What is the recommended slope for drain pipe carrying excess rain water from pool and deck around pool?

Answer 1

Since you are dealing with rainwater runoff, you want to refer to Table 1106.3 "Size of Horizontal Storm Drainage Piping" from the International Plumbing Code along with any local amendments This code will allow you to use the maximum slope you have available at your site and this can reduce the required drain pipe size and cost. For example: Let's assume your pool deck area is 5 feet above and 100 ft from the street gutter. Allowing 1 ft for a drain grate and elbow under your deck, you have 4 ft of elevation left to use over the 100 ft run to the street; this is very close to 1/2 inch per ft slope. To determine the drain pipe size required, we need to know how large an area will be drained. Let's assume your deck and pool area is 50 ft by 75 ft which is 3,750 sq ft but you also have roof area of 1,250 sq ft (horizontal projection only) that also drains onto your deck, bringing the total area to be drained up to 5000 sq ft. The next step is to determine the "Maximum Probable Rainfall Rate" for your location. In the US this information is typically available from the National Weather Service. For example, the East Coast area including Philadelphia and New York City has a 1 in 100 years probability of having rain falling at about 3 inches per hour. Many areas in Florida have values 50% higher. With this data we can refer to Table 1106.3 and find the column for 3 inch per hour rainfall rate. Reading down the column we can see the areas that can be drained for various pipe sizes and slopes. At a low 1/8 inch per ft slope a 6 inch drain line would be required; at 1/4 inch per ft slope a 5 inch line (uncommon size) would be required and finally at 1/2 inch per ft slope a 4 inch drain line would be adequate. Note: If you used a commonly available drain line size of 3 inch, this will adequately drain only 1,096 sq ft when sloped at 1/8 inch per foot (1,546 sq ft when sloped at 1/4 inch per foot or 2,295 sq ft when sloped at 1/2 inch per foot.) During a moderately heavy rain exceeding only 1 inch per hour, this line will allow water to accumulate in the above example and flood the pool deck; perhaps backing up into the house.