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Gutter Sizing Guide

Sizing Methodology

This Gutter Sizing section provides 4 steps to properly size a commercial gutter & downspout configuration for SAF Perimeter Systems commercial gutter designers, contractors, and building owners:

Gutter Sizing Step 1 – Determine roof area to be drained
Gutter Sizing Step 2 – Research historical rainfall data
Gutter Sizing Step 3 – Calculate water flow during rainfall extremes
Gutter Sizing Step 4 – Calculate water handling capacity for gutter & downspout options

1 – Determine Roof Area to Be Drained

Calculate roof area to be drained by your gutter system and note roof pitches.

Commercial roof structures can be complex, consisting of many areas, shapes, and configurations. When selecting and sizing gutters for such structures consider the following:

As a safety factor, always design or select gutters and downspouts for worst case conditions.

For roof levels that drain onto lower roofs, the lowest roof area in elevation should include the area of roofs above that drain onto it. For example: a 1st story roof is 10,000 square feet, a 2nd story roof drains onto the 1st story roof and its area is 5,000 square feet. Total area for 1st story roof should be calculated as 15,000 square feet.

The roof pitch (slope) has to be accounted for when using our GPM tables. Their values are formulated on a roof’s pitch. For multi-story roofs draining onto one another (as mentioned above), you will select your GPM table based on whichever roof has the greatest pitch. Taking from the example above, the 1st story roof has a pitch of <1″in 12″, the 2nd story roof has a pitch of 1″-6″ in 12″, and so you will use the 1″-6″ in 12″ GPM Table for both roof calculations.

For a canopy as shown in the photograph above, you would only take into account the area of the upper roof that drains onto the canopy.

For roofs that intersect at the same elevation with different roof slopes, you will always use the GPM tables for the greater pitch.

2 – Research Historical Rainfall Data

NOAA (National Oceanic & Atmospheric Association) has produced historical rainfall Isopluvial Maps (in color) for most of the U.S. Unfortunately, due to government budget constraints, mapping for all the U.S. has not been completed. Visit the NOAA Database and climatic maps for the United States.

Note: When reviewing data from the NOAA Databases, you will need 60 minute duration data to use our SAF GPM Calculation Tables.

 3 – Calculate Water Flow During Rainfall Extremes

The SAF Perimeter Systems Roof Gallons per Minute Table allows simple calculation of Gallons/Minute/Square Foot of Roof Area.

Roof GPM table

Relate historical rainfall values and calculated theoretical gallons per minute per square foot of roof area, according to a project roof slope. Look up the value(s) applicable to your project and multiply by your total roof area(s) to be drained.

For example, assume you are draining a 20,000 square foot roof with a roof slope between 6″ and 12″. Your project location’s historic rainfall is 1.8″. Your GPM calculation is 20,000 x .094 = 1880 gpm (gallons per minute). You will use this GPM calculation to select the proper size gutter & downspout in Step 4 – Calculate Water Handling Capacity for Gutter & Downspout Options.

 4 – Calculate Water Handling Capacity for Gutter & Downspout Options

The SAF Perimeter Systems Gutter/Downspout Capacity Table relates Downspout Size to Gutter Sizes.

gutter/downspout capacity table

Water capacity values of a gutter & downspout system are calculated using the height (head) of water a gutter can hold, the outlet (orifice) size of the downspout, and the quantity of downspouts used.  It is possible to use a mix of gutter sizes, a mix of downspout sizes, and quantities to accommodate the theoretical gallons per minute draining from a roof during in-climate weather.  Practically speaking though, a common gutter size is usually selected and downspout spacing is equal for most buildings.

Continuing with our example in the last step, we know our gutter & downspout system may be subject to as much as 1,880 gallons per minute.  For design purposes and aesthetics, let’s assume you have selected an 8″ gutter.  Also assume our guttered eave is 335 linear feet.  From the table, we find that a 5″x5″ downspout will discharge 260 gallons per minute.  At 1,880 gpm’s, we will need 8 downspouts to handle the rainfall (1,880/260=8 downspouts, rounded up).

Next we need to check spacing, 335lf / 8 downspouts = 40′ centers (approximate).  Note that gutters require expansion joints at 40′ centers. 

In this example your maximum spacing of downspouts should be 40′.

By reducing the downspout spacing to perhaps 35lf, you could use a 6″ gutter with (9) 4″x6″ downspouts in lieu of an 8″ system.   It is easy to see that you can vary gutter size, downspout size, and downspout spacing to properly size a gutter & downspout system.