When sourcing waterproof canvas, the primary technical metric is the Hydrostatic Head (HH) rating, which should be at least 1500mm for light rain and above 3000mm for heavy-duty outdoor use. Additionally, check the Denier (D) count; a higher denier (e.g., 600D or 1000D) indicates superior thickness and durability. Ensure the fabric features a PU (Polyurethane) or PVC coating on the reverse side for absolute water impermeability, and verify the UV resistance rating (UPF 50+) if the product is intended for prolonged sun exposure.