East-West Solar Arrays: How Dual-Pitch Layout Affects Structural Loading vs South-Facing Arrays

East-west (E-W) solar array layouts, where panels face east on one slope and west on the opposing slope of each row, offer a distinctive combination of benefits for flat commercial rooftops: more uniform generation across the day, lower wind uplift than south-facing arrays at equivalent tilt, and significantly higher roof coverage density. These structural and spatial advantages make E-W arrays increasingly the default choice for large flat-roof commercial installations.

However, E-W arrays have their own structural loading characteristics that differ from south-facing layouts, and a structural assessment for an E-W installation requires specific attention to these differences. This article explains the structural loading profile of E-W arrays, the wind load calculation approach, and the structural implications of the higher roof coverage achievable with this layout.

How East-West Arrays Differ from South-Facing Arrays

A south-facing rooftop array typically uses tilts of 20-35° to maximise annual energy yield. E-W arrays use lower tilts, typically 10-15°, because generation efficiency is less sensitive to sub-optimal orientation at low tilt angles, and the lower tilt significantly reduces the inter-row shadow casting distance. This allows rows to be packed more closely, increasing the proportion of the roof covered by panels.

The structural implications of this difference are:

• Lower wind uplift: Panels at 10-15° tilt present a smaller obstruction to wind than panels at 25-35° tilt. Wind pressure coefficients decrease with lower tilt angles under BS EN 1991-1-4. For a given site, E-W arrays typically experience 20-35% lower peak wind uplift forces than south-facing arrays of equivalent size.
• Symmetric wind loading: A south-facing array has a single windward face (usually the west face). An E-W array has both east-facing and west-facing panels; the structural assessment must consider wind from both directions and the load reversals that occur as the dominant wind direction changes.
• Higher dead load per unit roof area: Because E-W arrays cover more of the roof area than south-facing arrays (40-50% more coverage at equivalent row setback), the total dead load on the roof structure is proportionally higher. The distributed dead load per m² of array is similar or slightly lower (lower tilt allows slightly denser panel packing with lighter mounting frames), but the total load on the roof is greater because more of the roof is covered.
• Snow accumulation in the inter-row valley: The valley between opposing E-W rows creates a natural snow trap. Snow drift calculations per BS EN 1991-1-3 must account for drift accumulation in the row valleys, this can create concentrated line loads at the valley of each double row that exceed the uniform snow load.

Wind Load Calculation for E-W Arrays

The wind loading on E-W solar panels on flat roofs is calculated to BS EN 1991-1-4, with additional consideration for the specific geometry of E-W layouts. The key aspects:

Zone-based pressure coefficients: As for any flat-roof installation, the roof is divided into wind pressure zones (F corner, G edge, H internal). The pressure coefficients for each zone apply to the E-W panels in those zones. Panels in corner zones (F) experience the highest uplift regardless of their orientation.

Low tilt angle coefficients: BS EN 1991-1-4 provides pressure coefficients for pitched roof panels as a function of pitch angle. At 10-15°, coefficients are lower than at 25-35°, confirming the reduced uplift advantage of E-W layouts.

Dominant wind direction: For E-W arrays, the structural assessment should consider both east-facing and west-facing panels as the windward face. The UK prevailing wind is from the south-west, meaning west-facing panels are typically windward more frequently, but the east-facing panels are windward in easterly conditions, and design loads must account for both.

Structural Element Check for E-W Loading

The structural element checks for E-W arrays follow the same methodology as for south-facing arrays, with specific attention to the load cases unique to E-W geometry:

Uplift in corner and edge zones: Even at lower tilt angles, E-W panels in corner and edge zones experience significant uplift. Ballast quantities in these zones must be calculated for the specific tilt angle, zone dimensions, and site wind speed, not estimated from a south-facing array table.

Downward push in the windward row: When wind strikes the east or west face of the array, it exerts a downward pressure on the windward panel face as well as an uplift on the leeward face. The combined loading, compression on the windward rail, tension on the leeward rail, creates a net moment in the mounting frame. The mounting frame must have adequate moment capacity for this in-plane loading.

