Solar Surveys Ltd: Created page with "= Ballasted solar PV on flat roofs = Ballasted solar photovoltaic (PV) systems are mounted on flat or low-pitch roofs without penetrating the roof covering. Instead of being fix..."

2026-08-17T14:48:38Z

Created page with "= Ballasted solar PV on flat roofs = Ballasted solar photovoltaic (PV) systems are mounted on flat or low-pitch roofs without penetrating the roof covering. Instead of being fix..."

New page

= Ballasted solar PV on flat roofs =

Ballasted solar photovoltaic (PV) systems are mounted on flat or low-pitch roofs without penetrating the roof covering. Instead of being fixed through the waterproofing into the structure below, the array is held in position by its own weight and by added ballast, typically concrete blocks or pavers placed on the mounting trays or frames.

= Why ballast is used =

Penetrating fixings create a route for water ingress and can invalidate a roof covering warranty. On single-ply membranes, built-up felt and asphalt roofs, the owner or the covering manufacturer will often prefer that the waterproofing is left intact. A ballasted system avoids penetrations altogether, which is why it is common on commercial and industrial flat roofs.

= The structural consequence =

Avoiding penetrations transfers the problem from the roof covering to the roof structure. A ballasted array must resist wind uplift purely through weight, and the weight required can be substantial. The result is that the assessment is rarely governed by the modules themselves, which are comparatively light, but by the ballast added to hold them down.

Three consequences follow.

The load is concentrated, not uniform. Ballast is placed where the mounting system requires it, which is frequently at the array perimeter and at the corners of the roof where wind uplift is highest. Those are not necessarily the positions where the structure has spare capacity.

Roof edges and corners govern. Local wind pressures are significantly higher at edges, corners and around upstands than over the central field of a roof, so ballast requirements increase in exactly those zones.

The array cannot always sit where the designer placed it. Where the structure cannot carry the ballast at the intended position, either the layout is revised, the array is moved inboard, or the structure is strengthened.

== Wind action ==

Wind actions on roof-mounted arrays are derived from BS EN 1991-1-4 and its UK National Annex. BRE Digest 489, revised in 2014, provides specific guidance on wind loads on roof-mounted photovoltaic and solar thermal systems and is widely used to establish uplift, overturning and sliding on ballasted systems. It divides the UK into five wind zones. For stability checks it suggests a coefficient of static friction of 0.3 where the roof surface is not known, and gives a partial factor of 1.35 for roof-mounted solar, derived from the partial factor for wind action of 1.5 reduced by a consequence class factor of 0.9.

A ballasted array must be checked for three separate wind effects: uplift, the array being lifted off the roof; sliding, the array being displaced horizontally across the roof surface; and overturning, individual rows or trays rotating about their downwind edge.

Each can govern the ballast requirement independently, and the controlling case varies with array tilt, row spacing, roof height and the building's exposure.

== Snow ==

Snow actions are derived from BS EN 1991-1-3 and its UK National Annex. On flat roofs with arrays, drifting is the principal concern: snow can accumulate against the upwind face of a row, against parapets, and in the channels between rows. Drifted snow is a local, concentrated action rather than a uniform one, and it acts in combination with the permanent load of the array and its ballast.

== Interaction with the roof covering and drainage ==

Ballast blocks bear directly on the covering, and the array obstructs the roof surface. An assessment therefore also considers point bearing on the covering and any insulation beneath it, and whether protection layers or spreader pads are needed; whether the array or its ballast obstructs falls, outlets or gutters; whether ponding is created or worsened, since standing water is itself a load; and access for maintenance of both the array and the roof beneath it.

= Regulatory position =

Ballasted systems are one of the cases where the applicable standards are specific about who carries out the assessment. Under MCS MIS 3002 Version 6.0, which applies to certified systems up to 50 kWp, ballasted or weighted flat-roof systems require the design to be verified by a structural engineer. Separately, and irrespective of system size, BRE Digest 489 identifies that where a retrofit system increases the applied roof loading by more than 15%, Approved Document A requires the structural integrity of the roof and its supporting structure to be assessed. This applies to building works in England, and in Wales in some circumstances, with equivalent provisions in Scotland and Northern Ireland. Because ballast is added weight by definition, ballasted installations cross that threshold more readily than mechanically fixed ones.

= What an assessment establishes =

A structural assessment for a ballasted array on a flat roof will normally establish the existing structural arrangement and its spare capacity, the ballast required for stability under the governing wind case, whether the structure can carry that ballast in the positions required, and where it cannot, what change to the layout or what strengthening would be needed.

= Related articles on Designing Buildings =

Structural assessment for rooftop solar PV · Flat roof · Single ply roofing · Wind loading · Snow load · Solar photovoltaics

= External references =

BRE, Digest 489: Wind loads on roof-mounted photovoltaic and solar thermal systems, revised 2014.

BS EN 1991-1-4, Eurocode 1: Actions on structures. Part 1-4: Wind actions.

BS EN 1991-1-3, Eurocode 1: Actions on structures. Part 1-3: Snow loads.

MCS, MIS 3002 Issue 6.0: Solar photovoltaic microgeneration systems.

[https://solarsurveys.co.uk/assessment-specification Structural Feasibility Assessment Specification], An open specification for the scope and content of a structural assessment for rooftop solar, free to cite and to require in a scope of works.

Ballasted solar PV on flat roofs - Revision history

Solar Surveys Ltd: Created page with "= Ballasted solar PV on flat roofs = Ballasted solar photovoltaic (PV) systems are mounted on flat or low-pitch roofs without penetrating the roof covering. Instead of being fix..."