Asbestos siding explained

5-10% of the homes I inspect have older cement asbestos siding. Though it can be brittle, it is generally quite durable, offers some fire protection and can be painted.

broken cement asbestos siding
Cement Asbestos siding needing repair


The question that arises is: Is this kind of siding safe or is it a health hazard? What can be done to repair the siding? Should I have it replaced?Is there a way to safely dispose of it?


The following article from InterNachi is a useful tool for home buyers who are facing these questions:

Asbestos Cement-Based Siding

Asbestos and cement were first combined in the United States in the early 1900s to form an innovative, new building material.  Asbestos cement is a composite material that consists of cement reinforced with asbestos fibers.  Asbestos-cement siding shingles can be made to imitate the appearance of wood siding shingles in shape and appearance. 
Asbestos fibers are a health hazard when inhaled.  Asbestosis is a form of lung cancer that is caused by inhaling asbestos fibers.  Because of the health risk, strict environmental regulations for working with asbestos were established in the U.S.  Health risks were shown to be greatest during mining and production processes, but minimal during the installation and use of asbestos-cement products.

According to the U.S. EPA, a material containing asbestos is deemed potentially hazardous only in its friable state, which is when the material can be crumbled, pulverized, or reduced to a powder by hand pressure.  Asbestos cement is not considered friable and, therefore, not hazardous because the cement binds the asbestos fibers and prevents their release into the air under normal-use conditions.  However, asbestos-cement products are classified as friable when deterioration disturbs the asbestos.  Asbestos-cement products are classified as friable when mechanical means are used for chipping, grinding, sawing or sanding, thereby allowing particles to become airborne.

If the asbestos-cement siding material is not disturbed, no hazard exists and no precautions are required.  It is highly recommended that periodic inspections be conducted.  Be sure to advise your clients about the need for this.

Maintain and Manage

Maintenance of asbestos-cement siding material includes performing visual inspections to evaluate its condition, keeping the siding clean, and making minor repairs, as necessary.  It is important to maintain the environment around the house and to protect the asbestos-cement siding materials. Asbestos cement is brittle.  It has little resistance to impact.  The siding is susceptible to cracking and chipping.  To protect the asbestos-cement siding material, one could plant small shrubs or flower beds between the bottom of the wall and the grass lawn.  This landscaping feature will serve to protect the siding from lawnmower damage.  A piece of trim detail could be added to the bottom of the asbestos-cement siding to reduce its vulnerability to cracking and chipping.  Trimming the branches from nearby trees and bushes will protect the siding from damage.


When repair to asbestos-cement siding is needed, the least amount of siding should be discarded and the greatest possible amount of original material should be retained.


If you see hairline cracks in the asbestos-cement siding, clear epoxy can be used in the cracks.  Epoxy is susceptible to UV radiation from the sun and may need periodic maintenance.  A grout of cement and water can be used to repair slightly larger cracks.  To repair cracks greater than 1/8-inch, a thick grout with sand added to the mix can be used.


If the fasteners for the asbestos-cement product have become deteriorated or have broken due to corrosion, they should be replaced with a more durable metal.  Stainless steel is generally recommended because of its superior corrosion resistance.  Fasteners, such as nails, should be long enough to hold the materials securely.


Discoloration can be caused by surface contamination.  Stains can result from a leaching of other material, such as corrosion runoff.  A change in color can be caused directly by the environment, such as UV radiation from the sun.  Discoloration could be normal, but it may indicate a chemical reaction, decreasing the durability of the material.

Cleaning involves the use of solutions of varying strengths while using the gentlest physical means possible, without causing adverse conditions to the material.  Mechanical methods for cleaning can promote asbestos fibers to become airborne and, therefore, should be used only by following strict asbestos regulations.

If discoloration or staining cannot be removed, the asbestos-cement siding could be painted, but this adds an additional maintenance factor.


Efflorescence appears on many cement products that are exposed to weathering.  This form of crystalline growth indicates that water is passing through the material, which can promote deterioration of the asbestos cement.  Efflorescence is usually observed at the beginning of the material’s life.

Biological Growth

Biological growth on the exterior of asbestos cement can be a problem in sheltered environments and on northern exposures.  Shade trees located close to a building can shield sunlight and result in prolonged dampness of the asbestos-cement building product, promoting biological formations, such as moss and algae.  These growths can stimulate surface deterioration and staining.


Asbestos-cement siding material commonly deteriorates by cracking and chipping.  Repairs are not usually performed on cracked or chipped pieces.  Replacement is usually recommended.  The replacement piece should be of a non-asbestos, fiber-cement type.

Replacing several pieces of asbestos-cement siding is easy to do because it has been  manufactured in standard sizes, shapes, colors and textures.  There are siding materials that have been manufactured to replicate asbestos-cement siding pieces.  There are non-asbestos reinforced cement, fiberboard with asphalt, fiberglass, metal and vinyl available, too.

Inspection Tips for Asbestos-Cement Siding

Where siding is damaged, moisture can enter the building.  Look for cracked or mechanically damaged pieces of siding.  Mechanical damage can be caused by balls, stones, ladders or children.  Be sure to check along the bottom edge and corners of the siding.

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