Lime Mortars for Pointing

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.

Lime-pointing-enhanced

Joint Preparation

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.

Lime Mortar

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.

Capture

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.

Tools

There are a wide variety of tools that can be suitable for pointing and for most applications it will come down to what is suitable to the joint size and that of personal preference and what feels comfortable etc.  Using too large a tool will result in mortar being smeared over the stone work or prove difficult to apply in tighter joints.

A (Plasterer’s) small tool, also known as a leaf & square & trowel & square, can be very useful for where you have smaller joints.

In our opinion the churn brush is the most important tool when it comes to pointing with its main role to beat the mortar back after application; this will be discussed in more detail shortly.

Filling the Joints

When filling joints, it is important that the mortar is thoroughly pressed home applying sufficient pressure to the tool forcing mortar into the whole of the joint.   When placing the mortar, avoid filling large voids with just mortar, instead think about packing them out with appropriately sized stones that act as aggregate thus reducing the mortar mass which will in turn aid the curing process; regardless of the type of lime used. While there are many ways that pointing can be finished we only advise that of “Fully Flush Pointing” or “Pointing to the Weathered Edge,” where the mortar is finished in such a way that allows water to run down over the face of the stone.  We never advocate or recognise recessed pointing where a ledge is created which in turn can allow easier access for water into the masonry fabric from wind driven rain.

Finishing the Joints

The best tool and method for finishing is to use the Churn Brush to strike the mortar so as to compact it within the joint in a tamping action, throwing the brush square onto the mortar, in such a manner that you compact the mortar closing up any initial shrinkage that may have occurred, thus improving the contact between mortar and masonry.  This action serves several purposes, firstly it improves the contact between the stone and mortar, it also helps clean any material that may be on the edge of the stone and finally it leaves a textured surface. A textured surface increases the surface area of the mortar and allows for greater evaporation from the mortar joint, as it opens the surface. This tamping operation can and should be further repeated at an appropriate time (12 -48 hours) while the mortar is in a suitable state to receive such treatment.

Buckland+shit+repointing

The action of finishing the mortar is extremely important for the obvious technical reasons but probably more so because of the visual impact of pointing works; this is after all what you will be displaying for many years to come.

As for the right time to carry out this task it should be done when the mortar has reached a state where it has started to set to the point that it’s “leather dry” e.g. when your thumbnail can just indent the mortar; too soon and you’ll simply smear the mortar over the masonry and too late you’ll just wear the brush out quicker.  It can be as soon as a couple of hours or as much as 48 hours, or longer during colder weather, especially with lower strength limes.

The exact time mortar will take to reach this point will vary considerably on weather conditions at the time (temperature especially), with the mortar taking longer during colder conditions and greatly accelerated during the summer.   Therefore with the explanation offered above the best advice we can provide here, is that you should wait for what we hope will be relatively apparent, but please don’t hesitate to contact us for further advice.

Aftercare

While dampening the wall is a vital process for pointing, as it helps cure the mortar there are further measures that need to be considered once the mortar has been placed.  Weather in the UK is dynamic and while there are several factors to manage the two main concerns to watch out for especially will be frost and drying out.  Most importantly, if a lime mortar is allowed to dry out too quickly (rapid moisture loss) the result will likely be a mortar that is chemically deficient and vulnerable to accelerated weathering, and one of the most common causes of mortar failures.  Lime mortars demand baby sitting in their infancy with appropriate measures adopted for good curing and appropriate to the prevailing conditions at the time they are placed.

The mortar should then be protected; this is usually achieved with Hessian sheeting. Hessian has a dual purpose, firstly it protects the mortar from the elements and secondly it helps keep the mortar damp to aid its cure. For further more specific information please refer to our guidance sheet titled “Curing Lime Based Renders & Mortars

This article is to provide general guidance and an overview of pointing, we strongly advise that you contact us for more specific information for individual applications.

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