Yes, a deck should have a slope. Building experts in New Zealand recommend having a sloped deck for efficient shading of water. During the building process, it should be designed with a 3mm per 300mm perpendicular slope. Regardless of the direction of the deck, the slope should be a minimum of 3mm per 300mm. However, if the contractor used a solid deck surface, the ideal slope orientation would be 6mm per 300mm. The 3mm-6mm slope range isn't noticeable even to the individual standing on it.