Components

• Chimney mortar is a mixture of water, sand, portland cement and hydrated lime. The water holds the ingredients together and provides the hydration that sets the mortar. The sand gives the mortar volume and aeration for strength. The cement provides the chimney with compressive strength and the addition of lime gives the adhesive compound flexibility so it is less likely to crack due to shrinkage, compression and stress.
  
  

Mixture Ratio and Classification

• Mortar is classified by its application, as different mixtures require various levels of strength and resistance to elements. Type N, chimney mortar, has a mid-level compressive strength of 750 psi (pounds per square inch) that is achieved through mixing equal parts of portland cement and hydrated lime with six parts sand, with the water amount variable depending on the desired consistency. The type N mortar formula is best for above-grade walls regularly exposed to inclement weather.
  
  

Pointing and Repointing

• Pointing is the term for applying mortar to a new chimney. Tuck pointing is a cosmetic process by which two colors of mortar are mixed to make the mortar joints appear finer than they are. Repointing is the repair and restabilization of chimneys with old or damaged mortar that has rendered them unstable, leaky or moldy. Minor chimney damage can be repaired by homeowners, but if the chimney has deteriorated to the point of leaning, sagging or is missing bricks or stones, hire a masonry contractor who specializes in chimney restoration. Chimneys with major problems may have to be torn out and rebuilt.
  
  

Preventive Measures

• To avoid the need for chimney repointing, have the brick or stone and mortar regularly inspected for hairline cracks and seal them with a water-repellent solution specially formulated for chimneys. Repair or replace damaged chimney crowns and install a chimney cap to reduce the water exposure of the chimney. Check the efficiency of the flashing, the material between the bottom of the chimney and the rooftop, and replace it if it is poorly sloped or has tears in it that expose the chimney to excessive climatic moisture.