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Design the roof for rainwater catchment and solar PV

On an off-grid estate the roof is your water catchment and your power station. Its shape, pitch, material and orientation must be agreed with the water and power designs before the structure is fixed.

How

1. Get the target catchment area from the water module and the target PV array size and design tilt from the power module (power-4 finds the tilt that best serves the worst month).
2. Give the roof one large, unshaded plane facing the equator for PV. Avoid dormers, vents, flues and chimneys on that plane.
3. Keep catchment roofs simple, with few valleys, and route all of them to gutters that reach the tank location shown on the water plan.
4. Choose a roofing material suited to drinking-water catchment; check the water module and your local guidance before using coatings or treated materials.
5. Make the roof structure strong enough for panel racking, maintenance access and, in cold climates, snow load (design-13).
6. Generous overhangs protect earth and straw walls from rain (Your Home, HUD), so do not trim eaves to save money.
7. In dry climates HUD advises shed roofs slope at least 2:12 to drain.

Do these first

• power-1
• water-1

Sources

1. Straw bale. Your Home, Australian Government, accessed 2026-11
2. Southwest Housing Traditions, Chapter 3: Adobe, Rammed Earth and Straw Bale Wall Systems. US Department of Housing and Urban Development (HUD User), accessed 2026-11
3. Cool Roofs. US Department of Energy, Energy Saver, accessed 2026-11

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