Calculate Rainwater Gutter Size

Calculating effective roof areas where gutter angles are installed effect of angles if a length of eaves gutter includes an angle.

Calculate rainwater gutter size.

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. An inch of rain is not much but it if falls in half an hour it could flood a gutter. 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. The downspout and gutter sizing calculator is located on the tools cad and apps page of the smacna website. If a roof s various drainage areas call for different size gutters go for the biggest one k style. 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.

Adding additional downspouts will require less gutter and downspout volume resulting in smaller gutters downspouts. This can be calculated by using the following formula see diagram below. The total rainfall and how rapidly it falls will affect the runoff from the roof. Match your downspouts to the gutter size you chose in step 7.

To limit the effects of thermal expansion in gutters 50 ft. Please lower the maximum spacing between downspouts to see this effect. There are two things you need to know. 15 m is a practical maximum gutter length to be served by a downspout.

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. Multiply the drainage area by the roof pitch factor and rainfall intensity to find out the adjusted square footage. 6 inch 7 960 square feet. If a roof s various drainage areas call for different size gutters go for the biggest one k style.

5 inch 5 520 square feet. The effective roof area that will collect the rain which will need to be removed by the guttering. Then use the information 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.

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. As most roofs are not flat this needs to be calculated we ll show you a simple way. 5 inch 5 520 square feet.

Box gutter with rain water head at end of box gutter. Gather some basic weather information to calculate the proper size for gutters and downspouts for a house. The area that your chosen guttering system can cope with. Then use the chart below to see what size gutter you need.