Gutter sizing is an aspect of rainwater collection that has been studied extensively and can be calculated based on published guidelines in the plumbing codes. For example, the Uniform Plumbing Code (UPC) recommends that a gutter system be able to carry the runoff of the heaviest 60 minute downpour recorded in the last 100 years. The International Plumbing Code (IPC) has a similar, but not exact, published sizing recommendation. Both plumbing code manuals include the necessary sizing calculations to properly size a gutter system.
With water weighing over 8 pounds a gallon, it poses a significant threat to a house and it’s inhabitants. The weight of rainwater standing on a roof has caused roofs to collapse. The gutter system must be capable of draining the roof fast enough so that the structural limits of the roof are not exceeded. Both plumbing codes use the highest hourly rainfall event recorded in the last 100 years as a way to ensure the gutter system planned for and ultimately installed can handle the largest known rain event to occur in recent times.
In properly sizing the gutter system (i.e. sometimes also referred to as the drainage system), any of the following can be varied: roof size, number of downspouts, slope of the gutters and the size of the gutters and downspouts. In most situations, the roof size will be a given and the examples below assume the roof size is fixed and has been constructed to meet building codes.
To determine system sizing alternatives, know the roof size to be drained. In a typical home with two slopes, each side will have gutters and be sized separately. Next, find the amount of rainfall that needs to be carried off the roof. This number can be located by calling the local building department or by looking the number up in either Appendix D, Table D1, of the UPC or Appendix B of the IPC manual. Copies of these manuals are generally available at the local library.
For example, for Central Virginia it is 3.96 inches of rain per hour or .044 Gallons Per Minute Per Square Foot of Roof Area. After locating the rainfall number, determine the total rainfall that needs to be drained by multiplying the Gallons Per Minute Per Square Foot of Roof Area by the Square Footage of Roof area. On a 2,500 square foot roof, this works out to be: .044 Gallons Per Minute Per Square Foot of Roof Area x 2,500 square feet = 110 gallons per minute.
Consequently, the guttering system needs to be designed to handle up to 110 gallons per minute or 6600 gallons per hour.
That is over 26 tons of water!
So what size of gutters need to be installed to handle this incredible volume of rainfall? The width of the gutters, the slope of the gutters and the number of downspouts all come into play to determine the correct system sizing. For example, the greater the width of the gutter (i.e. going from a 3” gutter to a 5” gutter), the lower the slope required (i.e. going from ½” to ¼”) to handle the same amount of rain. Your project planner from A Plus Roofing can assess your home and determine the size and configuration of your gutter system.
