In older fiber-cement residential siding that contained asbestos, the most common percentage by mass was approximately 10–15%, though some products could contain up to 25% or even exceed 50% depending on manufacturer and intended application.[1][2][3]
Typical Asbestos Content
- Most standard fiber-cement siding from the early to mid-20th century contained about 10% asbestos mixed with 90% Portland cement and sand.[2]
- In some cases, laboratory analysis has identified certain products with 25% asbestos content.[1]
- Original expert sources (such as Rosato’s industry research) have documented specific asbestos-cement products with asbestos levels as high as 50% or even more, especially in specialized industrial applications.[3]
Product and Manufacturer Variation
- Differences in manufacturing processes, preferred fiber length (long fibers for strength, dust for filler), and product function created substantial variation in asbestos percentage in siding shingles and panels.[3]
- Products aimed specifically at fire resistance or structural reinforcement typically had higher asbestos content.
Identification and Safety
- Asbestos-cement siding was especially popular from the 1920s to the late 1970s; nearly all fiber-cement siding from that era potentially contains asbestos if untested.[4][3]
- Modern fiber-cement siding is formulated without asbestos, using cellulose fibers and safe fillers instead.[5][4]
Always verify actual content through analytical testing, as the proportion can vary widely depending on manufacturer and year of production.[3]