For many tiny house dwellers, “Water Independence” is the ultimate goal. You want to park off-grid and live off the sky.
While buying a tank and a pump is easy, designing your roof for optimal collection is often overlooked. The Pitch (Slope) and Shape of your roof dictate not just how much water you catch, but how clean that water is.
This guide explores the physics of roof angles for rainwater harvesting on a Tiny House on Wheels (THOW).
The Golden Rule: Metal is King
Before debating angles, we must settle the material. For rainwater harvesting, Metal Roofing (Standing Seam or Corrugated) is the only serious choice.
- Asphalt Shingles: Shed petroleum granules and grit into your water. Hard to clean.
- Cedar Shakes: Release tannins that turn your water brown (like tea).
- Metal: Slick, non-porous, and sheds debris instantly. It provides the cleanest “catch.”
The Pitch Debate: Steep vs. Shallow
Roof pitch is measured in “rise over run” (e.g., 4:12 means the roof rises 4 inches for every 12 inches of horizontal length).
1. The Steep Pitch (6:12 to 12:12)
- The Pro: The “Self-Cleaning” Effect. Gravity moves water so fast that it scours the roof surface. Leaves, bird droppings, and pollen are washed into the first-flush diverter almost immediately.
- The Con: Height Penalty. On a tiny house, you are limited to 13′ 6″ total height. A steep roof eats up precious vertical space, drastically reducing the headroom in your sleeping loft.
2. The Shallow Pitch (1:12 to 3:12)
- The Pro: Loft Space. A flatter roof maximizes interior volume. You get a comfortable sit-up loft while keeping the exterior low.
- The Con: The “Debris Trap.” Water moves slowly. Leaves tend to stick, and pollen settles in a thick paste that doesn’t wash off easily. This organic matter can rot on the roof, feeding bacteria into your tank.
- The Risk: If you have horizontal corrugated roofing on a shallow pitch, the ribs act as dams, holding water and dirt.
3. The Sweet Spot for THOWs (3:12 to 4:12)
This is the compromise. It is steep enough to shed bulk water and prevent pooling, but shallow enough to allow for a functional loft.
- Maintenance: With this pitch, you will need to manually brush/wash your roof with a broom periodically if you are parked under trees.
Roof Shape: Shed vs. Gable
The shape of your roof determines the complexity of your plumbing.
Option A: The Shed Roof (Single Slope)
This is the gold standard for rainwater collection.
- Efficiency: 100% of the rain that hits your house is directed to one single gutter.
- Plumbing: You only need one downspout and one first-flush diverter. It is simple, elegant, and fewer parts mean fewer leaks.
- Setup: Perfect for a “high side” wall full of windows and a “low side” for services.
Option B: The Gable Roof (Classic Peak)
- Inefficiency: The rain is split 50/50.
- Plumbing Nightmare: You need gutters on both sides of the house. To catch 100% of the water, you have to plumb the two downspouts together (often running a pipe under the trailer) to feed a single tank.
- Result: Most gable-roof tiny house owners only gutter one side, instantly losing 50% of their potential water yield.
Calculating Your Yield
Does the pitch change the amount of water you catch? Technically, no. It is the “footprint” of the roof that matters, not the surface area of the slope.
- The Formula: 1 square foot of footprint x 1 inch of rain ≈ 0.623 gallons.
- Example: A 8.5′ x 24′ tiny house (204 sq ft footprint) in a 1-inch rainstorm.
204 x 0.623 = 127 Gallons
- Whether your roof is flat or steep, if it covers 204 sq ft of ground, it catches the same volume of rain (assuming no wind loss).
Conclusion
For the ultimate off-grid water setup, design a Shed Roof with a 3:12 or 4:12 pitch using Standing Seam Metal. This combination gives you a single collection point, decent self-cleaning properties, and enough interior headroom to live comfortably while filling your tanks.