The effective roof area that will collect the rain which will need to be removed by the guttering.
Calculate rainwater gutter size.
The simple way to calculate gutter sizes.
Multiply the drainage area by the roof pitch factor and rainfall intensity to find out the adjusted square footage.
Gather some basic weather information to calculate the proper size for gutters and downspouts for a house.
6 inch 7 960 square feet.
Calculating effective roof areas where gutter angles are installed effect of angles if a length of eaves gutter includes an angle.
The total rainfall and how rapidly it falls will affect the runoff from the roof.
5 inch 5 520 square feet.
The area that your chosen guttering system can cope with.
There are two things you need to know.
Then use the information below to see what size gutter you need.
Box gutter with rain water head at end of box gutter.
Match your downspouts to the gutter size you chose in step 7.
As most roofs are not flat this needs to be calculated we ll show you a simple way.
5 inch 5 520 square feet.
Total roof area also affects runoff.
Multiply the drainage area by the roof pitch factor and rainfall intensity to find out the adjusted square footage.
This can be calculated by using the following formula see diagram below.
Now architects engineers designers and contractors can easily and accurately size downspouts and gutters according to the specifications in smacna s architectural sheet metal manual 7th edition 2012.
Rainwater calculations on face value guttering installation is a piece of cake but it is amazing how many installers get it wrong.
Determine the size of your downspout.
15 m is a practical maximum gutter length to be served by a downspout.
If a roof s various drainage areas call for different size gutters go for the biggest one k style.
To limit the effects of thermal expansion in gutters 50 ft.
Then use the chart below to see what size gutter you need.
An inch of rain is not much but it if falls in half an hour it could flood a gutter.
Simplified suitable for flattish roofs with little or no vertical faces or steep roofs draining to one side of box gutter only or any roof if you are in a hurry and make the necessary allowances for all catchments.
Adding additional downspouts will require less gutter and downspout volume resulting in smaller gutters downspouts.
For example a 5 inch gutter system requires at least 2 x 3 downspouts and a 6 inch gutter system requires 3 x 4 downspouts.
Downspout sizes must not exceed the bottom width of the gutter.
Refer to the gutter packaging or specification for the recommended downspout size.
Calculations are based on level gutter capacity as experimentally determined by the national institute of standards and technology nist.
Please lower the maximum spacing between downspouts to see this effect.
