Lime pointing of traditional masonry using lime mortar based on lime putty or natural hydraulic lime (NHL) offers many advantages over cement based mortars.
Lime mortars for lime pointing are generally softer and more porous compared to mixes using cement therefore allowing moisture to evaporate from the joints more freely. This can help to lower moisture levels in the wall and reduce the build up of soluble salts in the stone face, therefore reducing potential damage or ‘spalling’ of the masonry.
As with all lime based materials the best outcome requires patience and careful control of drying and suction.
Our video and guidance notes in this section will help you to achieve aesthetically pleasing and long lasting lime pointing.
Any existing defective pointing must be raked out to a depth usually equal to twice the width of the joint, but not less than 20 mm. The back of the joint should be roughly square in profile. The use of a plugging chisel ensures that the stone or bricks aren’t forced apart.
Damping.
After brushing out any loose material the joints must be dampened, with enough time left for the stone or brick faces to dry to prevent smearing. The mortar should be as dry as it is practicable to point with. This allows maximum compaction, reduces shrinkage and reduces the tendency to smear on the face of the masonry.
Premixing.
Traditional lime putty based lime mortar benefits from being pre-mixed for a minimum of a couple of weeks and then “knocked up” (agitated/mixed) again prior to use to plasticise them – this reduces shrinkage in the mortar. To save waiting, we supply premixed lime mortar which can be used immediately.
For NHL based mortars it can help to mix the mortar and let it rest for an hour or two before knocking up prior to use.
Gauging.
We suggest our pre-mixed lime mortar for internal and external lime pointing applications.
In damp, frost prone or very exposed situations it may be appropriate to add a pozzolan to a lime putty mortar to increase its compressive strength and frost resistance. The pozzolan will slowly begin to add a little extra compressive strength to the mortar after a few days.
If NHL mortar is to be used the appropriate strength will depend on the substrate and degree of exposure.
Start at the top of a wall to avoid raking out over finished work. Use a small tool or pointing spatula and force the mortar in from a hawk. Joints deeper than 20 mm will need an initial dubbing out as shrinkage can occur otherwise.
Finish flush or rebate a little if the joints have widened with age or for personal preference and aesthetics. Weather struck joints rebate under the stone above and are flush to the edge of the stone below the joint.
Brushing.
When the lime pointing is “green hard” (firm enough to brush without smearing but still malleable enough to work), brush or tamp the joints with a churn brush to remove the patina of lime on the surface of the mortar. This enhances the appearance by exposing the coloured aggregate in the mortar.
Protection.
Hessian cloth is recommended for protecting new lime pointing. Hessian protects from direct sunlight, rain and wind, and from frost if essential work is carried out during the winter.
Time of year.
Please note that the application of lime mortars should be avoided during the colder months for risk of frost damage.
For pure lime putty mortars the hardening process through carbonation takes up to a month for each mm of thickness. Therefore it may take 20 months before a lime putty mortar has carbonated to a depth of 20 mm. If a pozzolan has been added, it will provide an initial hardening after a few days and will have fully reacted after about a month.
NHL based mortars will cure more quickly due to their hydraulic set, the speed will vary depending on the strength of the NHL chosen.
Exposed elevations and chimney stacks.
Letting rising moisture wick away to the outside is one of the important functions of lime pointing. On exposed locations it’s important to recognise that there is a balance between letting rising moisture wick away and avoiding excessive rain ingress through more porous lime mortars.
Rain can easily penetrate some types of masonry and a porous lime mortar may add to this problem. In this instance you may consider adding a little (at least 4% by volume) raw or double boiled linseed oil to the lime mortar to improve water repellency.
If the composition or thickness of the wall means that rain ingress is still a problem, there are vapour permeable water repellents available that can help.
Windows and Door Frames.
See our separate guide to mortars based on mastic sand and double boiled linseed oil
Our advice and information are given in good faith. It’s important that users satisfy themselves that they’ve chosen an appropriate product and have a suitably skilled workforce.
20 kg of Lime Mortar will point 2-3 m² of average sized stonework or 1- 1.5 m² of brickwork based on a 10 mm joint and 20 mm depth
Safety
Limes are caustic. Always wear eye protection and protective gloves and clothing and follow the safety instructions on the labels.
