Methods for repointing brick and stone walls
Rubble Stone Masonry
The process can be considered as 4 stages and it is important that sample panels are executed prior to commencement.
Raking out and removal of old or inappropriate mortar to a minimum depth of 25-40 mm or until sound mortar. Depth of the joint is relative to the width between stones. As a guide, joints should be raked out to twice the width of the joint – so a 1cm wide joint should be raked out to at least 2 cm, preferably 2.5 cm. The exposed joints are then cleaned with water or compressed air. The joint and masonry should then be moistened to ensure water in the mortar is not drawn into the surrounding masonry causing the pointing to dry out too fast.
Filling of voids and tamping out to provide uniform depth prior to pointing, where necessary. Pinnings can be used to reduce volume of mortar, this is sometimes a decorative feature of a historic mortar and should be reproduced. For pure lime work mortar joints should not be filled more than 15mm at a time and the joints allowed to firm up before the next application. Once complete the joint and stonework should be moistened again as above.
Pointing using previously described mortars. Care needs to be taken to avoid ‘laitence’ or the creation of a hard outer surface by overworking the pointing and drawing lime water to the face. After initial set the pointing should be finished of by scraping/brushing to achieve desired appearance and remove residual laitence.
Protection of the joint and monitoring for a minimum seven day period. Remoistening by spray after 24 hours and working out any hairline cracks. For pure lime work protection is essential to avoid damaging the material during the period it is curing (or hardening) in air and dampened sacks hung in front of a wall are a common solution in hot weather. Pure lime mortars are also vulnerable to frost for at least three months after application. This should be taken into consideration when programming such
work. Work must start at the top of the wall and the activities can normally proceed in an orderly sequence downwards. All joints should be pre-wetted and well filled with the mortar rammed solid and left partially set. (This is usually the following day, however, climatic conditions, mortar type and exposure can vary this). The mortar after initial set is scraped or brushed back to the arrises of the stones creating a slightly ‘hungry’ joint. This joint can then be washed using low pressure water in order to rinse the brush and remove the debris and expose the aggregate, creating a naturally weathered
appearance. Care needs to be taken that water is directed out of the joint. On most historic buildings a brush finish is appropriate. On some buildings there may be a different historic style such as tuck pointing where a white line of mortar is inserted into a mortar coloured to match surrounding brick to create a more regular appearance. Specific historic styles of pointing should be followed where encountered.
Ashlar stonework is inevitably finely jointed and repointing requires a fine mortar without grit aggregate. The joints are often so narrow that repointing is unnecessary and any attempt to repoint usually results in some form of inappropriate strap pointing on the surface of the stone, doing consequent damage to the stone. If repointing is necessary, care must be taken to avoid damage and the joints raked out to a 10-12 mm depth and fully packed with mortar using a variety of fine edge pointing irons or tools with the joint being left slightly recessed from the face.
Only those joints where the absence or failure of mortar is adversly affecting stones or walls or where strong sound mortar is causing decay or is visually disruptive should be repointed.
Beds where work is agreed should be raked out using hacksaw blades or other similar instruments, the joints should not be widened or the stone damaged as a result of this process and on no account should mechanical disc cutters be permitted on fine ashlar work. The joint should be raked out to a depth of 10-12 mm.
Twisted wax string should be inserted along the joint by the hacksaw blade at the 10-12 mm depth.
Masking tape should then be placed along each side of the joint to avoid discolouration by action of the lime mortar. The joint should be thoroughly rewetted prior to application of the new pointing material.
The mortar should be applied to the joint and pushed in to meet the string backing using a suitably thin implement. Care should be taken that the mortar finishes flush with the ashlar. New mortar should be based on the existing but it is common to find a very rich lime mortar used either pure lime or with a fine aggregate such as 1:1½ lime to white sand (silver silica) or crushed portland stone. Linseed oil was sometimes added historically.
After pointing the stone should be cleaned down and all traces of masking tape removed.
The joint should be rewetted, kept damp, and protected for at least seven days as normal for pure lime work. Any fine hair cracks noted should be worked over during this period.