What is it?

Ground water contains various dissolved salts in the form of nitrates, chlorides and sulphates. These pass up the wall with the water and are left behind as solid salts when the water evaporates, accumulating in the fabric of the wall and on its surface.

• The sulphates form ‘efflorescence’ that appears as a floury crust on the wall, which does no damage other than appearing unsightly.
• The nitrates and chlorides are somewhat more troublesome; they are ‘hygroscopic’ which means that they absorb moisture from the atmosphere surrounding them, and therefore generally remain invisible. If these salts are left on the wall, even if the original cause of the rising damp has been fixed, the dampness will not abate as these salts continue to draw moisture in.

Although technically not rising damp since it doesn’t come from ground water, penetrating damp can occur in the walls of a building, often on higher stories, and is usually caused by a leak from rainwater goods, plumbing or condensation.

Penetrating damp can be a problem in chimney breasts. This is because the gases given off from a fire contain traces of acids that form the hygroscopic nitrate and chloride salts, and these absorb moisture from the atmospheric air in the chimney, much higher than any damp proof course, causing the surrounding wall to become damp.

How to spot it

A wall that has experienced damp can develop one or several symptoms, and you should investigate further if you see evidence of any of these:

• Wallpaper blistering or peeling easily from a wall, possibly with black mould growing behind it
• Paint blistering and peeling from the wall
• A tide mark developing on the wall, possibly with a salty edge
• Dark staining to the wall
• Fungal decay in skirting boards
• Damp or wet patches or water droplets on the wall

Using a moisture meter to determine type of damp

Because water can conduct an electrical current, the moisture meter reveals relative changes in the conductivity and therefore moisture content of the wall. It is the relative pattern of readings that is important, not the precise readings themselves.

Take readings at intervals of 250mm (10in) up the height of the wall, up to the boundaries of the moisture (where readings are consistently zero).

• If the readings decrease steadily rising up the wall, active rising damp is in place.
• If the readings peak half way up the dampness in the wall (instead of at the bottom), rising damp is inactive, but old plasterwork that still contains hygroscopic salts is still absorbing moisture into the wall.
• If readings tend to increase moving up the wall, or are quite consistent, you most probably have a condensation problem.
• If the readings are uniformly high, check that the wall is not covered in a electrically conducting foil-backed wallpaper which is providing false readings.

How to fix it

The first course of action when curing a damp problem is to establish where the water source is and how it is gaining entry in to the walls. Use the following link to find out the areas to check:

• Where does damp come from? What to check

Having located the source of moisture, two stages of repairs must be made.

The primary repair is that of removing the source of moisture; leaks in plumbing or rainwater goods must be fixed, good ventilation through rooms and beneath floors should be restored, good drainage must be re-established and if necessary a new damp-proof course should be inserted into the affected area.

After the source of moisture has been removed, the decorative finishes applied to the walls will need to be repaired and/or replaced.

Repairing decorative finishes in the home

If the damp is caused by condensation alone, there is no need to remove plasterwork unless it has been significantly damaged. Simply clean it down, allow it to dry out by ventilating and apply a coat of fungicidal paint that includes zinc oxychloride (ZOC) to inhibit mould growth prior to adding the decorative finish.

To repair rising damp completely, it is imperative that plasterwork permeated by hygroscopic salts is removed. If it is left, these salts will continue to absorb moisture from the atmosphere into the wall allowing the problem to persist and the symptoms to get worse.

• When the affected plaster is removed, it will not take with it all of the hygroscopic salts as plenty will still be sitting in the core brickwork of the wall, and these salts will continue to work their way out into any new plaster and decoration thereon.
• Any moisture remaining in the core of the wall will also continue to work its way out, which can damage new plaster and paintwork.
• When replastering, you should use a plaster mix, which includes a silicone-based waterproofing and fungicidal additive that is specifically designed to resist the passage of these hygroscopic salts and moisture.
• Alternatively a layer of Platon plaster base can be fixed on top of the existing plasterwork, to which new rendering and plasterwork bonds. Platon plaster base is a clear, high-density polyethylene (HDPE) membrane which is moulded such that new plasterwork is kept physically separate from old hygroscopic masonry.

Applying damp resistant plasterwork

• Remove skirting boards and architraves.
• Remove plaster back to the masonry across the full width of dampness, and 50cm (19¾in) above the maximum height of the moisture damage.
• Rake out mortar joints in the brickwork by 15mm (½in) as this will help new cement rendering resist shrinkage.
• Mix a first render coat using 3 parts washed, sharp, loam-free sand to 1 part cement mixed with minimal water containing the waterproof and fungicidal additive (diluted to manufactures guidelines). Apply this to a depth of 12mm (½in) and scratch when setting to key for the next layer. The rendering and plasterwork should not carry all the way down to meet the floor to avoid drawing water from a damp floor upwards.
• Mix a second coat as for the first but with clean water containing no additive. Apply another thickness of 12mm (½in) and key.
• Apply a final 3mm (⅛in) layer of finishing plaster.

The wall should be ventilated (NOT heated) as much as possible to encourage the dampness inside and the new rendering and plasterwork to dry out:

• Allow approximately 1 month for every 25mm (1in) thickness of wall, and do not decorate the wall for the first 4 weeks.
• Wallpaper should not be hung for a year.

Further information and useful links

• Find a Chartered Surveyor near you
• Find a Damp Proofing Specialist near you
• Find a Water Proofing Specialist near you
• Find a Masonry Repairs Service near you