When designing a gutter system, first you must take into account the amount of rainfall, roof size, gutter volume, and the downspout size. If these factors are not properly considered first, the gutter system may be useless during bursts of heavy rainfall when the gutter system is needed the most. Here we have provided you with a quick ste-by-step summary of how to properly size your gutter system.
1. Calculate the roof’s watershed area
A roof’s watershed area isn’t always obvious. Often the maximum rainfall is wind driven rain leading to steeper roofs collecting more rain than lower sloped roofs. To calculate the watershed area of a roof, you must first determine the pitch of the roof, and then multiply the surface area of the roof by the appropriate factor in the associated table.
2. Find the maximum likely rainfall
Residential gutters are designed to handle the most intense 5-minute burst of rain, measured in inches per hour, that is likely to happen within a 10 year period. See the associated map to find yours.
3. Find the appropriate gutters for your home’s watershed
To determine the maximum level of watershed gutters can handle between downspouts, divide your gutter’s 1-in.-per hour watershed by the five minute rainfall intensity. Now you have determined the maximum watershed levels gutters can handle between downspouts. Now pitch your gutters by 1/8 in. per foot and then you can multiply your gutters maximum watershed by 1.4.
A square inch of the downspouts drain hole can drain 100 sqft. of watershed. Therefore a 2 inch x 3 inch downspout can drain up to 600 sqft. and a 3 inch x 4 inch downspout can drain 1200 sqft. If you increase the number of downspouts, you increase the amout of watershed a gutter system can handle.
SAMPLE
A 5-in-12 pitch roof in Tampa Florida is 60 ft. wide and it rafter length is 30 ft. It’s roof’s area is 1800 sqft. The pitch factor for a 5-in-12 roof is around 1.05. When multiplying our roof area by our pitch factor we get 1890 sqft. The 7,960 sqft watershed drained by a 6-in K-style gutter, divided by Tampa’s 7.8 in.-per-hr. rainfall intensity, shows a maximum watershed of 1020.5 sqft meaning they can’t handle the the 1890 sqft of watershed. We could pitch the gutters by 1/8 inch per ft. but this would only increase our watershed to 1428.7. Still not enough. To make 6-inch K-style gutter work for this home, we would need to add another downspout which would double the watershed 20 2041 sqft. allowing our gutters to safely handle the maximum waterhshed of this home.
[…] smaller sized gutter systems. Before installing K-Style gutters, you need to be sure you know how to size gutters properly for the building they are being installed on, and the intensity of rainfall where they are […]