Usable area, not roof area
A designer does not care how big your roof is. They care how many rectangles they can fit on it.
Plumbing vents, chimneys, skylights, dormers, hips and valleys all break a plane into fragments, and fire codes require clear setbacks at ridges and eaves for roof access. A large, complicated roof can yield less usable area than a small plain one.
This is why two houses on the same street get very different proposals.
Orientation changes when, not only how much
In the northern hemisphere a south-facing plane produces the most over a year. That is the familiar rule, and it is incomplete.
- South. Highest annual total. Best when you are paid a flat rate for everything you produce.
- West. Lower annual total, but output shifts into the late afternoon and evening. Under a time-of-use tariff that is when electricity is most expensive, so west can be worth more per kilowatt-hour.
- East. Morning-weighted. Useful for households with a morning load, and for pairing with a battery that then carries the evening.
- North. Rarely worth it in the US unless the pitch is very shallow.
Pitch, and the flat-roof exception
Most residential pitches between roughly 15 and 40 degrees are fine, and the difference between them is smaller than people expect.
Low-slope and flat roofs are a different job. Panels are usually tilted on frames, which means spacing rows so they do not shade each other, which means fitting fewer panels than the raw area suggests. Flat roofs also shed dirt and snow poorly, so soiling matters more.
Roof material decides the labour
Asphalt shingle is the straightforward case and most US racking is designed around it.
Tile is slower: tiles must be lifted, sometimes cut, and specific flashings used, and breakage is normal and should be priced in. Slate is slower still and unforgiving of foot traffic. Standing-seam metal is the outlier in the good direction, because clamps grip the seam and the roof is never penetrated at all.
Ask which flashing system is being used by name, and how many penetrations the design makes.
How the work runs
- Plane inventoryEvery usable rectangle measured, with setbacks and obstructions removed. This is the real constraint on system size.
- Shade measurementMeasured across the year, not judged by eye on the day of the visit.
- Structural checkThat the framing takes the added dead load, especially on older or long-span roofs.
- Attachment and flashing designWhere the holes go, and what keeps water out of them for twenty-five years.
- LayoutPanels placed to fill planes efficiently while respecting fire setbacks and access paths.
What moves the price
We do not publish a national average and present it as your price. These are the variables that separate two quotes for the same job — run your own numbers with the cost guides and calculators, and check what incentives you actually qualify for.
| Factor | Why it moves the number |
|---|---|
| Number of separate planes | Each plane needs its own rails and often its own wiring run. Four small planes cost more than one large one for the same panel count. |
| Roof covering | Tile and slate carry real labour premiums. Standing-seam metal can be cheaper because it needs no penetrations. |
| Pitch and height | Steep or two-storey work needs more fall protection and slows everything down. |
| Obstruction density | Vents and skylights that have to be worked around, or occasionally relocated. |
| Remaining roof life | Not a line item, but the largest hidden cost if it is ignored. |
What to ask before you commit
Take this list to the conversation. A professional who answers these directly is telling you a lot; one who deflects is telling you more.
- How much usable plane area did you find, and where?
- Which flashing system are you using, by name?
- How many roof penetrations does this design make?
- What did you measure the shade with, and across what period?
- What is the remaining life of my roof covering, in your assessment?
Common questions
Does my roof have to face south?
No. South gives the highest annual production, but west-facing arrays shift output into the late afternoon, which is worth more under a time-of-use tariff. East suits morning loads. North is rarely worth it.
Can solar be installed on a tile roof?
Yes, with tile-specific flashings and more labour. Some tile breakage during installation is normal and should be discussed and priced before work starts.
Will panels damage my roof or cause leaks?
A correctly flashed penetration should not leak for the life of the roof. Leaks around solar are almost always a flashing workmanship problem, which is why the workmanship warranty and who holds it matter.
How many panels will fit on my roof?
It depends on usable plane area after setbacks and obstructions, not total roof area. A complicated large roof often fits fewer panels than a plain smaller one.