About Cement Asbestos Siding

  • During the 1940s construction boom, a wider variety of colors became available, including a spackled look, where the colors were impregnated throughout the sheet so they would not powder or peel off. When color change became desirable after installation, owners were encouraged to paint the asbestos-cement products. It was the “attractive home safeguarded with modern asbestos siding, fireproof, rot proof, termite proof” idea that intrigued many Americans during this era. By 1950, approximately one billion square feet of asbestos-cement products had been produced for use in the building industry.
  • By the time the Environmental Protection Agency (EPA) was established in 1970, the commercial world of asbestos-cement products had expanded into many markets. Annual use of asbestos-cement in the United States continued to climb for another three years before reaching the peak of its popularity, only to plummet to a quick death in 1973 when the EPA implemented the initial ban on asbestos. Asbestos-cement products are still produced in a few countries outside of the United States and are considered a beneficial resource; however now they carry the label “hazardous material” and not “miracle mineral” as they once did.
  • Asbestos-cement products were developed in an era of ingenuity for creating easy to install and economic building materials. Although asbestos-cement has acquired a poor reputation by association of its title, it has not gained that reputation through a lack of durability or utility. In order to preserve this twentieth-century material, understanding what makes, or does not make, asbestos a hazard is truly important. In this case, no hazard is created when asbestos-cement building materials are sound and left in place, or when treatments incorporate non-abrasive means.

Asbestos Regulations

  • Some asbestos fibers, when inhaled, constitute a health hazard leading to asbestosis, a form of lung cancer. These health risks prompted the establishment of strict environmental regulations on working with asbestos.
  • Health risks were shown to be greatest during mining and production processes, but minimal during installation and use of asbestos-cement products.
  • According to the EPA, a material containing asbestos is deemed potentially hazardous only in a friable state, which means when it 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 severe deterioration disturbs the asbestos or mechanical means are used for chipping, grinding, sawing, or sanding, therefore allowing particles to become airborne.


  • Due to the abundance of buildings clad in asbestos-cement products, and the low health risk of the non-friable material, it is necessary to know how to preserve and rejuvenate the material back to a vibrant and usable life.
  • The primary conservation options for asbestos-cement building materials are to maintain and manage in place, repair, replace in part, or abate.
  • The level of deterioration determines the appropriate option to be employed. Abatement, including full removal or encapsulation, should only be used as a final course of action.

Maintain and Manage

  • Deterioration of asbestos-cement is inevitable, as is eventually the case with most all materials. Maintenance procedures can decelerate deterioration, such as conducting visual inspections to evaluate condition, keeping the material clean, making minor repairs as necessary, and organizing treatment practices that minimize the extent and impact on the material.
  • Also, it is important to maintain the environment that surrounds the structure and protect the asbestos-cement materials.
  • Examples of protective measures include the planting of shrubs or flower beds between the skirt of the wall and lawns to protect from lawnmower damage, adding a bumper material to the bottom row of siding to reduce vulnerability to cracking and chipping, and keeping branches and debris away from the roof and out of gutters.


  • When repair is the necessary treatment for a deteriorated asbestos-cement product, the least amount of material should be discarded and the most possible amount of original material should be retained.
  • The type and extent of material deterioration associated with asbestos-cement products depends on the cause of the distress and therefore requires investigation and the tailoring of a solution for each case.
  • When repairing the deteriorated materials the gentlest means possible should be used following applicable asbestos regulations, along with repair techniques sympathetic to the existing fabric.
  • Asbestos-cement is inherently a brittle material with low impact resistance, so even with the added reinforcing given by the long thin asbestos fibers, the material is susceptible to cracking and chipping as generally induced by low impact forces, repeated cyclical loadings, or deteriorated fasteners. In addition to this primary deterioration tendency, manufactured asbestos-cement products can potentially discolor, erode, spall, flake, form efflorescence, and create an environment for biological growth.
  • Guidance for repairing asbestos-cement products is given here for these several distress manifestations.


