Materials / Roof Cement and Mastic
Roof Cement and Mastic
accessoryRoof cement, also called plastic cement or flashing cement, is a trowel-grade asphalt-based mastic used to seal flashing laps, felt seams, and penetrations on steep- and low-slope roofs. Modern products are formulated asbestos-free under ASTM D4586, Standard Specification for Asphalt Roof Cement, Asbestos-Free, in Type I or Type II asphalt and Class I (dry) or Class II (wet) grades. Its older sibling, ASTM D2822, covered the asbestos-containing predecessor product and was withdrawn in 2016, though IRC Table R905.9.2 still lists D2822, D3019, and D4586 together as accepted asphalt cements for built-up roof coverings. IRC Section R905.6.6 separately requires a plastic-cement top coating on cap flashing where slate roofing meets chimneys or masonry walls.
- governed by the active asbestos-free ASTM D4586 spec, unlike the older withdrawn D2822 asbestos-containing spec
- type and class system lets installers match the cement to dry or wet/underwater application conditions
- code-recognized material for built-up roof coverings and flashing seals under IRC Table R905.9.2
- trowel-grade consistency works for both steep-slope flashing details and low-slope built-up seams
- widely available from multiple manufacturers who publish explicit d4586 conformance statements
- a mastic patch is a maintenance measure, not a substitute for correctly lapped and fastened flashing
- service life is shorter than the roof covering it seals, and it requires periodic inspection and reapplication
- surface prep (dry, clean, sound substrate) is critical and easy to shortcut in the field
- excess or sloppy application traps water against the substrate instead of shedding it
Sources
- up.codes verified 2026-07-28