Tile Backer Board Washer & Screws x 100 We recommend 15 washers & Screws per backer board. The discs are black plastic to protect them from corrosion and minimize any chance of degradation over time. The discs have a counter sunk mid-section to allow the screw head to pull into the board but the outer rim of the disc to remain flush with the board surface. This is to maintain the flat board surface ready for easy unhindered application of the subsequent layers. They are compatible with Marmox / Wedi / Aquapanel / Thermopanel / Plasterboard / Duccaboard /Celotex Insulation board / Insulation Board / and Plasterboard Sample will be 1 sent out int he post.