Cracks are among the most common defects found in stucco finishes. They may be merely hair cracks or they may be large enough to admit moisture which may damage the underlying structure and interior walls.
Hair cracks may develop if a stucco mixture is too rich or if the stucco material is inferior. They may also be caused by too rapid drying of the stucco. Large cracks are usually the result of settlement of the walls of a house or movement within the walls caused by improperly constructed foundations or poorly designed framing in the superstructure.
Stucco over brick, stone, or similar materials is liable to crack, especially around chimneys, because the stucco has a different rate of expansion and contraction than the material that it covers, and a shearing stress or “crawling” effect takes place in the plane of contact between the two materials.
If cracks are unsightly and large enough to admit moisture, they should be repaired. If they are not noticeable and seem to be doing no damage, repairs may be postponed, since the plastered cracks are liable to look worse than the open cracks. The following tools and materials are needed: A hammer, a sharp-edged and pointed instrument (such as a cold chisel or screw driver), a wire brush or whisk broom, a mixing board, a mason’s trowel, and a water bucket; Portland cement, clean sharp sand, hydrated lime, water, and mineral pigments for color if necessary.
Before pointing, clean out the cracks thoroughly and chip them out to the shape of an inverted V in order to key the mortar securely to the old work. The cracks should be brushed to remove all dust and loose particles, and the cleaned surface and adjoining stucco dampened before new mortar is applied, to prevent the water in the mixture from being absorbed.
In pointing, it is desirable to use the same brand of cement and the same mix proportions as the original work. If the previous mixture cannot be determined, it is usually safe to use a 1 to 3 mixture, containing 1 part cement, 3 parts sand, and one-tenth part finely divided materials, such as hydrated lime, measured by volume.
The mortar should contain just enough water to make a fairly dry mixture of about the consistency of putty. It should be applied like a calking material; that is, rammed and tamped in well to make complete contact and form a secure bond. If the cracks show up badly after pointing work is finished, it may be necessary to paint the entire surface with a cement-water paint. The new work should be kept wet for several days after it has hardened to increase the strength of the mortar. It is advisable to hang a tarpaulin or similar covering over the completed work to protect it from direct exposure to the sun and drying winds.