Quick answer
Often, yes. Many building departments ask for a letter from a licensed engineer confirming that the existing roof framing can carry the added weight of the solar array and resist wind uplift at the attachments. Older homes, long rafter spans, heavy tile roofs and areas with heavy snow or high wind are the most likely to need one, and some roofs need reinforcement before panels go on.
Solar installers usually handle the electrical design and the racking layout, but the structural question belongs to an engineer: can this roof safely support the system for the next 25 years?
When a solar permit needs a structural letter
- Your department requires one for every rooftop system, which many do
- The house is older, or the roof framing is undersized, damaged or undocumented
- Rafters or trusses span long distances, or the roof has been modified before
- Heavy roofing such as clay or concrete tile, or multiple layers of shingles
- High design wind speeds, heavy ground snow or a steep roof
- Ground-mounted or carport systems, which need foundation and frame design
Some jurisdictions, including many in California, use automated permitting tools for standard residential systems. Even there, homes outside the standard criteria still need an engineer’s review.
What the engineer checks
- Rafter or truss size, spacing, span and condition
- Added dead load from panels and racking, combined with roof live load, snow and wind
- Wind uplift at each attachment and the attachment spacing
- Load path down through walls to the foundation where loads are concentrated
What you get
A sealed letter or short calculation package stating that the existing framing is adequate for the proposed system, with the attachment spacing it relies on. If the roof is not adequate, you get a reinforcement detail, such as sistered rafters or added supports, that your installer can build and the department will approve.
Cost and turnaround
A structural letter for a typical residential array falls in our review and stamp tier, starting at $249 as a fixed fee. Most are returned in 48 to 72 hours once we have the information below. Installers with regular volume can ask about ongoing support. See pricing.
What to send us
- The solar layout or plan set showing array location and attachment spacing
- Panel and racking specification sheets
- Attic photos showing rafters or trusses with a tape measure for size and spacing
- Roofing type and number of layers
- Property address
Common questions
How much weight do solar panels add to a roof?
A typical rooftop array adds only a few pounds per square foot, but it concentrates load at the attachments and adds wind uplift, which is why the attachments and framing are checked together.
Can my solar installer write the structural letter?
Only if a licensed engineer on their team prepares and seals it. Departments that require a structural letter want an engineer’s seal.
What if my roof fails the check?
The engineer specifies reinforcement, usually straightforward work in the attic, and the letter is issued based on the reinforced framing.
Do I need a new letter if I add more panels later?
Usually yes, because the loads and layout change.