What is the gradient required to prevent water pooling on flat surfaces, such as sidewalks?

To prevent water pooling on flat surfaces like sidewalks, a slight gradient is necessary to ensure proper drainage. The recommended gradient is generally around 1:20, which provides a balance between effective drainage and user comfort. This means that for every 20 units of horizontal distance, the surface should slope by 1 unit vertically.

A gradient of 1:20 allows water to flow away from the surface efficiently, reducing the chances of pooling and creating hazards such as slippery surfaces or water damage. In contrast, steeper gradients could be uncomfortable for users, creating a slope that's too difficult to walk on, especially for those with mobility challenges.

Other options represent steeper gradients, which may not be necessary or advisable for sidewalks. A gradient of 1:10, for example, would result in a very steep path unsuitable for pedestrian use, while gradients like 1:15 and 1:25 are either too steep or may not provide adequate drainage. Thus, the 1:20 slope is the standard that effectively balances drainage needs and accessibility.