The pointing of stone can significantly affect both the appearance and weathering of the stone itself.
Mortars in stone walling are traditionally composed of lime, sand and stone dust in varying proportions. The vapour permeability of this traditional mortar mix is the key characteristic. It is often claimed that lime based mortars are “too soft”.
Repointing should only be carried out when the existing pointing has failed and where water penetration is taking place. Where repointing is necessary, it is vital that the correct style of pointing is used as well as the appropriate materials. The use of inappropriate joint finishes and cement mortars can seriously alter the appearance of a historic building, will detract from its character, and can result in accelerated decay of the stone over time.
When repointing, decayed mortar should be raked out using appropriate hand tools. The use of mechanical equipment such as angle grinders should be avoided at all times as they are difficult to control and can damage the stone arises and widen joints. Mortar that proves resistant to hand tools should be regarded as sound and should be left in-situ.
Joints should be raked out to the full width of the joint and should be square in profile. The depth will depend on the friability of the pointing but should be at least twice the width of the joint.
Careful consideration needs to be given to the style of joint finish. Raised or weatherstruck finishes should never be used as they spoil the appearance of stonework. Instead joints should be either flush with the stonework or be slightly recessed where the arises are weathered back or rounded. When repointing rubble stone walls, small stones known as pinnings should be inserted into any wide joints to reduce the amount of mortar needed and the apparent size of the joint, and the mortar should be contoured to follow the contours of the stone.
Particular care needs to be taken when repointing ashlar work where the joints are very fine. Loose mortar should be raked out using a hacksaw blade. New mortar should never be coated over the surrounding stonework, and very fine jointing tools are required to maintain a precise finish. It is recommended that masking –tape is placed over the surrounding stonework to protect it from mortar staining. In some cases it may be necessary to insert the new mortar with the aid of a syringe or mastic gun.