  • If a crack forms from either impact or fatigue and mandates repair, several techniques can be used depending on the size of the crack.
  • For hairline cracks, work clear epoxy into cracks with a thin object. Epoxy can be susceptible to UV attack and may need to be restored periodically by removing and replacing.
  • For slightly larger gaps, use a grout of Portland cement and water, mixed to a flowing consistency, and tinted to match.
  • For cracks greater than an eighth of an inch, use a thicker grout consistency or add sand to the mix. The crack may need to be widened to rake out the loose material. Soak the crack with water, then trowel patch the grout into it. Keep the repair damp for a week to promote slow and proper curing and reduce shrinkage.
  • If the fasteners for the asbestos-cement product have become deteriorated or have broken from corrosion, they should be replaced with a more durable metal. Various metals can be considered for the replacement, but should be compatible with the sheathing. Stainless steel is generally recommended because of its superior corrosion resistance. Fasteners such as nails should be long enough to hold the materials securely (self-clinching nails can help with this).


  • Discoloration of asbestos-cement products stems from a build-up of surface contamination (such as soiling), stains produced by leaching of other material byproducts (such as corrosion run-off), or a direct change in color due to the environment (such as ultra-violet sun bleaching). These discoloration occurrences typically result from normal weathering, but indicate a chemical reaction that may decrease the strength or durability of the material when neglected over time.
  • Discoloration should be removed from the asbestos-cement products, and cleaning recommendations generally suggest trying several solutions of varying strengths. After evaluating the results of the trials select the alternative that provides the needed results while using the gentlest means possible without causing adverse reactions to the substrate.
  • Mechanical methods for cleaning can promote asbestos fibers to become airborne, therefore should only be used following asbestos regulations.
  • To clean light stains, such as dirt, the asbestos-cement products should be washed with a detergent solution or a mixture of one half cup of trisodium phosphate dissolved in a gallon of hot water. Rinsing with plenty of clear water helps to remove all trace of the cleaning solution. Start the cleaning at the bottom of the wall, working upward in small sections, rinsing immediately, and keeping the shingles below wet, otherwise, dirty water can drip down over dry surfaces and leave streaks almost impossible to remove later.
  • Recommendations for stains such as rust, are to dissolve one part of sodium citrate in six parts of commercial glycerin. Mix part of this with inert dry clay, such as diatomaceous earth, to form a poultice and apply as a thick layer. When the paste is dry, replace with fresh paste or moisten with the remaining liquid. Complete removal of the stains may require a week or longer. A ten percent oxalic acid solution has also been found to successfully remove rust from cementious products.
  • If the substrate, metal fixtures, or other adjacent objects are causing staining they should be cleaned and coated or replaced.
  • If the stain cannot be removed, another option is painting the asbestos-cement products. Painting is an especially good solution if the material was originally painted, but adds a maintenance factor. Oil based paints and varnishes are not chemically compatible with cementious materials. High quality alkali-resistant and weather resistant exterior paint (i.e., 100% acrylic coating) should be used on exterior asbestos-cement materials, or use pigmented shingle stain.
  • Before being painted, asbestos-cement surfaces should be cleaned, then primed with an alkali-resistant primer.

Eroding, Spalling, or Flaking

  • Erosion removes cement particles and can result in the release of asbestos fibers, leaving the material with less reinforcement. Due to the high density, low permeability, and low porosity of the material, this tendency is virtually unnoticeable. However, erosion can become a more serious problem under regular and extremely harsh weather conditions. If intense erosion occurs, the durability of the material can be compromised.
  • Although rare, spalling or flaking occurs when elements permeate beneath the surface of the asbestos-cement material and then expand, causing a portion of the material to be released due to the resulting stress. As the moisture content increases, more severe deterioration can occur. This deterioration is more likely to occur in products that were cured at lower temperatures and therefore are more vulnerable to water penetration.
  • To control eroding, spalling, or flaking, chemical consolidants and/or breathable sealers (most commonly silane) can be applied to strengthen the material while adding water protection. Testing is critical since consolidants and sealers can promote spalling if water is getting in through the backside of the material.
  • A grout or latex-patch may also be considered, but must be compatible with, and typically softer than, the asbestos-cement material to form a good bond and not promote increased spalling. This repair procedure can be tricky and may lead to constant patching, and may be unsightly if not done with extreme care. For these types of deterioration tendencies, the material may be better off left alone or partially replaced.


