Lime Mortars for Pointing

Technical advice note for the application of Lime Mortars

The preparation for pointing works is of the utmost importance requiring close attention to detail. Observe exactly how the wall was built originally and take note of what is ‘actually’ there. Look at ‘any’ original rubble wall and note the small pinning stones, supporting and spaced between the larger stones, known as gallets and galleting (also known as snecks or pinning). All too often walls that are in need of re pointing end up being finished with all resemblance of its previous detail gone forever, with all the big stones left floating in a sea of cement custard, all for the sake of a little additional care. These small pieces of stone play an important role in the wall as they act as a large piece of aggregate, cutting down the amount of mortar required to complete the works and something readily available and costing nothing at the time of building.

Joint Preparation

The joints should be raked out rather than cut out; using any form of appropriate hand tool to suit both the joints and the operator (the use of angle grinder/disc cutters should be strictly avoided). As a rule, the depth of new pointing should be at least twice the width of the joint, with an absolute minimum of 25 mm. This is crucial as insufficient mortar depth could probably result in the mortar falling out owing to the lack of ‘grab’ on the original wall. Joints should be cleaned out thoroughly, left square and cleaned of all loose and friable material.

Prior to the pointing works being carried out the joints should be dampened, along with any very absorbent stones or bricks that make up the wall. Lime mortars are very different to cement mortars in the way they set, and require a little help sometimes in order to control the process. It is this setting (carbonation) process that is probably the most important aspect of all when working with lime mortars and will be greatly helped by regularly applying a fine damp mist spray. A very handy tool for this operation would be a pump up type sprayer or similar. Very suitable as the volume of water is far better controlled delivering a fog mist type spray. The use of hose pipes or other appliances delivering large volumes of water should be avoided.

The use of water is very important as it helps to control the loss of moisture (from the new mortar) being absorbed into the original and surviving joint, too rapid a loss of moisture will result in a total failure.

Once the work is finished or as work proceeds, the use of damp Hessian sacking draped over the work surface and then covered with polythene is very effective. On larger projects with scaffolding the use of protective scaffold sheets is always to be recommended.
Whatever method used will be greatly determined by the time of year and the weather, but always remember that the longer it takes the mortar to make a set the better it will be.

Another point worthy of note, reference the damping down, is that the lime in the mix being composed of much finer materials than the aggregate can migrate to the surface quite freely unless checked, and could well result in an excess of lime being deposited at the surface creating a much denser outer layer (case hardening). This freedom of the lime can be made worse by over tooling the work with steel tools, pulling the free lime to the surface, it is also therefore important that lime mortars are not overworked or too wet at the time of application.
Other major factors governing the set will be the weather with both wind and sun affecting the rate at which the mortar dries, if allowed to dry too quickly it will invariably result in a failure owing to the mortars inability to achieve a proper set.
(Note: Winter working externally, is not recommended with lime putty mortars owing to the risk of frost damage)

The Mortar

Mortars should be workable without being too wet and the stiffer they are the cleaner the work will be executed. Too wet a mortar may lead to excessive staining of the wall and ‘will’ create greater shrinkage drying problems, in the form of (plastic tension) crack's etc.

When filling the joints it is important to apply plenty of pressure to the tool being used and press the mortar well into the joint. There are various types of tool to apply the mortar but the only criteria as to the kind of tool to use, is that it should not have too large a surface area (to smear mortar all over the stone with) for the size of joint being worked on, and that it Is comfortable for the person carrying out the work.

Whilst placing mortars, avoid filling large voids in one operation, bulk out first using pieces of stone, aggregate etc. to fill the gap and cut down the amount of lime mortar required (large volumes of mortar also take much longer to carbonate).

After the mortar has started to set, a churn brush (stiff bristled brush) should be used to compact the mortar by ‘tamping’ it into the joint. The action of tamping serves three purposes, it improves the contact between stone and mortar, it helps to clean any material on the edge of the stone and thirdly it textures the surface increasing the actual surface area of the joint allowing for better evaporation. It will also expose the aggregate within the mortar improving the overall appearance. The timing of this being fairly critical, if it is done too soon it will simply smear the stones and could well result in the free lime being pulled to the surface of the joint (case hardening). If on the other hand the mortar is left too long it will be too hard and all that will be achieved is to wear the bristles on the brush out.

We strongly advise, and assume here that the correct choice of sands has been sought.
The sand is critical to both the overall performance and appearance of the finished work.

The information contained here is for guidance purposes only and is in essence applicable to non-hydraulic lime mortars. However, should you have any doubts or queries please don't hesitate to contact us for further advice