Planning & safety
Is your roof solar-ready? Area, pitch, setbacks and panels
Solar panels last 25 years or more. Before they go up, the roof under them needs to last as long, have room after the setbacks and face the right way.
Four questions before panels
- Will the roof outlast the panels? Panels are warranted for 25 years and often last longer. A roof with less than 10 to 15 years left should be replaced first: removing and reinstalling an array for a reroof costs thousands.
- Is there room after the setbacks? Fire codes reserve clear paths on the roof.
- Does a face point the right way, at a workable pitch? Orientation and shade matter more than area.
- Can the structure carry it? Usually yes, but it is checked.
Remaining life
Start with the roof’s age against its expected life. Architectural shingles last about 25 to 30 years; a roof installed 15 years ago has 10 to 15 left, which is the borderline. The replacement planner estimates the window from the install date, material, climate and visible signs, and how long roofs last gives the ranges. Many homeowners reroof and add solar in the same year so the two clocks start together; repair or replace discusses that timing.
Setbacks: the space you cannot use
The International Fire Code and the residential code’s solar provisions (IRC R324), as adopted locally, keep paths on the roof clear so firefighters can walk it and cut ventilation openings. Typical requirements:
| Path | Typical width | Notes |
|---|---|---|
| At the ridge | 18 or 36 in | Depends on the array’s share of the roof area and on sprinklers |
| Eave to ridge access paths | 36 in | On one or more roof faces, depending on layout |
| Around vents and skylights | as required | Arrays cannot cover plumbing vents |
| At the eave | 6 to 12 in | Installer practice and wind |
Local amendments vary, so the building department or an installer has the final word. On a small roof face, setbacks can take a third of the area.
How many panels fit
Subtract the setbacks from the face, then fit the panels in rows in both orientations; the better one wins.
A 40 by 15 foot south face
36 in at the ridge, 18 in at each side, 12 in at the eave, 69 × 41 in panels of 400 W, 1 in gaps.
- Usable: 444 × 132 in
- Portrait: 10 across × 1 up = 10; landscape: 6 across × 3 up = 18
- 18 panels, 7.2 kW, covering 59% of the face
From the solar roof fit calculator. The face height is along the slope: the pitch calculator converts a plan measurement.
Orientation, pitch and shade
In the continental US, south-facing faces produce the most, east and west faces somewhat less, and north faces little except on low pitches. Pitches from about 4/12 to 9/12 (18 to 37 degrees) are close to ideal at most latitudes; flatter roofs use tilted racking. Shade from trees, chimneys and neighbouring buildings cuts output more than any of these. Production depends on all of it and on local sun; the National Renewable Energy Laboratory’s free PVWatts tool estimates it, and installers model it from satellite imagery. This site counts panels, not kilowatt-hours, and links no solar company.
Roof pitch explained converts between rise per 12 and degrees.
Weight and attachment
Panels and racking add roughly 2.5 to 4 pounds per square foot, much less than a change from shingles to tile. Most roofs carry it; the installer’s permit package includes a structural check, and older or modified framing may need reinforcement. The roof weight calculator shows the roofing load the panels add to.
Panels attach with flashed mounts screwed into rafters or the deck. Each mount is a penetration, flashed into the shingles like a vent; on metal roofs, clamps on standing seams avoid penetrations entirely, which is one reason standing seam pairs well with solar. Tile and slate need special mounts and careful handling.
Total roof area and the right faces
The total roof area, from the roof area calculator, is less useful for solar than the area of each good face. A hip roof spreads its area over four smaller faces with long setbacks along the hips; a simple gable with one long south face often holds more panels from less roof. Measure each candidate face separately.
Vents, skylights and the reroof
Plumbing vents, exhaust fans and skylights cannot sit under panels, and they often end up exactly where the array wants to go. During a reroof, many can be moved: plumbing vents rerouted through the attic to exit on a north face, bath fans vented through soffits, a ridge vent replacing scattered box vents. Doing it while the roof is open is far cheaper than working around them afterwards, and it can add several panels to a small face. Ask the roofer and the solar installer to look at the same plan before the reroof starts.
Ready-to-go checklist
- Roof with 15+ years left, or reroofed first.
- A face within about 90 degrees of south, mostly unshaded, at a workable pitch.
- Enough area after local setbacks for the system you want.
- Vents and skylights clear of the array, or relocated during a reroof.
- A structural check in the permit package.