9283 Sutton Place

West Chester, OH 45011

Give us a call

513-554-6120

Email us

info@metalpanelsystems.com

9283 Sutton Place

West Chester, OH 45011

Give us a call

513-554-6120

Email us

info@metalpanelsystems.com

Gutters and Downspouts

Gutters and Downspouts Information

Gutter sizing is much more important with interior gutters than it is with exterior gutters located at the perimeter of the building.  A failure with an interior gutter could be catastrophic while a failure at an exterior gutter could simply cause an inconvenience for the building owner.

It is important to know how gutters and downspout sizes are specified in order to meet building codes and perform to their intended use.  Gutter and downspout sizes are determined by using historical rainfall data and are calculated by determining the drainage capacity in gallons per minute.  Metal Panel Systems can assist in performing drainage calculations for our customers when we fabricate your gutters and downspouts.

The type of roof system is an important factor when considering what size gutters and downspouts are required.  For example, low sloped roof systems and steep sloped roof systems present different challenges. Low sloped roof membrane systems like TPO, PVC, BUR, EPDM, and modified bitumen drain water very slowly.  On the other hand, steep sloped roof systems like standing seam metal or asphalt shingles drain water very quickly. For this reason, the drainage capacity for steep sloped systems must be greater than those of low sloped systems.  The rainfall data that is used to determine building codes, specifically the International Plumbing Code, is provided by the National Weather Service. Two important data points that we use are the 100 year storm occurrences and rainfall intensity durations.

Historical rainfall data across the state of Ohio ranges between 2.4 to 2.8 inches per hour for a 100 year storm.  Using the high end of this range and a safety factor means that gutters and downspouts on a low sloped roof system should be designed to drain .029 gallons per minute (per sq foot).  During intense storms, 5 minute durations can average 6.8 inches per hour of rainfall. For this reason, Metal Panel Systems uses this higher number when calculating drainage capacity for steep sloped roof systems.

The pitch of the gutter determines how quickly water will drain.  For instance, a gutter with a pitch of 1/16” in 12 will drain nearly 3 times slower than a gutter that has a pitch of ½” in 12.

The next important information to consider is tributary area or the amount of roof area that drains to each downspout.  For low sloped roofing systems throughout the State of Ohio, the following downspout sizes will be sufficient for the following roof areas:

  2”x3” rectangular 1,840 sq feet

3” round 2,930 sq feet

3”x4” rectangular 4,400 sq feet

4” round 6,130 sq feet

3.5”x5” rectangular 7,100 sq feet

4”x6” rectangular 10,990 sq ft

5” round 11,530 sq feet

It is also important to note that at least 1 downspout should be installed for every 50 feet of gutter.

For steep sloped roofs, larger gutter widths are required as the pitch of the roof increases. As mentioned above, Metal Panel Systems uses a much larger rainfall intensity number of 6.8 inches per hour (over a 5 minute duration) to size gutters and downspouts.  This is due to the more rapid run off compared to low sloped roofs. For steep sloped roofing systems throughout the State of Ohio, the following downspout sizes will be sufficient for the following roof areas:

  2”x3” rectangular 740 sq feet

3” round 1,260 sq feet

3”x4” rectangular 1,750 sq feet

4” round 2,630 sq feet

4”x6” rectangular 4,400 sq ft

5” round 4,945 sq feet

For help determining the sizes of gutters and downspouts on your project, please contact us.

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