Solar Roof Certification Letters That Pass Plan Check

For residential solar, the structural letter is usually the shortest document in the permit set and the one most likely to hold it up. A structural engineer reviews the existing roof framing, checks it against the added loads, and stamps a solar roof certification letter the AHJ can accept. Done well, it is invisible. Done poorly, it means a correction notice, a site revisit and a customer asking why their install date moved.

This article covers what the letter certifies, when you need one, what the engineer actually checks, and how developers, EPCs and installers can get letters back in 24 hours. For the broader code framework, see our solar structural engineering guide. For our full residential scope, see residential solar structural engineering.

What a Solar Structural Letter Certifies

A solar roof certification letter (also called a structural letter or roof letter) is a signed and stamped statement that:

• the existing roof structure has been evaluated for the proposed PV system,
• it meets the governing code under a stated basis, for example the 2021 IRC with ASCE 7-16 loads, and
• any required reinforcement is shown in the attached details.

The letter should name the code editions, the design loads used (dead, roof live, snow and wind) and the framing assumptions. A letter that just says "adequate" without a basis invites plan check comments.

When AHJs Require a Letter

Requirements vary by jurisdiction, but most fall into three groups. For a full breakdown, see do I need a structural engineer for solar panels.

Prescriptive or Automated Paths

SolarAPP+ lets participating AHJs issue permits automatically when a project meets eligibility rules. Its structural criteria include a PV system and hardware weight of 4.0 psf or less, flush-mounted arrays no more than 10 in. above pitched roofs, no wood shake roofs, and no structural modifications. Local AHJs add their own limits, such as maximum ground snow load or wind speed. California's Solar Permitting Guidebook offers a similar simplified structural checklist for qualifying one- and two-family dwellings.

State-Level Deemed-Adequate Rules

Some states amend IRC R324 to deem roofs adequate under defined conditions. Washington, for example, accepts roofs without engineering where ground snow is 70 psf or less, the PV dead load is 4 psf or less, modules sit 18 in. or less above the roof and no point load exceeds 50 lb, among other conditions.

Engineered Letter Required

Everywhere else, or whenever a project falls outside the prescriptive limits, a stamped letter is required. That includes tile roofs, roofs with multiple reroof layers, low-slope roofs in snow country, older homes with nonstandard framing, and any roof that needs reinforcement.

What the Structural Engineer Checks

Load Cases

IRC R324.4 requires two gravity checks: dead load including the PV system plus snow load, and dead load without the PV system plus roof live load or snow, whichever is greater. Wind uplift on the array and attachments is checked separately using ASCE 7 component and cladding pressures with the Section 29.4.4 factors for flush-mounted arrays.

Dead Loads

The existing roofing matters as much as the array. Composition shingle is light. Concrete and clay tile can weigh several times more, and each reroof layer left in place adds weight. PV systems typically add 3 to 4 psf on flush-mounted residential arrays. We use the actual roofing and layer count reported from the field.

Rafters and Trusses

• Rafters: bending, shear and deflection using actual size (a 1950s 2x6 is often larger than a modern one), spacing, span and species, with NDS adjustment factors such as load duration and repetitive member.
• Manufactured trusses: the top chord is checked for the added load between panel points. Damaged, altered or notched truss members are flagged. Truss modification always requires engineering.
• Existing condition: sag, cracked members, previous fire or water damage and missing collar ties all change the answer.

Attachments

Each attachment must resist uplift through lag screw or structural screw withdrawal into the rafter or truss, checked with NDS withdrawal values and the wind load duration factor. Attachment spacing is set by the lower of the racking manufacturer's rail span and the withdrawal capacity at the governing wind zone. Edge and corner zones usually need tighter spacing.

The IEBC 5% Gravity Rule

When a home is altered under the IEBC, Section 503.3 requires existing elements whose gravity load increases by more than 5% to meet current code. The 2024 IEBC exempts many small Group R buildings of light-frame construction from this threshold. Where it applies, the engineer must show the framing meets current code with the PV system in place, not just that the increase is small.

