Before deciding to re-point a basic understanding of the function of mortars is required. Mortar is used for jointing individual units in a mass of masonry. The final structure must have certain characteristics to function satisfactorily. It must carry the load for which it was designed, it must be durable and it must give protection against wind, rain and frost. See Protecting your work,
Mortar should develop sufficient strength and at such a rate as to be capable of withstanding the stresses to which it will be subjected during the construction of the building and subsequently when the structure is fully loaded. It should not however set and harden so quickly that it becomes inflexible at any stage and cannot accommodate slight movement. There is normally no requirement for significant structural strength in the mortar of traditional masonry buildings, particularly in re-pointing work.
Mortar should be permeable in itself, both so that the quantity of free water on the face of the building is reduced, thus reducing the possibility of wind-driven water penetration and so that moisture evaporation is not concentrated in the masonry, which may then be vulnerable to accelerated breakdown in the vicinity of the joints. Mortar should bond firmly to the units so that a tight joint is obtained through which rain will find it difficult to penetrate.
Mortar should be workable, so that the material may be applied easily and to ensure that the vertical as well as the horizontal joints can be adequately filled. Masonry buildings rely on their mass and the interlocking of individual units for their stability and the mortar in a masonry building serves in the main to provide a bedding medium for often very irregular components, filling the voids and maintaining the wind and watertight integrity of the building.