Rooftop Solar Structural Assessment: Load Calcs

Last updated: November 2026

A roof that looks fine from the street can still fail under a new array. Rafters, fasteners and wind uplift decide that, not the shingles. A rooftop solar structural assessment is the engineering work that shows whether the building can carry the array for its whole life without sagging, leaking or losing panels in a storm.

This guide gives a load summary table, the governing codes and a seven-step process with a worked rafter check. It also covers roof-type notes and where engineering is required. See the solar mounting structure design guide for racking detail.

Load Quick Reference

The values are screening numbers, which means rough first checks. The engineer’s calculation uses the project’s actual loads and members.

What a Rooftop Solar Structural Assessment Verifies

A structural assessment confirms three things about the existing building. First, the roof framing has spare capacity for the new dead load (the permanent weight of the array). Second, every member and connection passes the governing load combination (the worst mix of loads that applies). Third, the attachment hardware carries force in an unbroken path from the panel clamp into the framing.

It differs from a site survey and a roof inspection. A survey records dimensions, obstructions and tilt. An inspection checks waterproofing and roofing wear. The structural assessment is the calculation layer on top of both. It is the document the building department wants for the permit.

An array changes the roof in four ways. It adds dead load. It concentrates load at each attachment point. It changes how wind loads the roof, especially at edges and corners. And it changes how snow drifts and sheds. The accepted output depends on the authority. It can be a stamped calculation package, a manufacturer or engineer letter for a pre-engineered system, or a simplified checklist. A stamped package is sealed by an engineer. A pre-engineered system is racking the maker has already designed for set loads. See the glossary entries on structural letter and load analysis.

The Four Load Categories Every Assessment Quantifies

ASCE 7 sorts the loads on a rooftop PV array into dead, live, wind and snow loads. Seismic (earthquake) load is added where the site requires it. Each load is calculated separately. The worst-case combination governs. ASCE 7 is published by ASCE, the American Society of Civil Engineers. ASCE/SEI 7-22 is the edition listed on ASCE’s ASCE 7 page.

Dead Load

Dead load is the permanent weight of the array. NREL’s Solar Ready Buildings guide puts PV panels and racking at about 3 pounds per square foot of collector area. Ballasted mounting (weights that hold the array in place) is 4 to 6 pounds. A 400 W panel of about 21 square feet is only part of that weight. Rails and clamps add weight too.

Dead load is not even across the roof. Brackets create point loads at each attachment. A point load is weight pressing on one small spot. If attachments land on every other rafter, each loaded rafter carries twice the load per square foot of the array.

Live Load

Live load is the temporary load from people on the roof. Under the array, it depends on the code and the clearance below the panels. In California, the California Solar Permitting Guidebook says roof live load is not imposed where there is less than 42 inches clear under roof-mounted panels. Roof surfaces not covered by panels keep the roof live load. Check the rule your jurisdiction applies.

Wind Load

Wind pushes down on the array and also lifts it. This lift is called uplift. Uplift often governs, which means it decides the design, because panels are light. Wind pressure depends on the basic wind speed, building height, exposure category, the position of the array on the roof and the racking manufacturer’s test data. Use the ASCE Hazard Tool for site values under the ASCE 7 edition your code adopts. Exposure categories run from B to D. B means suburban areas with closely spaced obstructions. D means flat, unobstructed areas such as coastlines. The higher categories give higher pressures.

Snow and Seismic Load

Snow load starts from the ground snow load for the site. It is then adjusted for roof slope, exposure and thermal conditions. Panels can shed snow onto lower roof areas. They can also trap drifts along racking rows. So the engineer checks drift as well as uniform load. Seismic load matters where ground shaking is a design concern. The added mass raises the sideways (lateral) demand on the whole load path from rafter to foundation. A load path is the route a force takes through the structure to the ground. The California guidebook calls for seismic and snow design where they apply. It requires the applicable load combinations of CBC 1605 (the California Building Code).

Codes That Govern the Calculations

In the US, the International Building Code, the International Residential Code (IRC) and ASCE 7 govern rooftop PV structure. Each jurisdiction adopts a specific edition. The 2021 IRC provides a deemed-adequate exception for rooftop PV. Deemed-adequate means the code accepts the roof without a separate structural analysis. No structural analysis is needed if all of these are true. The system is designed for wind and installed to the manufacturer’s specification. Ground snow load is 70 psf or less. Total dead load is 4 psf or less. Modules sit no higher than 18 inches above the roof. Supports are spaced so no point load exceeds 50 pounds. That wording is as printed in the MyBuildingPermit.com Construction Tip Sheet 24 (14 April 2025, with Washington State amendments). So check your own jurisdiction’s adopted text.

