Asbestos Putty and Mastic

Asbestos putty and mastic are sealing compounds that were formulated with asbestos fibres to improve durability, flexibility, fire resistance, and adhesion. These products were used extensively in construction from the 1930s through to the mid-1990s for a wide range of sealing and filling applications including window glazing, pipe penetration seals, bath and shower seals, expansion joints, and duct connections.

The asbestos content in putty and mastic products varies widely, from as little as 2 per cent to over 15 per cent, depending on the manufacturer and intended application. Chrysotile was the most common fibre type used, though some industrial-grade products contained amosite or crocidolite for enhanced heat resistance.

Asbestos putty and mastic are among the most frequently overlooked asbestos-containing materials in buildings. They are often hidden beneath paint, behind trim, or packed into joints where they are not immediately visible. During asbestos surveys, these materials are sometimes missed because they are small in volume and easily mistaken for conventional, non-asbestos alternatives.

The risk from asbestos putty depends on its condition and the type of disturbance. When soft and intact, the fibres are bound within the matrix and present a low risk. However, as putty ages, it hardens, becomes brittle, and eventually cracks and crumbles. Old, dry putty can be easily scraped or broken, releasing asbestos fibres. Window replacement work, in particular, poses a significant risk because old putty must be chiselled or heated out of the frame — a process that readily generates asbestos-contaminated dust and debris.

Anyone undertaking window replacement, re-glazing, or renovation work on buildings constructed before 1995 should consider the possibility that putty and mastic materials contain asbestos. Samples should be tested before any work that would disturb these materials. If asbestos is confirmed, a licensed contractor should carry out the removal using wet methods and appropriate respiratory protection.