Damp proof courses (dpc) are first found commonly from about the 1860s in the more expensive house and then by the 1920s were common in any status of house.  Early forms can be of dense clay bricks or hard stone and later consisted of layers of slate built into a joint or even a mixture of bitumen and mortar.  These developed into the flexible sheet materials now used during the 20th C.

Damp proof courses (dpc) are built into the base of walls to prevent moisture rising up through their structures by capillary action in the stone or brickwork.  In normal conditions, without a dpc, damp can climb to a height equivalent of about one atmosphere (28” or 712 mm) above the adjoining ground level.  In locations where there is higher ground water pressure such, as at the bottom of a slope, then damp may be found higher up the wall.

It should be remembered that the height of rising damp in a wall is dependent on the level of the external ground level, so if the internal floor level is below that of the ground externally then the damp may be evident higher up the internal face of the wall.  The same can arise when there is a change in the ground floor levels within a building and this can result in rising damp affecting internal walls.  In such a situation a DPC will not hold back what is then penetrating damp and some other form of barrier protection will be necessary.

Damp in a wall can be reduced by allowing it to ‘breathe’ in a natural manner.  This is the term that refers to the external face of a wall being able to lose moisture to the adjacent air through evaporation and the movement of the external air.  That is necessary because moisture moves through a traditional solid wall from inside to out of a building due to normally higher temperature and vapour pressure internally.

Therefore it is important that outer face of the wall is not ‘sealed’ with coatings that prevent this movement.  Such coatings can include modern plastic or polymer based masonry paints, cement based renders, hard cement mortar pointing and other applied materials.  If the wall is unable to lose moisture the fabric becomes damp and will become cold and then condensation starts to form on the internal face.  Once that happens fungal and mould spores, of which there are millions floating around in the air, settle and grow on the surface and black, green, grey or other shades of mould develop.  Washing or treating with a fungicide can reduce the symptoms, but not the underlying cause and so the problem will keep on returning.

Dry Rot is a fungal growth that consumes timber, mostly softwood, and will spread with specific growing conditions, especially somewhere that is dark with a warm and very moist atmosphere, such as cellars or poorly ventilated underfloor and flat roof voids.  Its treatment varies by situation where it is and needs to be considered carefully as to its suitability for the affected timber, position and the importance of what is affected.  There can also be a difference in the method of remediation between a structural and non structural position.

One method of treating the timber is just to isolate it from the source of the damp that the fungus needs to survive, but can be very difficult to achieve.  This is usually only used if the infestation is very minor and the fungus is not in a structural timber member.

Traditionally the method of treatment is a combination of curing the source of the damp that allowed the initial growth, the replacement of the infected timber with new suitably treated material, and the application of fungicides to eradicate growth.  Whilst this is quite drastic, with the loss of the historic timber and probably adjacent decorative finishes such as plaster, paint, etc, it can often be backed by guarantees if specialists undertake the work.

It should be noted that the source of the dry rot can be some distance from where it is detected and so a thorough investigation of affected locations should be instigated.  The tendrils of dry rot are more than capable of penetrating brick and stone walls to get to a food source.

The majority of old properties were built with solid walls (they do not have cavities) and without a damp proof course in the base, but which need not be a problem.  However, the lack of proper maintenance and the use of inappropriate materials can, and frequently does, contribute to the cause of damp in the fabric.

All buildings whether ancient or modern if not heated or ventilated properly will appear to be damp, because the fabric will absorb moisture from the air even if it is well maintained.  As a result the level of moisture within the fabric can vary with the weather conditions and the season and will typically be around 12 – 16%, although can vary several percentage points either way.

When a property is regularly occupied, with a good flow of air through it and coupled with a reasonable level of heating, the moisture in the fabric will reflect the ambient air temperature and so the level of humidity.  That may typically be around 8 – 10% in a family house, although in rooms such as bathrooms or kitchens where humidity levels are higher the fabric will be damper.