Doubling the number of downspouts halves the required capacity for each.
How many downspouts for roof.
Calculate the total roof area by multiplying its length by width.
Use the chart below to figure out how many extra downspouts you need.
As with gutters a downspout s capacity must match or exceed the expected runoff.
A square inch of downspout can drain 1 200 square feet of roof during a rain with an intensity of one inch per hour.
The next factor to take into account is the pitch of your roof because the pitch will cause water to flow in a certain direction.
Rectangular 2 by 3 inches 600 square feet 3 by 4 inches 1 200.
Rain gutters and downspouts provide an efficient first step in controlling roof runoff.
Gutters come in many sizes.
A large roof is going to move a lot of water which means the downspout locations may need to be adjust to compensate.
The typical recommendation is one downspout for every 20 to 30 feet of gutters.
Size downspouts to fit the gutter.
But if the rain or melted snow channeled off the roof isn t carried far enough away from the house it will collect against the foundation wall or footing and seep into your basement or crawl space.
3 by 4 inches 1 200 square.
As with gutters a downspout s capacity must match or exceed the expected runoff.
2 by 3 inches 600 square feet.
Rectangular downspouts move slightly less water at 600 square feet for 2 by 3 inch or 1 200 square feet for a 3 by 4 inch downspout.
Buried or underground roof drainage downspout defects clogging overflowing holes corrosion leaks that cause wet basements or crawl areas proper installation of buried downspout extensions proper installation of french drains seepage pits at flat sites gutter downspout defects cause wet basements crawl areas questions answers about installing maintaining repairing buried roof.
Multiply the number of downspouts by square footage of each.
There are 5 easy steps to figure out how many drains your commercial roofing project will need.
A high slope moves water more quickly and if gutters have trouble dealing with a lot of water in a short period of time an extra downspout can alleviate that issue.
Smacna s new downspout and gutter sizing calculator enables the user to manually enter the design area of the roof or use the design area wizard for the calculation.
This wizard makes it easy to calculate the roof design area by providing various shape calculations and the calculation for vertical walls area per international plumbing code ipc.
Adding a 2 by 3 rectangular downspout for instance boosts your gutter s capacity by 600 square feet of drainage area.
First you must consider how many downspouts you will need.
Adding a 2 by 3 rectangular downspout for instance boosts your gutter s capacity by 600 square feet of drainage area.
Use the chart below to figure out how many extra downspouts you need.
