2.0 Roof and Gutters in Rainwater Harvesting: Overview
Consequences of Roofing Materials; Calculating Roof Area;
Calculating Tank Size; Calculating Gutter and Downspout Size
This is a big chapter because there are several things that you will need to consider; all at the same time. The presentation is meant to be linear, but you will probably find yourself referring back and forth, from one section to another, as your mind takes on new knowledge and choices. We will start by talking about what implications the roof and gutter material has on your collected rainwater quality – your health. Then we will discuss how to calculate the amount of rainwater that your roof can collect (wherever you are in the world). We will compare that to your water consumption to determine a tank size, and finally help you design your gutter and downspout system. Unless you are extremely lucky, you will need to go through the process more than once. You might find that you have too much or too little roof area to fill the tank. Or, you may find that your distance between gutters is too short to make it to a convenient corner, or that it would be wise to put in two small, rather than one large tank. That’s why you may need to go back and forth before making a final decision. Don’t worry, once you’ve gone through the exercise once, you’ll get comfortable with how it all fits together. Your decision-making will get easier, more confident, and you will have a great opportunity for a right-the-first-time design.
To help you figure out where you are in the process, as you finish reading a section, you will have the option of coming back to this page – this page is kind of like a table of contents for chapter 2, with abstracts. I’ll have some opening and closing comments on each section back here for you, just after this list.
2.1 Safe Materials: Selection and Consequences of roofing and gutter materials for rainwater harvesting.
This section is given so that you are able to get a good idea of how your roof and gutter material will affect your stored rainwater quality. You may be surprised to find several materials that you thought weren’t appropriate are actually good choices; and less expensive. Come back here when you have finished with . . .
Now that you are familiar with the health effects of your roof and gutter material, we’ll proceed to how to calculate your roof area.
2.2 Calculating Area: Calculating your roof area for rainwater harvesting
All of the following sections of this chapter require you to enter your roof area. The roof area that I am referring to is not necessarily the total area of your roof. It is the area that you are going to gutter. This section describes how to measure this roof area. Please note that there are two ways to measure this area depending on who you are talking to. If you talk to roof rainwater harvesting experts, most will calculate the roof area as if the rain is coming straight down and your roof is essentially flat. If you talk to the gutter manufacturers, installers, and many design codes, they will want you to take your roof pitch into account. They will get a bigger number every time. I will be presenting the “flat roof” technique because 1) my experience shows that it works quite well, 2) it is simple, and 3) the section on gutters will include a 25% over design. I’ll give you links that you can go to if you are determined to use the gutter manufacturer’s method. I’ll also link you to a correction table to convert from one to the other if you wish. However, I would not use these correction factors when sizing your tank.
You will need to visually divide your house into areas that you are going to gutter and then add up each area for the total. If in subsequent calculations you find that you don’t need this much total area, pick out which areas will add up to the total needed. Also, when you get to the gutter sizing section 2.4, remember that when it asks you for the roof area, it’s asking you for the longest gutter length before a downspout. This doesn’t necessarily mean the longest straight length. You might have a gutter that goes around a corner before it gets to a downspout. You’ll only need to enter the longest guttered area sharing a downspout. The other smaller areas will have no problem collecting water with that size gutter. This will become clearer as we continue. For now, have fun with . . .
This information is not meant to confuse you. The technique that I am giving you will work very well. The gutter manufacturers are simply adding an over size factor based on their gutter capture efficiency experience. We will build in our own over-design as you enter data in the following sections. However, do not enter too many over-design factors; they all add up!
2.3 Tank Sizing: Calculating the rainwater harvesting tank size based on your roof area, rainfall, and consumption
This section is a very powerful spreadsheet where you will enter your roof area that you calculated in the last step, your household water consumption, and your monthly rainfall data which we will help you find for your area of the world at a site called http://www.worldclimate.com. I strongly recommend that you open this site right now because if you wait to do it from within the spreadsheet, it will open in a window over the top of the spreadsheet which is a pain to use. If you'll open it from here, we will put it in a separate window for you so you can refer to it easier when you need to copy down the numbers on to the spreadsheet.
This spreadsheet will graphically allow you to size your tank(s), and simultaneously graphically depict what your tank inventory will be at the end of each month. Use your total roof area when you are asked to enter "Roof Area" the first time around. If you’re collected rainwater turns out to be consistently greater than your consumption each month, consider using fewer roof areas, and then recalculate a new tank size. If you use too much gutter (roof area), you’ll simply overflow the tank each month. Once you've got your tank sized, have fun with the spreadsheet by plugging in different numbers just to see what happens, then come back here . . .
Now that you have your tank(s) sized, we will move on to designing and sizing the gutters and downspouts.
2.4 Designing Gutters: Calculating gutter width and length between downspouts for semicircular, box, V, or trapezoid gutters made of any building material
For some of you, designing gutters will be left to an installer. They are perfectly capable of designing a successful system. I would suggest that you ask them to install the gutters at a ¼-degree slope for the first 2/3 of the gutter length before the downspout, and an angle of ½-degree for the last 1/3 of gutter length. This will get the water flow fast enough to help move debris along and help prevent pooling which can draw insects, harbor biota, and increase corrosion.
For those of you that will design or install your own gutters, this link will open another powerful spreadsheet that will help you design a gutter with materials at hand, and determine how far you can go between downspouts. . .