Outside the US, structural loads follow national standards and approval processes. See Europe solar compliance and solar permit requirements in Europe for the European context.

Step-by-Step Structural Assessment Process

The assessment follows a fixed sequence. Skipping a step usually shows up as a permit comment later.

Step 1: Collect Building Documentation

Start with the original construction drawings. Framing plans show rafter or truss size, spacing and span. A truss is a ready-made frame of triangles. Span is the distance a member crosses between supports. Permit history shows past roof replacements, additions or equipment that changed the load. If drawings are missing, a site survey recovers member dimensions (the sizes of beams and other framing pieces). A pre-1960 wood-framed building can be calculated on field-measured lumber sizes, as the California guidebook notes.

Step 2: Inspect the Roof on Site

Inspect two layers. These are the roofing surface and the framing under it. Note sagging, ponding on flat roofs (standing water), flashing wear and structural distress. Flashing is the metal that seals roof joints. Record the rafter or truss type, spacing, span and condition. Look for rot, splits, corroded fasteners and notched members. California’s checklist asks if the roof is a single layer without a reroof overlay (a second layer of roofing). It also asks if the roof shows alterations, deterioration or sagging. A LiDAR roof model or drone model speeds up capture of the roof shape. LiDAR uses laser scans. But neither replaces the attic inspection that confirms the framing.

Step 3: Calculate Existing Framing Capacity

Check each rafter, truss, beam and connection against the loads the code prescribes. Check for bending, shear and deflection. Bending is how much a beam flexes under load. Shear is the sliding force inside it. Deflection is how far it sags. The Sandia report Structural Considerations for Solar Installers (SAND2011-9066) gives span tables and a worked example. Sandia is a US national laboratory. Its rafter table is for Douglas Fir-Larch No. 2 (a wood species and grade) at 2 ft spacing. It assumes 20 psf dead and 30 psf live load, 50 psf in total. It allows a 2x8 to span 12 ft (at a modulus of elasticity of 1,700,000 to 1,800,000 psi). The modulus of elasticity measures how stiff the wood is, and psi means pounds per square inch.

Worked example: a residential rafter check. This is a hypothetical screening check, not a design.

The allowed span scales with the inverse square root of load. That means span falls as load rises. This is because bending demand grows with load times span squared. The check keeps the full live load. This is the conservative case (it errs on the safe side), because California’s simplified criteria assume the array displaces roof live load. A stamped calculation can show the rafter is adequate. It does this by treating live load, load sharing and the actual lumber grade properly. If the rafter still fails, the options are a 2x10, a lighter array, or reinforcement. The Sandia table lists 15.5 ft for a 2x10 at 1,800,000 psi. And 15.5 × √(50 ÷ 53) = 15.05 ft, which clears 12 ft.

Step 4: Determine Design Loads from the Array

Work out the new design loads. Dead load comes from panels, racking and ballast. Live load applies in some cases. Wind load depends on the site, roof zone and racking. Snow load includes drift. Seismic load applies where required. The racking manufacturer’s test data refines the wind uplift for ballasted systems.

Step 5: Combine Loads and Check Limit States

A limit state is a condition where a part would fail or become unfit for use. Apply the load combinations of the governing code. Compare each combination with the member and connection capacity at every point of the load path. This includes rafter bending, shear, deflection, sheathing (the boards on top of the rafters) and the roof diaphragm (the roof deck acting as one stiff plate). A passing assessment shows capacity above demand at every member, connection and load path. Demand is the load a part must carry. Capacity is the load it can carry. A failing assessment names the members that fall short.

Step 6: Design the Attachment and Document the Load Path

Specify the clamp, rail, standoff, fastener, embedment, flashing and attachment spacing. The standoff is the post that lifts the rail off the roof. Embedment is how deep the fastener goes into the wood. Every load must pass from the panel clamp to the rail, the standoff, the rafter and on to the structure. A break at any connection invalidates the calculation.

Withdrawal capacity is how much pull a fastener can take before it comes out of the wood. It depends on the fastener, the thread depth and the wood species. Sandia’s Table 10 comes from the 2005 NDS. NDS is the National Design Specification for wood construction. The table gives capacity per inch of thread depth for a 5/16 inch lag screw (a large wood screw). It is 266 lb in Douglas Fir-Larch, 307 lb in Southern Pine and 205 lb in Spruce-Pine-Fir. With 2.5 inches of thread in the rafter, that is 665 lb, 767.5 lb and 512.5 lb before adjustment factors (266 × 2.5, 307 × 2.5, 205 × 2.5). The table requires the lag screw in the middle third of the member. It notes the values do not cover wet service or shear.

