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WHY SHOULD WE
USE LIME ?
BY:
Ian MacLennan.
Managing Director
of the Building Lime Company
For the Property Care Magazine 2005
For the last 100 years modern artificial
cements have been replacing traditional lime based mortars
and plasters, to such a degree that now virtually all construction
is carried out using only modern materials. Whilst many of
these materials are perfectly suitable for modern buildings
they have been found to be incompatible with the construction
of old buildings.
All buildings move, especially traditional buildings on shallow
foundations or with timber frames. Cement
mortars and renders
are rigid and crack under stress. Often cement is stronger
than the masonry transferring fractures into surrounding brick
or stonework.
A typical example would be the Old Chapel in Wiltshire. The chalk stone and flint walls of the Old Chapel had been re-pointed with a very hard sand cement mortar during its conversion to a village house.
- The chalk had been laid in a lime chalk and grit mortar.
- The hard sand cement mortar used during the refurbishment was unsightly, causing cracks and water ingress.
- The Building Lime Company matched the original chalk mortar with they’re NHL2 Hydraulic lime and lime putty mixed with chalk and gritty sand.
- The contractor (LSE Building Preservation Ltd) removed all of the de-bonding hard sand cement mortar and re-pointed the chalk blocks with the chalk and lime mortar.
The need to understand the different technology
involved in historic and modern structures is essential ifsuccessful repair and maintenance strategies are to be carried
out.
Lime has been the primary binder used in mortars and plasters for many thousands of years and the vast majority of all buildings constructed before 1900 made use of lime.
Modern cements are harder and less permeable than lime mortars, the general aim when selecting mortar or render is that it should ‘breathe’ more freely than the material which it is applied to and that it should have less composite strength than the brick or stone which it is used with. This is essential if you are to prolong the life of the historic buildings.
Modern buildings generally rely on an outer
layer to prevent moisture penetrating the walls, whereas buildings
constructed before 1900 generally rely on allowing the moisture
which has been absorbed by the fabric to evaporate from the
surface. Old buildings exposed to the elements are continually
absorbing moisture and the ability for the moisture to evaporate
again is crucial to the well being of the structure, so they
must dry out so quickly that damp never penetrates the thickness
of the wall.
Historically rapid drying was aided by the use of open fires,
which remained alight throughout the year for cooking, providing
both heating and ventilation. Some idea of the rate of ventilation
this provides can be gained by watching the rate at which
smoke rises from the chimney, as air is drawn into the building
at exactly the same rate providing very regular air changes.
This ventilation ensured that any moisture present in the
building was expelled through the chimney in a very short
time.
Using cement based mortars and plasters
in traditional buildings runs the risk of locking- the moisture
into the walls which could result in dampness internally and
timber decay in timbers built into the walls. Problems generally
arise when the building has been 'repaired' with inappropriate
materials through lack of knowledge.
It is interesting to note that many structures built using lime technology 500+ years ago and maintained correctly are still in excellent condition today. It remains to be seen how modern cement built structures will fare in 500 years!
Where a traditionally built property has
been affected by ground water ingress and penetrating damp
for long periods of time the application of lime mortars will
not affect a cure for damp because the structure will be affected
by salts.
The Building Lime Company range of NHL mortar can be applied to damp masonry and will have some resistance to moisture and salt staining if blended with a well graded washed sharp sand.
The walls will remain damp and contaminated
with salts but if the plaster is lime washed the excess moisture
will be able to evaporate from the wall surface.
The Building Lime company does supply Natural Hydraulic lime mortars for use over ventilated meshed cavity drain membranes which have proved very successful on conservation projects where the walls are to damp to plaster with lime but need to be allowed to ‘breathe’.
Once the building is repaired and re rendered consideration should be given to decoration. Lime wash assists in maintaining the ability of the building to ‘breathe’ as it is one of the most vapour permeable decorative coatings.