirely. Boarding is finishing work. It does not require, and cannot substitute for, the damp-proofing decision.

Before any board goes up

1. Measure. A hygrometer costs under EUR 20 in Malta. Log the room, morning and evening, for a week. If you are consistently over 60% you have a problem to name before you spend on finishes.
2. Name the source. Rising damp, rain penetration, condensation or a structural defect such as a roof leak. These have different fixes and the wrong fix is money gone.
3. Fix the source, with a perit involved where the defect is structural or where damp-proof courses are in question.
4. Strip the salt-contaminated plaster and replace it. A damp-proof course does not neutralise salts already in the render.
5. Let the wall dry. Globigerina gives moisture back slowly, over weeks. Nobody publishes a target reading for Malta, so this has to be judged on the wall itself.
6. Then board — for level, for services, for insulation, for whatever reason you wanted board in the first place.

Flats, shell form and Gozo

In an apartment, the damp may not originate inside your demise. A roof defect or a failure in an external wall is likely to involve common parts, which means the condominium rather than you alone, and that conversation has to start before anyone quotes for finishes. Boarding your side of a wall fed from a common element does not make the moisture your private problem to hide.

Property sold in shell form is handed over before finishes, which is the ideal moment to look at bare walls honestly. It is also the moment at which a wall that will be damp in January looks perfectly dry in July.

For Gozo, note only that remediation materials come over on the ferry like everything else, which affects scheduling more than it affects method.

Living with the humidity afterwards

Even a correctly remediated wall sits in a climate averaging about 76% outdoor relative humidity. A mid-range dehumidifier costs EUR 120-200 and the target indoor band is 40-60%. Ventilation and moisture control are ongoing household management, not a one-off build item — and no amount of plasterboard changes that.