California’s checklist also uses a simplified downward-load check. Take a PV array of 4 psf or less on a roof from flat to 6:12 slope. Slope is written as rise to run. Table 1 lets anchors sit up to 6 ft apart horizontally on 24 inch rafters. It lets them sit 5 ft 4 in apart on 16 inch rafters. For roofs from 7:12 to 12:12, it limits them to 2 ft on 24 inch rafters.

Step 7: Produce the Stamped Report

The report normally contains these items. First, the project address and authority. Second, the design assumptions (code edition, exposure, wind speed, snow load, seismic category). Third, member and connection calculations and the combined load check. Fourth, attachment drawings and fastener specifications. Fifth, the racking manufacturer’s data. Sixth, a photo log of existing conditions. Last, the engineer’s seal where the authority requires one. Confirm early whether the authority takes a digital seal or a wet stamp (a physical ink stamp).

Roof Type and Material

The same array on different roofs gives different calculations.

• Asphalt shingle: lag screws go through the deck into the rafter. Flashing seals the hole. Published span tables apply directly to simple framing.
• Concrete and clay tile: tile hooks or replacement tiles carry the standoff. The fastener still goes into the rafter. Tile breakage during installation is a routine concern.
• Standing-seam metal: clamps grip the raised seam between metal sheets, so no hole in the roof is needed. The check focuses on seam spacing, the clamp manufacturer’s ratings and the framing beneath.
• Membrane and TPO flat roofs: TPO is a common plastic roofing membrane. These roofs usually use ballasted racking to avoid cutting the waterproofing. NREL puts ballasted systems at 4 to 6 psf. Wind uplift often governs. The deck must carry the added ballast.
• Concrete decks: these have high capacity. The check moves to the anchor pull-out and punching shear (a load pushing through the slab) at the anchors.
• Trusses, I-joists and solid rafters: the framing type matters as much as the surface. I-joists are engineered wood beams shaped like the letter I. Attach to the top chord of a truss (its top beam), not to a web member (a diagonal piece inside it). Keep concentrated loads over bearing points on I-joists. Bearing points are where the joist rests on a support. California’s checklist asks the installer to record whether the roof framing is rafter or manufactured truss.

Identify the framing type before specifying attachments. A roof that looks the same from above can have any of these systems underneath.

When a Roof Fails Assessment: Reinforcement and Alternatives

A failed assessment leaves three options: reinforce the roof, reduce the array, or move to a ground-mount system.

Get a quote for each option from a contractor. Costs depend on framing type, access and local labor. No published range is quoted here. Reducing the array lowers production in proportion, so rerun the energy model.

Where Structural Engineering Is Required (US Examples)

Requirements differ by state and by authority within a state. Municipal amendments override state defaults. The table covers the two sources read for this update and the model code they reflect. Check your own authority for other states.

The California simplified criteria assume four things. The area has zero snow load. The site is not in Wind Exposure D (within 200 yards of the ocean or a large coastal bay). The design wind speed is a standard 110 mph for open-terrain sites. The array displaces roof live load that the roof was designed to carry. The table is a starting point, not a state ranking.

Common Failure Modes That Stall Permits

These are the reasons a structural package gets sent back. Each has a direct fix.

1. Wrong code edition. Confirm the ASCE 7 and building code edition with the AHJ before the engineer starts.
2. Missing continuous load path. Trace every load from clamp to rail to standoff to rafter to the structure on an attachment detail.
3. Wrong wind exposure category. Assess the actual terrain, using the exposure criteria of the ASCE 7 edition in force.
4. Wrong pressure zone. Roofs divide into interior, perimeter and corner zones with different pressures. Map the array to the zones.
5. Missing snow drift analysis. Do not apply uniform snow load only.
6. Incomplete fastener specification. State diameter, length, embedment and material. The California checklist uses 5/16 inch lag screws with 2.5 inches of embedment as its simple case.
7. Outdated wind tunnel data. Confirm the racking vendor’s wind data is current for the code in force.
8. Ignored roof age and condition. Document existing conditions and adjust the analysis.
9. Wrong framing assumption. Confirm rafter, truss or I-joist in the attic before specifying attachments.
10. Missing seismic analysis where required. Check whether your jurisdiction needs it.

Software and Structural Assessment

Design software captures geometry and layout. The engineer does the structural calculation, either by hand or with engineering software. A solar design tool helps by producing a dimensioned roof model, a module layout and an equipment list that the engineer can review. SurgePV’s scope is roof modeling, layout, shading, yield, financial modeling, electrical workflow support, bill of materials and proposals. Its solar proposal software packages the project data for the customer. Its outputs do not replace approval by the responsible engineer or authority. See solar design software and Arka 360 vs SurgePV for a comparison. SurgePV publishes this guide and sells one of the tools mentioned in this section.

