Using mortars based on lime putty or natural hydraulic limes (NHL) for lime pointing offer many advantages over cement based mortars. They are generally a slightly softer, more porous material allowing moisture to evaporate from the joints in preference to the stone or brick. This should help to lower moisture levels in the wall and reduce the build up of soluble salts in the stone face therefore reducing damage or ‘spalling’ of the stone/brick.
As with all lime based materials the best outcome requires patience and careful control of drying and suction, the reward being a good looking and long lasting lime pointing mortar.
Preparation.
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. Plugging chisels ensure that the stone or bricks aren’t forced apart.
Damping.
After brushing out any loose material or dust, 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 in the joint, reduces shrinkage cracking and reduces the tendency to smear on the stone or brick faces.
Premixing.
Lime putty mortars benefit 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 for a couple of weeks to use your pointing mix, we supply a pre-mixed lime mortar which can be used straight away.
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 putty 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. It won’t give an overnight set but will slowly begin to add a little extra compressive strength to the mortar after a few days.
If an NHL mortar is to be used the appropriate strength will depend on the substrate and the degree of exposure.
Pointing.
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 as rebating highlights the stone more. A weather struck joint is rebated under the stone above the joint and brought 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 the protecting new lime pointing. At hotter times of the year it protects from direct sunlight, rain and wind, and in winter protects from rain and frost.
Quantities.
20 kg of lime putty 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
Time of year.
Please note that lime mortars should not be applied too late in the year or too soon in spring or else frost damage may occur. It is very important to prevent frost crystals forming within the mortar soon after application.
For pure lime putty mortars the ultimate hardening process of 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, this 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 stone and most bricks and a porous lime mortar can add to this problem. We recommend adding a little (at least 4% by volume) linseed oil (either raw or double boiled) to the lime mortar to improve water repellency whilst still retaining some breathability.
If the composition or thickness of the wall means that rain ingress is still a problem, then there are vapour permeable water repellents available that can help to reduce rain ingress. This is of course a cheaper option than lime rendering the stone or brickwork.
Windows and Door Frames.
See our separate guide to mortars based on mastic sand and double build 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.
Safety:
Limes are caustic. Always wear eye protection and protective gloves and clothing and follow the safety instructions on the labels.
Please contact us full more guidance and information.