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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.