COVERAGE A 22kg bag of STOPGAP 1100 GYPSUM mixed with 5.5 litres of clean water will cover approximately 5.0m² at a thickness of 2-3mm. STOPGAP 1100 GYPSUM is a self-levelling smoothing underlayment designed specifically for application over sound calcium sulphate (e.g. anhydrite) screeds prior to the installation of new floorcoverings.