  • Efflorescence appears on many Portland 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, in addition to making it unsightly. Generally this is seen at the beginning of the material’s life, where rain and weathering tend to remove it over time.
  • To clean efflorescence deposits, the surface should first be dry brushed with soft bristles, not scratching the surface. If efflorescence still remains, test to see if it is water soluble or acid soluble. If water soluble, the wall should be wiped with a wet sponge or brush (a light detergent can also be added). A hose can be used, but spray the water in a downward direction as perpendicular force will drive the efflorescence back into the material. If acid soluble, clear ‘white’ vinegar, acetic acid, phosphoric acid, or similar proprietary products diluted in water should be used. It is recommended to wet the surface with solution, then apply solution more liberally on the asbestos-cement. After two or three minutes, scrub using a fiber brush with more solution, then rinse extremely well with clear water.
  • Safety precautions provided on the product labels should be followed, and again tested before commencing extensive application as adverse effects or discoloration may occur. Pitting from chemicals will increase dirt buildup and water permeability, decreasing the durability of the material.

Biological Growth

  • Biological growth on the exterior of asbestos-cement can be a problem in sheltered environments or on northern exposures. Shade trees located close to a building can shield sunlight and result in prolonged dampness of the asbestos-cement building product and promote biological formations, such as moss and algae. Not only are the growths unsightly, but they can stimulate surface disintegration, dissolution, and staining.
  • The presence of moss and other fungi growth signals that the moisture content of the material is high and therefore an attack by a more damaging biological species could occur. It is not only important to remove the growth from the asbestos-cement material, but also to remove the environment that is causing the growth. To eliminate biological growth, a strong mixture of weed killer and water could be tested. If unsuccessful, a solution of four parts bleach, one part trisodium phosphate, and twelve parts warm water is recommended. After a week or so when the moss has turned brown and dry, it should be brushed off. In the case of ivy this technique is sometimes not helpful in removing the thousands of tiny roots left after the ivy has been pulled off; a stronger product may be needed (i.e., copper sulfate). It is important to remember that biological growths differ widely and so do the processes for their removal. Testing various products and selecting appropriately is highly encouraged.


  • Since asbestos-cement products were manufactured in standard sizes, shapes, colors, and textures, partial replacement is well suited for implementation. This process is acceptable when part of, or pieces of, the existing asbestos-cement building material have deteriorated to such a degree that it is much more feasible to replace than repair. Since the United States no longer produces asbestos-cement products, an alternative material should be selected to match the original. Some materials that have been manufactured to replicate asbestos-cement building components are non-asbestos reinforced cement, fiberboard with asphalt, fiberglass, metal, and vinyl. For the purposes of preservation, one of the non-asbestos reinforced cement products is most appropriate.
  • Many varieties of non-asbestos reinforced cement or fiber-cement are currently available. Fibers that have been introduced with cement include: steel, glass, polypropylene, wood (these four being the most common), acrylic, Akwara, alumina, carbon, cellulose, coconut, Kevlar, nylon, perlon, polyethylene, rock wool, and sisal. Combinations of fibers are currently undergoing research in order to get properties closely matching those of asbestos. Several companies manufacture products that replicate asbestos-cement roofing and siding shingles, flat sheets, and corrugated sheets. Some of these manufacturers include: Supradur Manufacturing Company, Cement Board Fabricators, U.S. Architectural Products, Inc., Re-Con Building Products, and GAF Materials Corp. The fiber-cement products replicate the size, shape, thickness, and structure, along with texture and color of many of the asbestos-cement products previously available. Where color matching is not found, an alternative is to replace in size and shape then paint over the entire structure for a uniform appearance. In addition, the hardware and the installation procedures for these products are similar to those for asbestos-cement products due to their similar characteristics and proportions.