When Reinforcement Is Required

Reinforcement should be the result of a calculation, not a default. When framing does not work, common fixes include:

• sistering a new member alongside an overstressed rafter,
• adding blocking or a mid-span strut to a bearing wall below,
• tightening attachment spacing so loads spread over more rafters,
• relocating the array to a stronger roof plane.

We design the minimum fix that works and show it on a one-page detail your crew can build.

Roof Types That Need Extra Attention

Tile Roofs

Concrete and clay tile add significant dead load before the array goes on, and tile hooks or replacement flashing change how loads reach the rafter. Tile roofs rarely qualify for prescriptive paths, so an engineered letter is the norm.

Low-Slope Residential Roofs

Flat and low-slope roofs on homes may use tilted or ballasted racking. Those systems bring higher wind demand at edges, possible snow drift between rows and, in seismic regions, the rooftop array provisions of ASCE 7 Section 13.6.12.

Older Homes and Additions

Homes built before modern span tables often have rough-sawn lumber, wider spacing or framing that changes direction at additions. Field data from each roof plane matters. One undersized plane can change the array layout.

Snow Country

In jurisdictions under the 2024 codes, ground snow loads come from the ASCE 7-22 reliability-targeted maps, and some cities have seen their residential design snow loads rise as a result. A letter should state the snow load basis so the plan checker can confirm it matches local tables.

Batteries and Inverters

Home battery and inverter installations add wall-mounted weight and seismic anchorage demands. Wall anchorage into studs or masonry, and floor-mounted battery anchorage in seismic regions, can be included in the same structural package so the permit goes in once.

Common Plan Check Comments, and How to Avoid Them

• "Provide framing size, spacing and span." Include them on the plan set and in the letter.
• "State the code basis." Name the IRC or IBC edition and the ASCE 7 edition.
• "Show attachment spacing by wind zone." Edge and corner spacing must match the calculations.
• "Address reroof layers." State the number of layers used in the dead load.
• "Letter does not match plan set." Module count, layout and racking must match exactly.

What to Send for a 24-Hour Letter

Dynamo returns residential structural letters within 24 hours of a complete submittal. Complete means:

• the prepared PV plan set with array layout and roof planes,
• module and racking specifications, including attachment type,
• framing information: rafter or truss size, spacing and span, and roofing type with number of layers,
• attic and roof photos showing framing, plus any visible damage.

For residential developers and EPCs running volume, a standard submittal format and a consistent design basis across jurisdictions keeps letters consistent across 100s of AHJs. For installers, it means fewer site revisits and faster install dates.

Missing framing data is the most common cause of delay. We flag gaps the same day.

What Drives the Cost of a Structural Letter

Pricing depends on roof complexity, the number of roof planes, whether reinforcement design is needed, and monthly volume. Developers, EPCs and installers with steady volume get per-project pricing. Submit a project for a quote.

Frequently Asked Questions

How much does a solar structural letter cost?

It depends on roof complexity, whether reinforcement is required and your monthly volume. Steady installers, EPCs and developers get per-project pricing. Submit a project for a quote.

Can I install solar without a structural letter?

Only where the AHJ allows a prescriptive or automated path, such as SolarAPP+ or a state deemed-adequate rule, and the project meets every condition. Otherwise a stamped letter is required.

What happens if my roof fails the structural check?

We design the minimum reinforcement needed, such as sistering, blocking or tighter attachment spacing, and include it as a stamped detail with the letter.

Can solar go on manufactured trusses?

Yes, in most cases. The top chord is checked for the added load between panel points. Trusses must not be cut or altered without engineering.

How fast can I get a residential solar structural letter?

Within 24 hours of a complete submittal: plan set, module and racking specs, framing information and photos.

This article is general technical guidance, not engineering for a specific project. Code editions and local amendments vary by jurisdiction.