Row-end connections: The ends of E-W rows, where east-facing and west-facing panels join at the ridge of the double row, create structural connections that must resist the load reversal forces. These connections are array-geometry specific and must be assessed against manufacturer data or the mounting frame structural calculation.

Snow valley load: Snow accumulates in the valley between adjacent E-W rows. The structural assessment must calculate the drift load in the valley using BS EN 1991-1-3 Annex B and verify that the roof deck and mounting structure can carry the concentrated valley load in combination with panel dead load.

Roof Coverage and Dead Load Implications

The ability to achieve 60-80% roof coverage with E-W arrays (compared to 40-55% for south-facing layouts at typical row setbacks) means that the total dead load on the roof structure is proportionally higher. This has implications for buildings with limited residual structural capacity.

For a building where the structural assessment shows marginal purlin capacity under the array dead load, an E-W layout that covers 70% of the roof may exceed structural capacity even if a south-facing layout at 45% coverage was within capacity. The structural assessment must check against the actual coverage fraction, not assume the same coverage as an alternative south-facing design.

On the other hand, for buildings with adequate structural reserve, E-W coverage maximises energy yield per unit of roof area, increasing system MWp without requiring a larger roof. This is particularly valuable in constrained urban and suburban sites where roof area is limited.

Ballast Design for E-W Systems

Most E-W flat-roof solar installations use ballasted mounting systems, because the low tilt angle makes ballasted systems structurally efficient (less ballast required per unit area than south-facing systems at the same tilt) and membrane-friendly (no penetrations). Ballast weights for E-W systems are typically lower than for south-facing systems of equivalent area, reinforcing the structural capacity advantage of this layout.

However, the ballast calculation must still be zone-specific. Corner and edge zone panels in an E-W array require higher ballast than interior panels, just as they do in south-facing arrays. The structural assessment should produce a ballast plan, a drawing showing required ballast weight per panel or per bay, by zone, rather than a single average ballast figure. Arrays installed with uniform ballast using an average figure will be under-ballasted in edge and corner zones.

E-W Arrays on Pitched Roofs

E-W arrays are primarily a flat-roof solution, but they can also be applied to low-pitched commercial roofs (2-10°). On a slightly pitched roof, E-W rows installed perpendicular to the ridge create a different loading geometry: the east-facing panels slope slightly downward (toward the valley) and the west-facing panels slope slightly upward (toward the ridge). This creates an asymmetry in the wind loading calculation that must be addressed.

For installations on roofs with pitches above 5°, the structural engineer should verify whether E-W mounting system geometry is consistent with the roof slope, some E-W mounting systems require a nominally flat surface and may not accommodate significant pitch without adjustment. The mounting system manufacturer should confirm the applicable pitch range for the proposed system.

Documentation Requirements for E-W Array Assessments

The structural report for an E-W array should contain all the standard desktop report elements, plus:

• Explicit confirmation that the wind load calculation used the correct pressure coefficients for the E-W tilt angle (not south-facing tilt angle coefficients)
• Snow valley drift calculation (BS EN 1991-1-3 Annex B) for the row geometry proposed
• Ballast plan showing zone-specific ballast weights
• Confirmation that total array dead load for the higher coverage fraction has been checked against structural capacity

An E-W structural assessment that uses standard south-facing pressure coefficients without adjustment for the lower tilt angle is technically incorrect, it will overestimate wind uplift and produce an overly conservative ballast requirement. While this is conservative rather than unsafe, it results in heavier ballast that may exceed structural capacity on marginal buildings where the correct calculation would show adequacy.

Row Spacing and Inter-Row Shading: Structural Efficiency Trade-offs

In east-west low-tilt racking systems, row spacing is primarily an energy yield optimisation decision: narrower row spacing increases array density and total installed capacity, while wider spacing reduces inter-row shading and increases the energy yield per panel at the cost of lower total capacity for a given roof area. However, row spacing also has structural implications that the structural assessment must address.

