Calculate Rainwater Gutter Size

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.

Calculate rainwater gutter size.

Multiply the drainage area by the roof pitch factor and rainfall intensity to find out the adjusted square footage. Calculations are based on level gutter capacity as experimentally determined by the national institute of standards and technology nist. Downspout sizes must not exceed the bottom width of the gutter. 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.

15 m is a practical maximum gutter length to be served by a downspout. Calculating effective roof areas where gutter angles are installed effect of angles if a length of eaves gutter includes an angle. To limit the effects of thermal expansion in gutters 50 ft. The total rainfall and how rapidly it falls will affect the runoff from the roof.

Determine the size of your downspout. If a roof s various drainage areas call for different size gutters go for the biggest one k style. 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. Rainwater calculations on face value guttering installation is a piece of cake but it is amazing how many installers get it wrong.

As most roofs are not flat this needs to be calculated we ll show you a simple way. Adding additional downspouts will require less gutter and downspout volume resulting in smaller gutters downspouts. 5 inch 5 520 square feet. Please lower the maximum spacing between downspouts to see this effect.

Match your downspouts to the gutter size you chose in step 7. Then use the information 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. The gutter and downspout size required is highly dependent on the amount of gutter and the spacing of the downspouts.

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. The simple way to calculate gutter sizes. The effective roof area that will collect the rain which will need to be removed by the guttering.

If a roof s various drainage areas call for different size gutters go for the biggest one k style. The downspout and gutter sizing calculator is located on the tools cad and apps page of the smacna website. Refer to the gutter packaging or specification for the recommended downspout size. Box gutter with rain water head at end of box gutter.

6 inch 7 960 square feet. 5 inch 5 520 square feet. Total roof area also affects runoff. The area that your chosen guttering system can cope with.