How do I prevent puddles (standing water) on the substrate?

Water pooling on an impermeable sub-base is prevented by providing adequate falls towards drainage and by filling existing depressions. Impermeable sub-bases include screed and concrete surfaces, as well as roof waterproofing formed from roofing felt, bituminous membranes or waterproofing membranes.

On these impermeable surfaces, areas where water collects are readily visible after rainfall. In dry weather, depressions can be identified by distributing water across the surface with a watering can or hose.

The depressions are filled by loosely placing accurately cut pieces of roofing felt into them. Several layers may be laid one over another where required, and the result can be checked with a long straightedge or straight metal bar.

The surface does not need to be completely level. The important points are that as little water as possible remains and that the existing falls are retained. Sand, grit and gravel are unsuitable because they can shift beneath the floor covering and be washed out by water. The cut pieces correct individual depressions, but cannot replace missing falls or inadequate drainage across the whole area.