Calculate Rainwater Gutter Size

This can be calculated by using the following formula see diagram below.

Calculate rainwater gutter size.

Then use the chart below to see what size gutter you need. The gutter and downspout size required is highly dependent on the amount of gutter and the spacing of the downspouts. 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. Calculations are based on level gutter capacity as experimentally determined by the national institute of standards and technology nist.

6 inch 7 960 square feet. Multiply the drainage area by the roof pitch factor and rainfall intensity to find out the adjusted square footage. 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. Gather some basic weather information to calculate the proper size for gutters and downspouts for a house.

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. Then use the information below to see what size gutter you need. Please lower the maximum spacing between downspouts to see this effect. The downspout and gutter sizing calculator is located on the tools cad and apps page of the smacna website.

As most roofs are not flat this needs to be calculated we ll show you a simple way. 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. Rainwater calculations on face value guttering installation is a piece of cake but it is amazing how many installers get it wrong.

An inch of rain is not much but it if falls in half an hour it could flood a gutter. The area that your chosen guttering system can cope with. Match your downspouts to the gutter size you chose in step 7. The total rainfall and how rapidly it falls will affect the runoff from the roof.

Box gutter with rain water head at end of box gutter. 5 inch 5 520 square feet. Calculating effective roof areas where gutter angles are installed effect of angles if a length of eaves gutter includes an angle. If a roof s various drainage areas call for different size gutters go for the biggest one k style.

There are two things you need to know. Refer to the gutter packaging or specification for the recommended downspout size. The simple way to calculate gutter sizes. 5 inch 5 520 square feet.

Determine the size of your downspout. Downspout sizes must not exceed the bottom width of the gutter. 15 m is a practical maximum gutter length to be served by a downspout. Adding additional downspouts will require less gutter and downspout volume resulting in smaller gutters downspouts.