The basic principle of the Flat Roof Ballasted System Racking is to mount solar panels and racking systems on flat or low-slope roofs by means of weight anchoring. At its core, it utilizes the weight of concrete blocks, rocks, or other heavy objects to press the solar modules against the roof surface for a solid fix without the need to penetrate the roof.

When considering roof load-bearing capacity in the design of ballast system supports, the following key factors need to be integrated and based on relevant codes and practical examples

1. Load-bearing capacity assessment and calculation

Distinguish between types of loads:

Dead loads: These include the self-weight of roofing materials, permanent equipment and ballast systems. For example, a standard concrete roof has a minimum permanent load of >3.6 kN/m² (approx. 367 kg/m²) and the ballast system load must be below this value.

Live loads: including temporary loads (e.g. construction personnel, equipment, snow). For example, flat roofs have a minimum concentrated load capacity of 488 kg/m², while pitched roofs may have a lower capacity due to structural differences.

Special loads: uplift forces due to wind pressure (to be resisted by ballast), snow loads (e.g. an additional 40 kg/m² in snowy areas) and equipment loads (e.g. photovoltaic panels).

Calculation method:

Calculate load combinations in accordance with building regulations (e.g. IBC, BS 6399), taking into account roof slope, span and material properties. Example:

Bending moments and axial forces for pitched roofs should be analyzed for the critical conditions of gravity and uplift forces to ensure that the maximum bending moment (e.g. 4.15 kip-ft) meets the design strength.

For flat roofs, the focus should be on localized loads to avoid overloading due to concentrated ballast.

2. Ballasted pv mounting system design elements

Non-penetrating fixing:

Priority is given to the use of cement ballast blocks to fix the bracket by self-weight to avoid damaging the waterproof layer. Ballast weight should be customized according to local wind speed, e.g. matrix arrangement can disperse the load and reduce the pressure per unit area of the roof .

Ballast optimization:

Placement of ballast blocks in different zones (e.g. differentiated arrangement of inner and outer perimeter) saves more than 30% of cement consumption while controlling the total weight.

Use of lightweight and high-strength materials (e.g. aluminum alloy) to reduce system weight.

3. Structural safety and code compliance

Compliance Verification:

Refer to local building codes (e.g. Florida Wind Load Code, SSTD-12) to ensure that the ballast system resists wind uncovering. Anchorage devices should resist lateral forces and purlin displacement should not exceed 1/360 of the span.

Older buildings need to be evaluated for risk of structural deterioration, which may reduce the load carrying capacity by 20-30% and require additional reinforcement.

Safety margin:

Designed to allow 15-20% load capacity for extreme weather (e.g., snowstorms, hurricanes).

4. Maintenance and monitoring

Regularly remove snow and water (2-3 kg/m² load per 1cm of snow).

Check for signs of roof deformation (e.g., sinking, cracks) and reinforce in a timely manner (e.g., strengthen trusses, repair damaged tiles).