Part I: Start HereChapter 1. Reading Your Site
Roof, Wall, or Ground? Choosing a Mounting Spot
Tradeoffs among roof pitch, a south wall, and a ground rack you can reach without a harness.
- Difficulty
- Beginner
- Cost
- Free
The sunny surface from the shade map still has to be a place you can build, flash, and fix. Roofs, south walls, and ground racks collect sun in different seasons and fail in different ways. Pick the place whose failure you can live with.
A roof is often the least shaded and the hardest to touch. A south wall is a strong winter heater and a weak summer collector, which is what space heat wants. A ground rack can sit at whatever tilt you choose, and it can also sit in the house's own shadow, in a snowdrift, or in the path of a mower.
Difficulty
- Level
- Beginner
- Time
- An hour with the site sketch
- Skills
- Comparing access, tilt, and water
Cost estimate
Free
Choosing is free. A ground rack adds footings and a trench. A roof adds flashing and, if the shingles are old, a new roof under the array. A wall collector adds two holes and a way to carry the weight.
Materials
| Item | Qty | Notes |
|---|---|---|
| Site sketch with true south and shade | 1 | A surface that fails the December shade test is not a candidate. |
| Notes on access | 1 | Can you reach it from the ground, from a short ladder, or only by walking the roof? |
| Notes on what is behind the surface | 1 | A living room, a shop, an attic full of wires, a yard with a buried line. |
Roof
A south roof tilted somewhere near your latitude is a common compromise for electricity across the year. Steeper faces favor winter. Shallow faces favor summer. You take the tilt the rafters already have unless you build a rack that fights the roof, and a rack that fights the roof catches more wind.
The costs that do not show up in a sun chart are penetrations, fire pathways, and the age of the roofing. Flash every hole the way the rest of the roof is flashed. Do not rely on caulk. Many fire codes want a walkway and a ridge setback left clear. If the shingles have only a few years left, recover the roof when the array goes on. Pulling an array off to reroof is a second project.
Snow slides off a steep roof and piles on whatever is below, including a lower roof, a door, and a ground rack. Plan the landing.
If you cannot walk that roof safely, it is not your first mounting spot. The safety guide is the test. Hiring out the roof work is a legitimate outcome.
Wall
A vertical south wall sees the low winter sun more squarely than a shallow roof does, and it sees little of the high summer sun. That is a good shape for a space-heating collector and a tolerable shape for a window heater. It is usually a weaker shape for maximum annual electricity than a tilted south roof.
The practical advantages are large. You can often build from a ladder or from the ground. Ducts or pipes into the room are short. You can see whether the glazing is dirty. The disadvantages are the wall itself: studs have to carry the weight, a fire separation between a garage and a house has to stay intact, and windows you still want cannot be covered. Water running down the siding has to be flashed over the top of the box, not into it.
East and west walls are for shade decisions. They are poor winter heaters.
Ground
A ground rack lets you choose tilt and aim, and you can work on it without a harness. That is why it is the friendliest place to learn. It needs a footing or an anchor that will not lift in wind, a clear southern sky the house itself does not block, and a path for pipe or wire that is buried deep enough for your climate and marked so the next shovel finds it.
Snow is the failure people miss: a rack set too low becomes a drift. Leaves and grass shade the bottom edge. People, balls, and animals can hit the glazing. A fence that protects the glazing can also shade it. Look at the winter altitude again after you draw the fence.
Theft and trip hazards are real for anything near a sidewalk. So is a permit, which some towns treat as a structure.
A short decision
Choose the ground if you are learning and the yard is actually sunny. Choose the wall if the project is winter heat and the wall faces near true south. Choose the roof when the goal is a lot of annual electricity, the roof is sound, and someone qualified will do the attachment and the electrical work. Do not choose a roof only because it is up high and therefore seems serious.
| Place | Best at | Watch for |
|---|---|---|
| South roof | Annual electricity, a tilt you already have | Falls, flashing, old shingles, fire pathways, wind |
| South wall | Winter heat, short ducts | Weight, fire separations, summer output |
| Ground rack | Learning, a tilt you choose | Shade from the house, snow, uplift, the trench |
Cross off surfaces that fail December shade
Use the shade map. A pretty roof in the shade of a cedar is not a candidate.
Cross off surfaces you cannot reach safely
If the only access is a steep wet roof, pick another surface or hire the work.
Match the season
Winter heat likes a wall or a steep south face. Annual electricity likes a clear tilt nearer your latitude.
Write the water and the wire path
A perfect face is no use if you cannot get the energy indoors.
Further reading
These links go to outside sources. This book cites them by link and does not copy their words or pictures.
- https://www.builditsolar.com/SiteSurvey/site_survey.htm
- https://www.builditsolar.com/SiteSurvey/FindingSouth.htm
- https://www.builditsolar.com/SiteSurvey/MagDecl.htm
- https://www.builditsolar.com/SiteSurvey/StepByStep.htm
- https://www.builditsolar.com/SiteSurvey/challenges.htm
- https://www.builditsolar.com/GettingStarted/GettingStarted.htm
- https://www.builditsolar.com/GettingStarted/ChoosingProjects.htm
- https://www.builditsolar.com/References/SunChartRS.htm
General education, not a permit or a stamped design. Follow the safety note in the guide, read the safety disclaimer, and hire a licensed trade when the work requires one.