However, if the shape of the roof allows for the occurrence of stagnating water, it stays on it much longer than on an open roof, allowing water to penetrate the welds, leading to leakage (and finding a leak on a ballasted roof is quite a challenge). To minimize these risks, Carlssons Plåt AB installed Grade Roof fittings on the roof of the Tollare apartment complex in Stockholm, achieving a perfect pitch and minimizing the number of welds in the roofing.