A large roof and a small footprint
An average roof load spreads weight mathematically across an area. A mounting foot initially transfers its force through a limited footprint. The layers beneath must carry that force onwards. Two arrays with the same total mass can therefore need different support details.
Support positions, contact geometry and the insulation beneath the membrane matter. Assess a board with its dimensions, joints and support conditions rather than its density alone.
A simple contact-pressure example
Assume a support transfers 1.0 kN in the load case being considered through a 100 × 100 mm footprint. That is 0.01 m², giving 100 kPa average contact pressure. If the same force were distributed uniformly over 200 × 200 mm, or 0.04 m², the result would be 25 kPa.
This only illustrates pressure = force / area. It is not an allowable product load and does not prove that an inserted board actually mobilises the entire larger area uniformly. Bending stiffness, substrate and edge position affect its behaviour.
| Assumed support force | Contact area | Average contact pressure |
|---|---|---|
| 1.0 kN | 100 × 100 mm = 0.01 m² | 100 kPa |
| 1.0 kN | 200 × 200 mm = 0.04 m² | 25 kPa |
The role of a mineral board
A suitable board provides a continuous substrate and can support load distribution in an agreed assembly. SHEET sits on insulation under new waterproofing. WEER lies above the existing membrane beneath PV. Their positions change the sequence of loaded layers.
A material compressive-strength value is not the load capacity of the complete roof. Softer insulation, an unfavourable joint or a support close to an edge may govern a particular detail.
Coordinate feet, joints and edges
A foot at a board centre differs from one beside an edge. Overlay the support layout on the board arrangement and examine roof openings, changes of falls and cut pieces. Include maintenance routes rather than showing only module rectangles.
Repeated access and later ballast changes also deserve consideration. An installation drawing should make these interfaces clear enough to check on site.
Provide the correct design inputs
Collect mounting-system forces and footprints, ballast arrangement, roof section and information about insulation and protective layers. RVO also identifies variable effects such as wind, ponding and snow accumulation in structural assessment. [1]
Check what each number represents: self-weight, support reaction or a design value with factors already included. Do not combine unlike values without explanation. Drawings, load sheets and photographs give the specialists a useful starting point.
Turn the assessment into an installation detail
Agree the load path with the responsible designers, then document support positions, board joints and connections. Site checks can compare the actual arrangement with this detail. Our additional roof weight guide complements the local assessment with the mass of the full protective layer.
A low average load can coexist with high local stresses. Follow the load path through every layer to the supporting structure.
Sources and technical documents
Next step for your roof
VlamShield WEERWeather-resistant mineral board above waterproofing beneath PV→