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Pointing Of Joints: Design – Tips – Technical Bulletin #44

Always rake and point mortar joints rather than full-bed setting and finishing in one operation. Mortar joints are best suited for masonry-bound trim items such as belt courses, lintels, window surrounds, date stones, inscription blocks, quoins, keystones and similar applications.

Not all joints between stones or between stone and other material should be filled with mortar. All head joints at coping stones and joints at column covers, cornices, platforms, soffits, window sills and in general, all stone sections with projecting profiles, exposed top joints or rigid suspension connections to the supporting structure should be “soft” sealant joints. See Technical Bulletin #43 on Sealants.

Pointing is required because mortar shrinks and settles as it cures. Since mortar beds harden from the face in, stresses can be applied to the edge of the stone which can cause spalling later. Shrinkage also can create cracks at the joints; a condition which causes leaking.

Regardless of whether the mortar or sealant is selected as the face joint material, the mortar must be raked out of the joint to a minimum depth of 3/4″. If sealant is to be used at the head joints, then mortar should not be used there at all.

Pointing is usually done in 1 or 2 stages to allow maximum sealing of shrinkage cracking in the mortar. It should not be done in areas exposed to hot sunshine and it is suggested that pointing be accomplished after patching of Cast Stone and before final wash-down.

Apply pointing mortar using proper tools to compress the material against the edges of the stone. A concave joint is recommended for the best protection against leakage although other joint types are often available in the stone setting trade.

Pointing mortar should be softer than the stone so that thermal stress will not cause spalling at the edges of the joints. It is usually slightly drier than normal setting mortar consistency to prevent shrinkage and is usually composed of the following:

1 part Portland cement, ASTM C150
1 part hydrated lime, ASTM C207
6 parts masonry sand, ASTM C144

Coloring may be added to achieve almost any hue, however pointing mortar which sharply contrast the color of the stone may cause staining. Excess pointing material must be sponged away from the face of the stone immediately. Colors added must be natural or synthetic mineral oxides which meet the requirements of ASTM C979 (sun-fast, lime-proof, alkali-resistant) and the dosage must not exceed 10% of the weight of the cement used. Carbon black or ultramarine blue pigments should not be used. In general, pigmentation types and amounts used in the manufacture of Cast Stone can also be used as a starting point when custom blending the pointing mortar to match or complement the color of the Cast Stone.

Always specify a mock-up wall when approving final colors and be sure that it has been properly cleaned because cleaning will usually affect the color of pigmented masonry materials.

This Technical Bulletin is provided by the Cast Stone Institute, and is intended for guidance only. Specific details should be obtained from the manufacturer or supplier of the Cast Stone units.


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