Sometimes bricks are quite loose. Individual bricks may be almost entirely missing. Internal plaster sometimes crumbles as well. 

The cause and the solutions are not always understood and attempted repairs often just make the problem worse. 

In this, the first of an occasional series of articles on conserving older buildings, Dubbo City Council heritage adviser Graham Hall outlines the fundamentals of "salt damp".

Probably the most common fault in older buildings in the central west is the erosion of masonry walls due to a combination of rising damp and salt attack.

Older bricks are relatively soft and porous.

The mortar used for the joints is made of sand and lime, with little or no cement.

It, too, is soft and porous.

So old walls absorb water - from rain, from leaky gutters, from the ground - more readily than modern walls built with denser bricks laid in strong cement mortar.

Even in dry weather, the soil contains some water.

In wet weather, the soil can be saturated.

Whenever there is any moisture in the soil it tends to seep into the base of a wall and is drawn upwards through the fine pores or voids within the bricks and mortar.

The wall acts like a wick.

Once the water reaches part of the wall exposed to heat and air, it tends to evaporate.

If the wall is painted, bubbles can form under the skin of paint, and eventually burst.

To stop this rising damp, walls are built with a damp-proof course about floor level.

Modern dampcourses are usually flexible polythene or bitumen-covered aluminium.

In older buildings there is sometimes no dampcourse at all.

More usually, slate, a trowelled mix of sand and bitumen, or a strip of bituminous felt "malthoid" was used.

These can fracture with slight movement in the wall, or can be bridged by soil or applied render, allowing the groundwater to rise higher into the wall.

Here it is more exposed to heat and air movement and more likely to evaporate.

As well as water, soils in the central west often contain a lot of salts (including common salt, sodium chloride) which dissolve in water, as many on the land will know.

When the water evaporates, the dissolved salts form crystals within the tiny voids in the bricks and mortar.

These take up more space than the liquid water and force the particles of masonry apart.

Over time the wall is eroded and eventually it can collapse.

Usually the mortar and the occasional soft brick erode first.

Frequently owners replace the soft lime mortar with harder cement mortar.

But this only makes the problem worse.

Because the cement render is denser than the bricks the salt now attacks the bricks themselves.

Sometimes the bottom part of the wall is cement rendered.

The dense render stops the water evaporating so it continues to rise until it is above the render.

And the erosion just continues higher up.

These treatments also go against the philosophy of heritage conservation.

They alter the historic appearance of the wall and destroy the patina of age that gives historic buildings their charm.

o o o

How to fix an eroded wall?

It is best to get an expert opinion because each case is different but here are some general rules.

Ideally a new dampcourse is inserted.

This can be a modern one, cut into the mortar joint brick by brick, but it will often be cheaper and just as effective to have an expert inject a chemical barrier into the brickwork.

This will stop any further salt being drawn up into the wall and if rising damp has been affecting internal wall finishes they can now be repaired.

Sometimes it will be necessary to remove the remaining salt from within the wall, by various means - get expert advice.

Often, however, a cheaper, although imperfect, solution is available.

If the bricks are reasonably sound, but the joints are decaying, simply re-point the joints.

But be sure the replacement mortar is lime mortar with no cement.

A good bricklayer should know the mixture and the techniques to use.

This doesn't cure the problem because the new mortar will be attacked.

But it should buy time - perhaps 20 years - before it has to be done again or more serious measures need to be taken.

Also, stop undue wetting and drying by making sure gutters and downpipes work and the ground is well drained, and carefully remove any render that has been applied above floor level.

Owners of heritage items can book an inspection by the heritage adviser by phoning Margie Clatworthy at the council on 6801 4000.

Repairs can be eligible for a grant from the council's local heritage fund.