Roof Solar Structural Input Form
Example Data Table
| Case | Modules | Added Load | Wind Speed | Attachments | Review Note |
|---|---|---|---|---|---|
| Small flush array | 16 | 3.2 psf | 105 mph | 24 | Often controlled by uplift. |
| Medium pitched roof | 32 | 3.0 psf | 115 mph | 48 | Check both roof reserve and anchors. |
| Ballasted low slope roof | 40 | 7.8 psf | 120 mph | 20 | Gravity load may control. |
Formula Used
Roof plane area = roof length × roof width × slope factor.
Slope factor = 1 ÷ cos(roof slope in degrees).
Module count = rows × columns.
Array area = module count × module length × module width.
Total added dead load = module weight + racking weight + ballast weight.
Added load psf = total added dead load ÷ array area.
Service roof demand = existing dead load + snow load + added load psf.
Velocity pressure = 0.00256 × exposure factor × wind speed².
Net uplift pressure = velocity pressure × net pressure coefficient.
Uplift per attachment = total uplift × safety factor ÷ attachment count.
How to Use This Calculator
Enter roof dimensions, slope, capacity, and existing loads.
Add local snow load, wind speed, and exposure values.
Enter module layout, module size, and component weights.
Add total attachment count and allowable uplift per attachment.
Press Calculate to view roof demand and attachment demand.
Use CSV or PDF buttons to save the calculation record.
Structural Checks Matter
A roof solar array adds useful energy value. It also adds permanent weight. Wind can pull the array upward. Snow can increase the downward load. A basic calculator helps screen these demands before detailed design. It does not replace a licensed engineer. It gives owners, electricians, and estimators a clearer first review.
What the Calculator Reviews
The tool starts with roof size, roof slope, and roof rated capacity. It then reads module count, module size, panel weight, racking weight, and ballast. These values create the added dead load. The calculator compares this load with the remaining roof capacity after existing dead load and snow load are included. This helps show whether the roof has a positive reserve.
Wind and Attachment Demand
Solar modules can act like small roof surfaces. Wind pressure rises quickly as wind speed increases. The calculator uses a velocity pressure formula with an exposure factor and a net pressure coefficient. It multiplies that pressure by array area. The result is total uplift force. That force is divided by the number of roof attachments. The answer estimates demand at each attachment.
Reading the Results
A pass result means the entered loads are within the chosen limits. A warning means one or more checks exceed the values entered. Review the added load, attachment count, roof capacity, and wind settings. Small input changes can create large differences. For example, higher wind speed greatly increases uplift.
Good Input Practice
Use measured roof dimensions. Use manufacturer weights for modules and racking. Include ballast when it is present. Use local snow load, design wind speed, and code exposure factors. Ask a structural professional to confirm final loads, load paths, waterproofing, and connection details.
Planning Benefits
Early structural checks save time. They help compare layouts, panel quantities, and attachment spacing. They also support cleaner conversations between installers, engineers, and building officials. Keep exported CSV and PDF results with the project file. They give a transparent record of assumptions. A careful first check lowers redesign risk and improves project coordination.
Use conservative values when uncertain. Do not ignore old framing, damaged sheathing, weak connections, or unusual roof shapes. These conditions may reduce capacity. Site inspection remains important before any final approval decision.
FAQs
1. Can this calculator approve a solar roof design?
No. It is a screening tool. Final approval should come from a qualified professional who can inspect framing, review code loads, and verify the complete load path.
2. What is added dead load?
Added dead load is the permanent weight added by solar modules, rails, clamps, ballast, and related hardware. It is divided over array area to estimate psf demand.
3. Why is wind speed important?
Wind pressure increases with the square of wind speed. A small wind speed increase can create a large uplift increase at attachments and roof connections.
4. What does attachment reserve mean?
Attachment reserve is the remaining capacity after calculated uplift demand is subtracted from allowable attachment capacity. A negative value means review is needed.
5. Should snow load always be included?
Use snow load when the project location requires it. Local building rules, roof slope, exposure, and drifting conditions may change the required snow value.
6. Why include ballast weight?
Ballast helps resist uplift on some systems. It also adds gravity load. Both effects should be checked because ballast can improve uplift resistance but stress the roof.
7. What is roof utilization?
Roof utilization compares calculated service demand with entered roof capacity. A high percentage means less reserve remains for uncertainty, future changes, or input error.
8. Can I use metric units?
This version uses feet, pounds, psf, and mph. Convert metric project data before entry, or modify the labels and formulas for metric engineering units.