It's less of a concern today, with so many types of floating (unattached) flooring available. But at one time, all resilient flooring was glued to the subfloor. While today you still can use adhesive-only flooring for a kitchen or bathroom—and it has its advantages—floating flooring and self-adhesive flooring are most common.
So, removing old linoleum or vinyl flooring may come as a surprise to some homeowners when they see smears of some type of black adhesive between the tiles and the subfloor. This is often called black mastic and it often contains asbestos.
What Is Black Mastic Asbestos?
Common in homes built in the 20th century, black mastic was used as an adhesive for ceramic tile, linoleum, and other resilient flooring materials. Mastic is a term that has largely become outdated, but you may still find it on old cans around the house.
The main concern is that some (but not all) black mastics may contain asbestos. Asbestos has been proven to cause a life-threatening condition called mesothelioma. So before you do anything with black mastic in your home, it is important to identify it and deal with it appropriately.
Mastic is a general term for a type of glue-like flooring adhesive. Many modern mastics are latex, or water-based, and can be softened with water. By contrast, asphaltic cutback adhesive is an older type of mastic made with asphalt-based cement.
Cutback is a term that refers to floor adhesives that are derived from asphalt, are not water-sensitive, and will not soften in water.
Some cutback adhesives contained asbestos. It is commonly thought that asbestos was added to these compounds for fire-resistance. While this would have a residual effect, the primary purpose of the asbestos was to make the product more durable. Asbestos is a highly fibrous material, and these fibers interlock to help strengthen mastic, insulation, and many other household building products.
What Does Black Mastic Asbestos Look Like?
You will only see black mastic asbestos after a floor covering has been removed. As the name suggests, black is its predominant color. But there may be traces of other colors, such as those from subflooring showing through the mastic or pieces of floor covering that were not completely removed.
Note that some older latex (water-based) adhesives also were black, so color alone is not a reliable identification characteristic.
In most cases, black mastic lies flat against the subfloor, with no ridges, bumps, or gaps of any significance. It often has faint embedded comb-like marks or swirls from its initial application.
No asbestos fibers in the mastic will be visible to the bare eye. While cutback mastic is not affected by water and many cleaners, if it is vigorously sanded (not recommended, due to the asbestos risk), it would become thick and tar-like from the heat.
How to Know If You Have Black Mastic Asbestos
Containing between 15- and 85-percent asbestos, black mastic adhesives were manufactured mostly in the first half of the 20th century. However, some companies produced asbestos adhesives as late as 1986. Therefore, if your house was built or remodeled around 1986 or before, there is a chance that black mastic adhesive on your floor may contain asbestos.
Short of testing the suspected material, one of the surest methods of determining whether it has asbestos is to find an old can. In many old houses, you'll find old cans of paint in attics, basements, under stairways, in outbuildings, or discarded in the yard.
Manufacturers of Black Mastic Asbestos
- 3M (Minnesota Mining and Manufacturing Company)
- American Biltrite
- Amtico Floors
- A.P. Green Industries
- Armstrong World Industries
- Asbestos Corporation, Ltd.
- Celotex Corporation
- Crown Cork and Seal
- Congoleum Corporation
- GAF Corporation
- Garlock Packing Company
- Johns Manville
- Mobile Oil Corporation
- National Gypsum Company
Prominent brands of asbestos-laced floor adhesive include:
- Armstrong S-89 Adhesive
- Armstrong S-90 Adhesive
- Atlas Stove & Furnace Cement
- Carey Fibrous Adhesive
- Empire Ace Fibrous Adhesives
- J-M Fibrous Adhesive Cement
- Crown Coat Cement
- Gold Bond Laminating Adhesive
Is Black Mastic Asbestos Safe?
Black mastic asbestos is safe if it is left alone, undisturbed, and encapsulated, such as by covering it with new flooring.
The material is unsafe if it is abraded so that its fibers are released into the air. Asbestos is most dangerous in its friable state, in which tiny, light fibers float through the air and can easily be spread or ingested or aspirated by humans.
The only state in which asbestos is truly safe is when it is not exposed to an activity that may abrade it, such as sanding it or even walking on it over time. It is best if the old flooring covering stays in place and is covered with new flooring. Many types of flooring can be installed over old flooring, provided the old flooring is flat and well-adhered. Unless a new floor is removed in the future, there should be no safety concerns about covering old black mastic.
The worst thing to do is to attempt to grind it off with a drum sander or to scrape it off manually. Because cutback mastic is thick, gummy, and difficult to remove, there is no practical way to get rid of it safely.
If you must remove a subfloor containing black mastic, have it tested for asbestos and follow all guidelines related to asbestos removal if it tests positive.
Serve, Kinta M, et al. Asbestos-associated mesothelial cell autoantibodies promote collagen deposition in vitro. Inhal Toxicol. vol. 25, 14; 774-84. Dec. 2013. doi:10.3109/08958378.2013.848249
Pira, Enrico et al. Exposure to asbestos: past, present and future. Journal of thoracic disease vol. 10,Suppl 2 (2018): S237-S245. doi:10.21037/jtd.2017.10.126