When we first picked out the spot to put our tiny house, I wanted to find a location where I’d be able to build some type of roof structure uphill from it. I was inspired by my buddy Joe from the Homesteadonomics YouTube channel where he built a rainwater harvesting surface using an old billboard tarp. He used the water collected and had it gravity fed down to an IBC tote where he’d use it for a garden area.
I loved the premise of doing a simple collection surface like that but since the water we collect would be used for drinking and cooking, I wanted the water to be as clean as possible. I started to play around with the idea of building a roof on the ground.
In order to collect enough rainwater for our domestic use, the catchment surface between our tiny house, awning and shed only totals about 600 sq ft. We need a minimum of 3000 sq ft in order to capture enough water in the desert to meet our needs. I based these calculations off of two things. I calculated about how much water use we’d use each year from Brad Lancaster’s book Rainwater Harvesting for Drylands and Beyond. I also modeled it after the size of Joe’s system. He has approximately 3600 sq ft of catchment surface from his roof and has lived primarily on rainwater for the last 7 years in the same climate.
Create a low-cost, easy to build, easy to maintain catchment surface.
How I Did It
I wanted to use the most basic materials to create this catchment surface. That means that others would be able to easily replicate it plus it would keep the costs down if they are widely used materials.
Through proper planning, I was able to use the slope of our land to our advantage. The entire roof structure sits on cinder blocks and there aren’t any major modifications to the grade of the land or addition of more cinder blocks to create a slope.
Main Construction Materials
- 8′ 2x4s
- Standard 8″x8″x16″ cinder blocks
- 8′ x 2′ corrugated roofing panels
- 3″ deck screws
- Roofing screws
- Galvanized uncoated wire rope
- Wire rope clamps (like u-bolts)
- High tensile wire
Like I said earlier, I wanted to use materials that are easily accessible. Everything was purchased at either Home Depot or Lowe’s.
Just from looking at the photos, you can see how basic the construction is. I generally laid out the cinder blocks and then found their specific position by laying out the 8′ 2x4s. I added more cinder blocks and 2x4s on the one end where the gutter would be attached to the framing to improve the strength of it.
One modification I had to make to the lumber was that the pieces that are going up the hill away from the camera in the photo above, is that I cut them about 2-3″ shorter. When you butt a 8′ piece of lumber up against each other and put an 8′ roofing panel over top, it doesn’t leave any room for the roof to overhang itself. By cutting the lumber short, you’ll have a few inches of over hang.
Anchoring The Roof
Another concern with this roof is keep it secured to the ground. I took a few preventative measures to ensure it doesn’t fly away. I secured the wood frame to the cinder blocks at about 50 points using wire rope and wire rope clamps (in the videos embedded above, I think I said a more specific number). This adds more weight to the roof. I also buried a number of dead man anchors using cinder blocks and high tensile wire (leftover from doing our fence). There was about 15 or so of these anchors that I attached to the roof framing.
The roof has been in place for about 7 months now and have not had any issues even through our monsoon season where we get some very high winds.
The roof dimensions are 72′ long by about 39′ tall. The dimensions worked well with the space I had and I only had to remove a few small trees. I also strategized such that if I wanted to add more catchment surface, I can do so by adding more roof surface going up the hill. I maxed out the length at 72′ long and that is where the gutter is located.
I had a company in Tucson called, Southern Arizona Rain Gutters do a seamless gutter install. They did gutters for our tiny house, awning, shed and the rain roof. Since rainwater harvesting is so critical out here for us, I wanted to have seamless gutters to ensure we wouldn’t be losing water through any seams.
The Transfer Pipes
From the gutter there are two 4″ PVC drain pipes going to our collection tanks. It’s recommended that for every 1500 sq ft of catchment surface, you want a 4″ pipe to transfer the water to your tanks. This just ensures that you won’t lose any water from an overflow at the gutter.
You might be thinking at this point as to how the water transfers in to the tanks by going up the pipe. This is just through the elevation difference between the gutter (inlet point) and the outlet in to the poly tanks. When we put the tanks in place, I had a 4′ hole dug out so that they would be partially buried. This was so that there would be enough elevation difference between the tanks and the roof such that the roof is higher than the tanks.
This is what is called a ‘wet system’. There is always water in the 4″ pipes and as water enters the pipes from the roof, it pushes the water through gravity in to the tanks.
I also put some clean outs and wyes in the photo above if I ever want to drain the water out of the pipes.
I’m really happy with the performance of the tank thus far. We recently just went through our first monsoon season and we captured at least 10,000 gallons of water in a period of about 3 weeks. This will last us quite a long time and we’ll get some more rains throughout our winter.
Rainwater harvesting can certainly be done in a desert climate where we get between 11-13″ of rain per year.
Enjoy all the photos below!