We’ll share how to calculate water amounts for roof catchment.

To try to make this as clear as possible I turned to one of the simplest books we have on Pc. Page 18 for the discussion and definition of Elements, like water. (The basics of permaculture design. Ross Mars)

General Design Principles

Elements: Plants, animals, structures and natural features are all elements.

Each element must be able to support at least 3 functions. Functions are numerous and interact with each other and other elements within the design. “We use elements to provide the many functions we desire, and that make the property productive and beneficial to all things which are present. These functions include windbreaks, food production, fire control, energy production and soil building and conditioning. Each of these functions is served by several elements; in this way each element contributes to the whole system. If one element fails there are others to perform the function. It is the large number of useful relationships which exist between the elements of a permaculture system that makes it so dynamic and functional. ”

Wrap you head around that.

lazer light

The following will show how to scratch out roof catchment calculations and then combine elements and functions to come up with the design for the area in focus. This particular area is ripe for water collection. First, rain falls on the upslope on the neighbor’s fields and the road. The rain sheds off the road and travels north and down onto our property. The rainfall on the neighbor’s field travels downhill to this location.

Final Line Draw

Three buildings act as moisture sinks simply by covering the ground and not allowing excessive evaporation. These three buildings plus a fourth all have metal roofs and produce water for catchment. Pockets of vegetation growing in the design area could be described as profuse (in dryland standards). Where windblown soil has deposited and the buildings have provided a modicum of shade the grass and weeds appear more prolific.

Last year I found three wild currant plants growing, one each, tucked next to the north side of each of the three separate buildings. They were making use of available water, shade and protection from the hot SW winds.

In dry lands like ours the water equation looks like this:

Together these two techniques allow the ground to take maximum advantage of road runoff and underground hydrology.

How here’s how we calculated roof water catchment one Geek Night last year

Water Catchment Class


House drawing pngCalculating roof water catchment is a fairly easy process. All that really matters is the foot print of the building.

RC = FP such that:


= FT X FT = INCHES / 12


= TOTAL RAIN falling on that foot print

You can’t actually get 100% due to: Blowing Snow, Leaks, Evaporation…

So best engineering estimate is: you can actually get 80%

Actual catchable precipitation = total falling rain in one year x .80

Cubic Feet x 7.48 = Gallons


Example 1:

House drawing png

Annual average rainfall is 14 inches

Main part of the house is 20’ x 60’

Mud room area is 12’ x 12’

20 x 60 = 1200, 12 x12 = 144 = 1344sq feet

1344 x 14/12 = 1344(14) /12  = 1568 feet cubed

1568 cubic feet x 7.48 gallons           = 11728.6 Total Gallons

So, 11,728.6 gallons of rain fall on the total foot print of the house in one year and 11,728.6 gallons x .80 = 9382.9 gallons


Example 2:

House drawing png

Annual average rainfall is 9 inches

Main part of the house is 20’ x 60’

Mud room area is 12’ x 12’

20 x 60 = 1200, 12 x12 = 144  = 1344sq feet

1344 x 9/12 = 1344(9) /12       = 1008 feet cubed

1008 cubic feet x 7.48               = 7539.84 Total Gallons

So, 7539.84 gallons of rain fall on the total foot print of the house in one year and 7539.84 x .80 = 6032 gallons.

In the new area between the three outbuildings how much water will the roof top catchment provide and thus augment the annual rainfall?

Final Line Draw

Four buildings set to contribute to water delivery via the gutters and down spouts:

  • Oil House
  • Old School House
  • Old Chicken Cottage
  • Barn

Water Catchment Table 1

First part of the water equation

In 2013 and 2014

  • Grasses and weeds were allowed to proliferate. Wood chips and coffee grounds were placed in small piles in the barest and driest spots within the design area.

Winter 2013/2014

  • A large mixed pile of wood chip and grass clippings from town was deposited and allowed to sit and decompose through all of 2014.

wood chip mix pile

Fall of 2014

  • In tandem with rain events the grasses/weeds were cut, dropped and covered with decomposed material from the pile. Then the smaller wet wood chip piles were raked over this and the whole area then covered with thin (2”) protective layer of wet straw.  This will sit through the Spring and Summer of 2015.

Fall 2015

    • Leave the area alone, other than chopping & dropping.


100% of our electrical use comes from the 8.4 KW Solar Array positioned NW and down slope from this area.

The sun angle on December 21st is 19.55o

The sun angle on June 21st is 66.38o



Visualize the design; no evergreen trees, no large over-story trees.

  • Dwarf fruit trees
    Nitrogen fixing shrubs
    Nitrogen fixing decidous trees
    Cover crops

Fall 2015

Leave the area alone except for chopping & dropping


Finish roof on school house

Gutters and downspouts directed towards swales

First round of Deciduous nitrogen fixing trees and shrubs are planted

Swales seeded in cover crops


Plant fruit trees


To reduce resource input ($) we will not be purchasing water storage tanks for this project. Instead each building, which has metal roofing, will be outfitted with gutters and down spouts. The down spouts will be directed to swales which have been designed on contour. Extensive chop & drop and mulching has been conducted for 4 years to increase the soil hydrology. The swales will be completed in the fall of 2015, cover cropped and planted to nitrogen fixing species. Because of the location (south and east of the Solar Array) only dwarf fruit trees are being selected.

If you live in a dry climate area, please have a look at this article:

Richard Perkins talks about dry climate Permaculture in a spectacular data dump http://permaculturenews.org/2012/09/15/reflections-on-dryland-water-management-in-portugal/

Thanks for reading my blog!