research roofing research roofing

Gutter and Downspout Water Capacity

So what's the trick to designing the perfect gutter system?  How to make sure your gutter is large enough and you have enough downspouts?  Use the helpful charts below and you'll be a near expert in no time!  These are "safe" numbers, meaning designed to work under the strongest rainfall not the average for your area.  You may be able to undersize based on these charts and be fine 90%+ amount of the time, but if you never want to worry better to be safe than sorry.

Gutter Sizing Water Capacity

Gutter Roof Area (Square Feet)
Per linear Ft of gutter underneath
ROOF PITCH Up to 6/12  7/12  8/12 9/12  10/12   11/12 12/12
5" K Style 22 20.79 18.71 16.84 16.17 15.28 14.05
6" Style 34 32.13 28.92 26.03 24.98 23.61 21.72
7" K Style 48 45.36 40.82 36.74 35.27 33.33 30.67
4" Box 18 17.01 15.31 13.78 13.23 12.50 11.50
5" Box 26 24.57 22.11 19.90 19.11 18.05 16.61
6" Box 40 37.80 34.02 30.62 29.39 27.78 25.55
7" Box 52 49.14 44.23 39.80 38.21 36.11 33.22
8" Box 60 56.70 51.03 45.93 44.09 41.66 38.33
Notes:  Max Roof Pitch 6/12 for these numbers
No Obstructions into gutter
Recommended Slope No Greater than 1/16" per ft

 

Downspout Capacity Based On Size

Down Spout Size Max Area of Roof draining to one down
2x3 400 Sq Ft
3x4 650 Sq Ft
4x5 1000 Sq Ft
3" Round 450 Sq Ft
4" Round 700 Sq Ft
5" Round 1200 Sq Ft
(Safe for maximum rainfall)  
* Reduce by 10% per 90 degree turn
Assumes proper pitch and no obstructions or turns

Problems with Gutter Sizing

Many factors can effect the proper size of gutter.  Most of all is location and your average rainfall during a large downpour.  For the vast majority of rains gutter and downspouts sizes considerably less than recommended will do the job.   

Primary cause for water spillage is DEBRI IN THE GUTTER.  It doesn't take many sticks, leaves, or needles to impede the proper flow of water in the eavestrough.  Make sure your gutters are clean at all times. 

Secondary cause of gutters not working properly is improperly maintained or failing gutter protection systems. Screens and filters can eventually clog. Covers that rely on surface tension get dirty and need cleaning.  You must maintain your gutter covers even more than the gutter itself many times.