Roofers Mastic Trowel This trowel has been specifically designed for the spreading of mastic on roofs and other surfaces by roofers and other construction professionals. This roofers mastic trowel features a thin, narrow yet long trowel blade, which allows both larger coverage of mastic and smaller, more detailed applications. Mastic can be applied in a number of situations, and this tool particularly helps with repointing brickwork and filling in gaps due to the narrow design. The width of the roofer's mastic trowel is 13mm.