Flat roof ballast systems

A ballasted solar mounting system sits on a flat roof without relying on fixings through the waterproof layer. The module frames, wind deflectors and carefully positioned weights work together to resist uplift, sliding and overturning.

Avoiding penetrations can protect a sound roof covering. It does not make the design simple or lightweight. The amount and position of ballast must come from a calculation for the actual building and mounting system, and the roof must be shown to carry the resulting loads.

Under MIS 3002, a roof below 10 degrees to the horizontal is treated as flat for these requirements.

The design starts with the building

A generic ballast schedule is not enough. Wind action changes with the site and with position on the roof. A useful design record identifies:

• the site location, altitude and exposure
• building height and plan dimensions
• roof level, shape, parapets, upstands and edge zones
• the proposed module layout, tilt and mounting-system geometry
• nearby buildings or terrain that affect wind exposure
• the mounting manufacturer’s calculation method and test evidence
• the roof build-up, waterproofing and proposed protection layer

The same array can require a different ballast arrangement on another building, or even in another part of the same roof. Moving rows after the calculation has been produced can invalidate it.

MIS 3002 requires the pressure coefficients for a ballasted system to come from BRE Digest 489 or recognised test data commissioned to establish wind loads on solar systems. A manufacturer may use wind-tunnel data for its mounting system, but the resulting design still has to match the site, layout and installed components.

Sliding and overturning

The system must resist both lifting or overturning and horizontal movement. More weight is not automatically the right answer: it can overload the roof while leaving an edge row or sliding path poorly controlled.

For the sliding calculation, MIS 3002 uses a coefficient of friction of 0.3 unless the actual combination of mounting foot, protection layer and roof surface has been tested using its specified method. A value above 0.3 needs a test report, and the calculation must declare the value used.

Gravel and green roofs need particular care because a loose surface can move independently of the waterproofing below. On these granular substrates, MIS 3002 requires either a tested friction value or mechanical restraint against sliding. A suitable kerb or tether may form part of a designed solution, but it must transfer load to something capable of taking it.

Ballast blocks should be the specified type and placed in the recorded positions. Substituting block sizes, omitting weights or spreading them evenly because that looks neater can undo the wind design.

The roof has to carry it

MIS 3002 requires a qualified structural engineer to be consulted to confirm that a flat roof can withstand the combined imposed load from the PV system and its ballast.

That assessment is not a check against one universal kilograms-per-square-metre figure. It considers the existing structure and roof build-up, the distribution and concentration of the new loads, snow, wind action, drainage falls and any existing plant or defects. A roof that carries the modules might not carry the ballast needed at an exposed edge.

The outcome should identify any limits on array position, ballast, access or material storage during installation. Temporary stacks of modules or blocks can create a more severe local load than the finished, distributed system.

Where ballast makes the design too heavy, the answer may be a mechanically fixed or hybrid system, a different aerodynamic layout, a smaller array or strengthening. It is not to reduce the weights without recalculating the system.

Protecting the waterproofing

A compatible protection or slip layer is required between the mounting system and the waterproofing. Compatibility matters because some rubbers, plastics and membranes can react, stain, soften or lose their warranty when placed together.

The roof-covering manufacturer or warranty provider should be consulted where a warranty remains in force. The design should also address:

• local bearing pressure beneath feet and ballast trays
• movement caused by thermal expansion and wind
• sharp edges and trapped debris
• drainage routes, outlets and overflow paths
• standing water and connectors or cables lying in it
• access for inspecting and repairing the membrane

Ballast must not obstruct outlets or turn a maintainable roof into one where leaks cannot be traced. If the covering is already brittle, blistered, split or near replacement, repairing or renewing it before the array is normally the sensible sequence.

Layout and access

Tilt and row spacing are design choices, not fixed national dimensions. They affect energy yield, self-shading, wind action, ballast, roof use and access. An east-west arrangement and a south-facing arrangement behave differently, so the chosen configuration needs its own calculation.

The layout should leave safe access to outlets, rooflights, plant, fire-safety features and any components that require maintenance. MIS 3002 also asks designers to consider perimeter access, corridors through large arrays and protection around fragile elements.

No one should assume that a flat roof is safe to walk on. HSE guidance says all roofs should be treated as fragile until a competent person confirms otherwise, and open edges need suitable protection during work.

When mechanical fixing is used

A mechanically fixed mounting transfers loads into the roof structure through designed fixings. It can reduce ballast, but penetrations have to be detailed and installed as part of the waterproofing system.

For an existing solid deck, MIS 3002 says a condition survey should be carried out and pull-out tests undertaken. Where pull-out resistance cannot be calculated reliably, the specified site test protocol is used. The fixing design, deck condition and weathering detail all belong in the project evidence.

Hybrid systems combine restraint and ballast. They still need one coherent calculation showing how every load reaches the structure.

Evidence to expect

A complete handover record should include:

• the mounting-system layout and component schedule
• site-specific wind, sliding and overturning calculations
• the declared friction coefficient and any supporting test report
• the structural engineer’s assessment
• ballast type, quantity and location plan
• membrane and slip-layer compatibility evidence
• fixing and pull-out evidence where mechanical restraint is used
• photographs before the array concealed the roof
• inspection and maintenance access instructions

If the proposal gives only the total ballast weight, it is missing the information needed to show where that weight goes and what it resists.

Related entries

• Roof loading survey
• Scaffold and roof access
• MCS 012 solar mounting standard
• MIS 3002 solar PV standard
• Shading analysis