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Building Lime Company | Tel: 01794 884294
Surveying for Damp Issues with Historic Buildings
Buildings fall into two categories for the purposes of surveying for damp:
I. Modern buildings built with a damp proof course and barriers to rainwater penetration.
II. Old buildings built prior to mid to late 19th century depending upon the region of the country. Old buildings relied on the walls breathing and shedding moisture before damp became a problem.
In old buildings, walls were built to such a thickness that normally damp would not penetrate to the inside. The joints were always of lime mortar or earth and were more porous than the building’s structural elements comprising brick, stone etc. Consequently the joints would drain and shed water by evaporation, therefore not allowing damage to these structural elements. The joints were the sacrificial element of the building.
Because lime and earth mortars are so porous, timber in contact with these mortars is less prone to decay than when bedded in cement mortar.
Historic houses, when built, were able to breathe and shed water. They were also heated by coal or log fires in an open fireplace which promoted rapid air changes by way of air being drawn out through the chimney. Windows and doors were not sealed as they are today allowing air movement into and out of the building
Internal finishes were lime washed which allowed surfaces to breathe and, although it would discolour when damp, it would not peal off the wall like wall paper nor blister like modern paint.
Over the years most of these houses and buildings have been altered to suit our modern lifestyle, nearly always to the detriment of the
building.
The heating has been changed and the windows and doors have been sealed and draught proofed. Consequently, we now no longer have the same amount of air movement.
From the day they were built many of these buildings have had moisture from the ground, rising into the walls by of capillary action. This moisture contains salts in solution. Initially for many years the moisture in the walls would evaporate harmlessly, outside during fine dry weather and inside,
as a result of the rapid air changes induced by the heat from the open fire drawing
air rapidly up the chimney.
When this moisture in the walls evaporates, the soluble salts it contains are left
in the walls at the point of evaporation which would be within the surface of
the masonry or plaster finishes.
Salts block the pores and cause moisture to rise further up the walls to a point
where evaporation can continue. A proportion of salts are also hydroscopic
(moisture attracting) and in humid conditions (modern living, no open fire, etc)
will attract moisture to the wall causing a damp area or band even when there is
no capillary moisture.
In many buildings the floors have been concreted on top of a plastic damp
proof membrane which reduces the area of evaporation of moisture from the
floor possibly causing more moisture to rise into the walls.
Hard cementitious internal plaster finishes have been applied, which are far less
porous than lime wash.
Ground levels are often higher than the original, either above the height of the
masonry plinth in earth structures and/or sometimes above the level of the
internal floors.
These problems along with defective roofing, joinery, rainwater goods,
chimneys, pointing etc., can cause excessive damp in old properties.
Also to consider, as Graham Coleman, a well respected expert from BRE
wrote:
“Another factor to consider is that, all things being equal, rising damp tends to
rise higher in thick walls than thin walls; this is due to the lower surface to
volume ratio of thicker walls, evaporation being mostly subject to surface area.
This is an important feature to consider when dealing with properties with
larger dimensioned walls - simply the so-called 'allowing walls to breathe'
syndrome to stop the rising water may prove of little effect in such cases”.