Lime pointing: getting it right
The dark and beautiful art of lime pointing can easily be overlooked when maintaining an old building.
There are many corners which can be cut, and alternatives that have been used in the past (and even today) which only lead to worse problems down the line. We wish to highlight some of these and draw attention to the passion behind each and every Highlife lime pointing job.
First up is the obvious cardinal sin number one. Replacing or covering Lime pointing with a cement based alternative. Alongside its lack of breathability, cement continues to harden indefinitely. This is caused by the cement particles reacting to moisture and creating new bonds. The issue with cement is that often the stone used to make an old building is more permeable than the actual mortar so any moisture is forced through the stone instead of the joints. This can then lead to fast tracked erosion and/or delimitation of the stone face.
A similar issue can also be caused by using a lime mortar which has too high a compression rating for the stone. Natural hydraulic lime (NHL) is graded according to its compression strength. NHL 2 would be exceptionally permeable but have a lower strength under compression whereas NHL 5 is exceptionally strong but is less permeable than a lot of stone. NHL 3.5 sits somewhere in the middle and is the most common grade for general building works. Each of the three NHL gradings has its own uses, and matching the correct lime, aggregate, and sand mix ratio is important to maximise the longevity and aesthetic of each individual building.
Most older buildings enjoy a more rustic and coarse finish to their lime pointing, a style which is only achieved if the lime mortar is correctly tended to. Making the aggregate SHINE is a proud achievement that we have become accustomed to.
Other problems which we see regularly include:
Poorly face pointed joints, which usually just fall out, and provide no structural support to the stones.
Tending lime when it is still too wet and leaving lime smears all over the stone.
Contractors offering cheaper quotes and employing very low skilled workers to work on scaffolding. You get what you pay for.
Not taking the time to back fill deep joints
Letting the lime dry out too quickly, leading to cracks and/or lime failure (turns to dust)
Pointing wide joints without the use of additional stone (galletting), which leads to weak joints.
Poorly colour matched lime mix without consistent use of one particular aggregate throughout a job.
Poor de-veg removal prior to pointing. Not removing all root systems and treating with biocide to prevent regrowth.
At Highlife we maintain a strict process and high standard to our pointing. Depending on the task it may initially involve a consultation with an architect/conservation officer to identify a specific mix ratio, colour match or even specific aggregate sourcing. This is often then followed by some patch pointing at low level to agree a final aesthetic.
For smaller jobs where an architect is not involved, upon inspection we would use our professional judgement to determine the best materials and mix for the job so as to best fit in with the surrounding mortar.
Pleased to get a great finish after doing a test patch for an architect
Whilst undertaking the job we make sure to remove enough of the old pointing so as to leave a deep enough recess for a good bond. Also taking care to remove any vegetation and applying a biocide prior to repointing.
After repointing we pride ourselves with our care of the lime, ensuring that the it doesn't dry out too quickly, until it is ready for tending to give a compact, neatly finished, and long lasting result.