Blog | Pointing with Lime Mortar
Pointing with Lime Mortar
Pointing is the action of filling the gap between masonry units (joints) with a mortar to protect the masonry from water ingress and associated decay.
Before any work begins it is important to observe how the wall was originally built and take note of this. If you observe traditional solid masonry construction, you will very often note the inclusion of small stones between larger stones. These pinning stones (also referred to as gallets or snecks) may be there to offer support or “chock” or act as large pieces of aggregate, reducing the amount of mortar used within the joint. This reduction in mortar mass aids curing and to some degree the cost. In order to maintain the visual integrity, it is important that these pinning stones are placed back within the wall as close as possible to that of the original.
In most repointing cases the least popular aspect of the process, although vital that it’s done as thoroughly as possible is the removal of the existing mortar joints which, in the case of original lime mortars, is relatively straight forward; however, more frequently this involves the removal of hard cement mortars and in order to mitigate further damage to the original masonry this should be removed as carefully as practical leaving as square a profile as can be practically achieved to the back of the joint. Exactly how it’s achieved will be very subjective, as will the tools used to achieve it. There may well be occasions where mechanical cutting out is deemed appropriate and when and where this method is suitable we would advise due diligence to mitigate unnecessary damage to the masonry units.
When removing existing pointing it’s important to keep any damage to the masonry to an absolute minimum, and themost obvious element to manage that is “common sense”. Remove by cutting out in a controlled manner where the existing is mortar is cut out to the open face of the joint, don’t direct energy from the chisel directly into the mortar. This reduces the energy and stress to the masonry unit. Tools should be appropriate to the joint size and chisels should be as sharp a chisel as possible, while it may be counter intuitive you don’t necessarily require big heavy hammers, and rarely would we advocate the use of mechanical type “breakers” or similar.
The joints need to be raked out to a suitable depth, with the general rule being that it should be at least one and a half times the width of the joint, in the case of wide joints common sense should prevail. Insufficient depth will result in nothing more than a token gesture with an increased risk of the mortar becoming loose or simply falling out within a relatively short time. Conversely too much mortar, can lead to masonry becoming looser while benefiting the vendor.
Once the joint has been cut out it should then be squared off at the back and further prepared by a thorough brushing out, with no loose material present to compromise the bond of the fresh mortar once it is placed.
Dampening the Wall
While this should be important when working with any mortar it’s far more so with lime, to ensure that once the mortar is placed that it’s not allowed to dry out too quickly. To best manage the process, before any mortar is applied the background of the joints will need to be dampened down. As different masonry will have different characteristics the advice we offer here is generic; however, existing lime mortars tend to be highly absorbent and assuming the work is being carried out on typical masonry with an average amount of residual moisture (approx. 20% in northern Europe), spray the wall using appropriate methods that will place sufficient moisture into the background to prevent desiccation of the fresh mortar after placing. NEVER place your mortar into a situation where there is liquid water present.
A suitable mortar should be selected for specific application.
The table below offers a simple overview of suitable mortars; however, we do advise that you contact us for more specific advice.
|Lime Type||Host Surface||Exposure||Time of Year||Examples|
|Lime Putty||Soft/Friable too Reasonably sound||Sheltered||April to September||Cob (earth) soft friable brick or stone. Well suited to other masonry types throughout the year|
|NHL 2||Soft/Friable too Reasonably sound||Sheltered||All year with adequate protection|
|NHL 3.5||Reasonably sound to Very Hard||Sheltered to Moderate||All year with adequate protection|
|NHL 5||Very durable hard||Exposed||All year with adequate protection|
The mortar for pointing should be workable without being too wet, as a rule the stiffer the mortar the cleaner the work can be executed. If the mortar is too wet, it can be difficult to apply and will readily stain the masonry; also, the wetter a mortar is the more prone it will be to shrinkage cracking.
This text has been provided by Cornish Lime, the trusted lime experts.
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