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Free Tool · Indicative Screening
Solar panel roof load calculator, in kg/m² and kN/m².
Work out the permanent dead load a rooftop array adds over its footprint, from the module weight, the module size, the mounting type and any ballast. No sign-up, nothing stored. This is the first number in a structural assessment, and on its own it never decides whether a roof can carry an array.
Indicative added dead load
Over the array footprint
15.4 kg/m²
0.15 kN/m²
How to read the number
Two things about this figure catch people out, and both matter more than the number itself.
It applies over the array footprint, not the whole roof. An array covering a third of a roof does not add its load evenly across the building. Structurally, what matters is where the load lands relative to spans, supports and existing plant, which is why an average figure spread over the full roof area understates the local demand.
Dead load is rarely what governs. On most commercial roofs the panels are comfortably within the frame's spare capacity, and the assessment turns on wind uplift at the edge and corner zones instead. That is the opposite of the intuition most people bring, which is that heavy is the danger. On a low-pitch roof, the load trying to remove the array is usually the more demanding case than the load pressing it down.
| System type | Typical added dead load | What sets it |
|---|---|---|
| Rail-mounted, in-plane | About 15 to 25 kg/m² (0.15 to 0.25 kN/m²) | Modules at roughly 11 to 13 kg/m², plus rails, clamps and fixings. The lightest framed systems sit near 12 kg/m². |
| Ballasted, flat roof | About 30 to 60 kg/m² (0.30 to 0.60 kN/m²) | The ballast quantity, which is set by the wind uplift calculation rather than chosen for convenience. |
| Ballasted, high tilt or exposed | Around 75 to 100 kg/m² (0.75 to 1.0 kN/m²) | Greater tilt and exposure raise uplift, so more ballast is required to restrain the same array. |
| A single module | Roughly 25 kg | Typical commercial module. Large-format modules run heavier. |
Conversion: 1 kN/m² is about 102 kg/m². Structural calculations to the Eurocodes are worked in kN/m², so this is usually the first step in reading a manufacturer's datasheet against an engineer's assessment. More detail on all of this is on the solar panel roof load page.
What this calculator does not tell you
Being explicit about the limits is the point of publishing it. This tool covers one input. A structural assessment covers all of the following, and any of them can be the reason a roof does not work:
- Wind uplift. Derived for your specific site exposure, with local pressure coefficients that rise sharply at edges and corners. On low-pitch roofs this usually governs.
- Snow loading. Varies by location and altitude, and can concentrate through drifting against arrays and upstands.
- The capacity of your actual structure. Purlin sizes and spacings, sheeting rail condition, frame type, span, and any strengthening or degradation since it was built.
- Load path and concentration. Where the load lands matters as much as how much of it there is. Point loads at feet and fixings behave differently from a distributed figure.
- Condition of the covering and the fixings. A structurally adequate frame under a failing covering is still not a roof you want to install on.
- The existing loads already on the roof. Plant, services, previous alterations and any accumulated ballast from earlier works all use up the same spare capacity.
If the indicative figure has told you what you needed, good. If it has raised a question, the free structural pre-check takes two minutes and points you to the right level of assessment.
Roof load questions
How much dead load does a rooftop solar array add?
A rail-mounted, in-plane commercial array typically adds about 15 to 25 kg/m² (0.15 to 0.25 kN/m²) over the array footprint: modules at roughly 11 to 13 kg/m² plus rails, clamps and fixings. A ballasted flat-roof system is heavier, typically 30 to 60 kg/m² (0.30 to 0.60 kN/m²), rising to around 75 to 100 kg/m² at higher tilt or exposure because the ballast quantity is set by the wind uplift calculation. These are indicative figures; the actual figure comes from the specific system datasheet.
How do I convert kg/m² to kN/m² for roof loading?
Divide the load in kg/m² by about 102 to get kN/m², or multiply by 9.81 and divide by 1000. One kN/m² is approximately 102 kg/m². Structural calculations to the Eurocodes are worked in kN/m², so this conversion is usually the first step in reading a manufacturer's datasheet against a structural assessment.
Does this calculator tell me if my roof can take solar panels?
No. It gives the indicative added dead load only, which is one of three load inputs. Dead load is combined with snow loading to BS EN 1991-1-3 and wind uplift to BS EN 1991-1-4 with PV uplift checked against BRE Digest 489, then assessed against the actual capacity of the roof structure. A roof that looks light enough on dead load alone can still fail once snow and wind are added. Whether a specific roof can carry a specific array requires a project-specific structural assessment by a qualified structural engineer.
Why is ballast the largest load on a flat roof system?
On a flat roof the array is usually held down by weight rather than fixed through the covering, so the ballast has to resist wind uplift. The quantity is set by the uplift calculation, which is highest at the edge and corner zones of the roof. That is why ballasted systems commonly add two to three times the dead load of a rail-mounted array, and why the ballast layout, not just the total, matters structurally.
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