For warehouse and commercial roofs, see the warehouse rooftop solar design guide. For high-wind sites, read high wind zone solar mounting.

How to Buy Solar Structural Engineering Services

Ask for a written design basis before you compare fees. A design basis is a written list of what the engineer will use and deliver. It should name the structure and mounting system. It should list the survey and record-drawing inputs, material condition and governing load criteria. It should also list equipment loads, attachment assumptions, the analysis method, the required calculations and drawings, and whether a professional seal is included.

The provider should name the engineer responsible for regulated work. It should also name the jurisdictions and disciplines (fields of engineering) in which that person is currently licensed. Check credentials with the relevant licensing authority. Have a qualified reviewer check load paths, combinations, member and connection capacity, attachment details, assumptions, and limitations. The reviewer should also check that the calculations match the drawings. Record missing field information as an assumption, or start a site investigation. Never guess silently.

Structural analysis does not certify concealed conditions, workmanship, waterproofing, or as-built compliance (that the finished work matches the plans). The exception is when those inspections are explicitly included. The installer or EPC must coordinate electrical and fire-access requirements and obtain the authority’s acceptance. EPC means engineering, procurement and construction, the firm that designs and builds the system. To buy a complete plan set, see the solar permit design services guide.

Disclosure: SurgePV is promoting Heaven Designs through a commercial relationship. Apply the same license verification, sample review, paid-pilot, and independent QA process to every provider.

Review the Heaven Designs rooftop engineering service, visit the Heaven Designs website, and request a structurally relevant sample before seeking a quote.

Conclusion

A structural assessment is the engineering check that decides whether a roof can carry an array. It goes smoothly when the documentation is complete and the code edition is confirmed.

Three action items for any new project:

• Call the AHJ before the engineer to confirm the code edition, the document type it accepts (full calculation, letter or simplified form) and the seal format.
• Pull the as-built drawings first, because the framing plans are what the engineer needs.
• Do the load pre-check early. The worked rafter example above takes minutes and shows whether the roof is marginal before you quote.

When the roof geometry is settled, book a SurgePV demo to see how the layout and bill of materials reach your engineer.

Frequently Asked Questions

What is a rooftop solar structural assessment?

It is the engineering evaluation that confirms an existing roof can carry the added weight, wind uplift and snow of a planned solar PV system. It combines four parts: a document review, a site inspection, framing capacity calculations and a load combination check. The check uses the ASCE 7 edition your local code adopts. The output is a calculation package, letter or checklist that the permit office accepts.

How much weight does a solar system add to a roof?

NREL’s Solar Ready Buildings guide puts PV panels and racking at about 3 pounds per square foot of collector area. It puts ballasted systems at 4 to 6 pounds. Simplified residential reviews, including California’s checklist, use 4 psf as the limit. Attachments concentrate that weight at points. So the assessment checks individual members.

Do I need a structural engineer for residential solar?

It depends on the authority. Under the 2021 IRC deemed-adequate exception, no structural analysis is needed if all of these hold: 4 psf or less, 70 psf ground snow or less, 18 inches height or less and a 50 lb point-load limit. California’s guidebook lets a checklist replace calculations for qualifying flush-mounted arrays. It requires an engineer’s stamped calculations when any item fails. Ask your building department which applies.

What roof age is too old for solar panels?

There is no single age limit. A roof should have enough remaining life to outlast the array. Otherwise replace it first, because removing and reinstalling the array costs money. California’s checklist asks for a single roof layer without signs of alterations, deterioration or sagging. Ask a roofer for the remaining life of the covering. Ask the engineer to check the framing.

What is the difference between a PE stamp and a structural letter for solar?

A PE stamp is the seal of a professional engineer. A stamped calculation package is a sealed set of calculations and attachment details for the specific project. A structural letter is a shorter document in which an engineer states that a pre-engineered racking system suits the site. Which one the authority accepts varies, so confirm with your building department.

How long does a solar structural assessment take?

It depends on the engineer, the drawings available and the project size. Ask the engineer for a turnaround estimate in writing. Pull the as-built drawings first, since the engineer needs the framing plans to start. As-built drawings show the building as it was actually built.

What happens if a roof fails the structural assessment?

There are three responses. You can reinforce the affected rafters or trusses. You can reduce the array on that roof plane. Or you can move to a ground-mount system. In the worked example above, switching to a 2x10 rafter clears the screening check. Get a contractor quote for each option before choosing.