Technical Q&A 21 : Inappropriate Cement Pointing
The type of mortar that someone proposes for repointing joints in the walls of
an old building is a good test of whether they should be working on such buildings
at all. Douglas Kent, the SPAB Technical Secretary, explains why - and what you
do if an inappropriate cement mortar has been used.
Q. Why is hard cement pointing inappropriate for an old building?
A. Simply because it can disfigure and hasten the decay of historic masonry. Portland
cement became particularly popular after the Second World War. Whereas modern
buildings rely on such materials to seal walls and exclude moisture, the permeable
lime mortars widely used on old buildings allowed them to ‘breathe’ – the ‘raincoat’
versus ‘overcoat’ effect. In some cases, lime mortar pointing weathers back to
the stage repointing is needed, but if this is done with impervious cement (a
common mistake), erosion of the bricks and stones themselves occurs in preference
as they become the most permeable part of the wall and suffer the greatest frost
and salt action.
Dense, cement-rich mortars also develop hairline cracks from shrinkage or, with
old buildings on shallow foundations, from minor structural movement. Lime mortar
better accommodates movement and, because it is ‘autogenous’, any minor cracks
that do develop will self-heal. Remember too that modern forms of joint finish
associated with cement can form ledges, trapping water.
Q. How do I tell whether inappropriate cement pointing has been used?
A. A very hard, smooth and dull grey mortar typically indicates an unsuitable mix
rich in ordinary Portland cement, as opposed to the softer, more open-textured
and lighter coloured mortar bound with lime that is compatible with old masonry.
Furthermore, the latter may contain impurities such as chalk from the lime-burning
process. Featheredging along margins is characteristic of cement, as is a weather-struck
(angled), bucket-handle (concave) or ribbon pointed (protruding) joint finish.
Weathering characteristics differ as well. Unlike lime-based pointing that gradually
erodes back, cementitious mortar cracks and drops out in short sections or is
left standing proud of decayed bricks or stones.
Q. If my building pre-dates the late 1800s, should all inappropriate cement pointing
be removed?
A. Not necessarily. Although remedial action should ideally involve removal, this may cause further
damage to the fabric. A small area should therefore be removed as a trial and
a pragmatic approach adopted.
Where the pointing adheres well, it is probably best to leave it to age naturally,
especially if, fortuitously, problems such as damp have not ensued. Sometimes
a compromise may be possible, perhaps just by removing selected areas where localised
deterioration has occurred, and patch pointing with a more suitable mix. Weathering
over time often breaks the bond of the remaining cementitious material, so patience
can be rewarded.
Q. What is the best method of removing inappropriate cement mortar?
A. Hard pointing can possibly be cut out with hand tools, such as chisels or quirks
and a light club hammer. The use of a hammer and bolster is not recommended. An
alternative technique involves the use of a handsaw, particularly for fine joints.
Cutting discs, reciprocating blades and air tools with adapted masonry carving
chisels can be helpful in removing detrimental cement mortar, but they must be
properly used, and only entrusted to highly experienced operatives. Incorrectly
used, power tools can cause untold damage to stone or brick surfaces. Regardless
of the technique, the importance of trials cannot be overemphasised, and great
care must be exercised to avoid damage to the edges, or ‘arrises’, of bricks and
stones.
Q. What should I use to replace inappropriate cement pointing?
A. Generally, a soft and permeable lime mortar that matches the colour, texture
and profile of any surviving lime pointing. Where there is none, and it is necessary
to design a new mortar, this must be weaker than the bricks or blocks and take
into account site exposure. Recently, cement has been added to modify lime mortars,
but this is discouraged now that a wider range of limes is obtainable. If in doubt
about the joint profile, a flush or nearly flush finish is usually sympathetic
or there may be local vernacular precedents to follow. It is important that work
is undertaken by someone familiar with lime-based materials. The SPAB may be able
to advise on suitable contractors or courses.
In exceptional cases, where the arrises of brickwork have been significantly
damaged, it may be appropriate to introduce the specialist technique known as
‘tuck pointing’ to replicate the original appearance of the masonry by giving
the impression of finer joints.
Further reading
Repointing Stone and Brick Walling, SPAB Technical Pamphlet 5 (2002)
© Douglas Kent 2005