Closer row spacing in an east-west array increases the tributary area per fixing point and the cumulative dead load on the roof zone covered by the array. For very high-density arrays on buildings with marginal residual structural capacity, row spacing may need to be increased not for energy yield reasons but to keep the distributed dead load within the structural envelope confirmed by the assessment. The structural report will typically state a maximum area load constraint rather than a minimum row spacing, allowing the racking designer to work within the structural limit using whatever geometry achieves the target energy yield.

Row spacing also affects the wind load regime. In east-west arrays, the gaps between rows allow pressure equalisation between the top and underside of the panels, which can reduce the net uplift force on individual panels compared to a continuous array without inter-row gaps. Some racking manufacturers publish aerodynamic performance data for their east-west systems demonstrating this pressure equalisation effect. Where this data is available and supported by wind tunnel testing or validated CFD analysis, the structural engineer may be able to use it to reduce the design wind uplift forces compared to a conservative BRE Digest 489 analysis, potentially allowing higher array density within the same structural capacity.

Ballasted Versus Mechanically Fixed East-West Systems: Structural Implications

East-west racking systems are available in both ballasted (non-penetrating) and mechanically fixed variants, and the structural assessment methodology differs materially between them. Understanding the structural implications of each system type is essential when specifying the racking approach for a project.

Ballasted east-west systems use concrete or rubber paving blocks as ballast weight to resist wind uplift, relying entirely on the self-weight of the ballast rather than fixing into the roof structure or membrane. The structural advantage is that no roof penetrations are required, preserving the waterproofing integrity of flat roof membranes and eliminating the risk of fixing-induced damage. The structural challenge is that ballast generates concentrated point loads at the racking feet, loads that may be higher per unit area than the distributed load of a through-fixed system of equivalent capacity, particularly at high-ballast zones designed for wind uplift at array corners. The structural assessment for a ballasted system must verify that the concentrated ballast loads at each racking foot are within the capacity of the roof deck and the underlying structure.

Mechanically fixed east-west systems attach to the roof substrate via screws or clamps and do not rely on ballast for wind uplift resistance. This approach generates lower dead loads at the attachment points but requires penetration of or engagement with the roof covering, which may affect warranty and waterproofing requirements. The structural assessment for mechanically fixed systems focuses on the pull-out capacity of fixings in the specific roof deck material and the wind uplift distribution across the array. On fibre-cement or metal profile roofs with purlins at standard centres, mechanical fixing is typically the more structurally efficient option because the fixing load path is direct and well-characterised.

Some east-west system suppliers offer hybrid designs that combine a small amount of ballast with limited mechanical fixing, providing uplift resistance primarily through ballast with mechanical fixings at high-uplift perimeter zones. This approach can be appropriate on flat roofs with limited structural capacity for concentrated ballast loads, and the structural assessment must verify both the ballast distribution and the fixing capacity at the hybrid zones.

Comparison of E-W Loading with Conventional Pitched Arrays

East-west arrays and conventional portrait pitched arrays on the same roof generate significantly different structural load profiles. Understanding this difference explains why the structural assessment methodology must be tailored to the system type rather than applying a single generic loading model.

A conventional portrait array at 10-15° tilt presents a significant wind uplift challenge at the upslope and downslope edges, where the panel angle creates a large differential between windward and leeward face pressures. BRE Digest 489 addresses this through zone-specific pressure coefficients that reflect the elevated uplift at array perimeter zones. For a pitched roof installation, the array pitch is additive to the roof pitch, and the combined angle determines the worst-case uplift scenario.

An east-west array at 10-15° tilt per side presents a fundamentally different wind load geometry. The panels face east and west rather than south, and the maximum wind load scenario is a north or south wind striking the system from the long elevation rather than the end. The inter-row gap between east and west panel banks allows pressure relief. The net result is that east-west systems typically exhibit lower wind uplift forces per unit area than equivalent south-facing pitched systems on the same roof, and the structural clearance for an east-west array may be achievable on buildings where a conventional pitched array would exceed structural capacity limits. This comparative advantage is most significant on flat roofs with marginal dead load capacity, where the lower self-weight of a balanced east-west rack compared to a heavy-ballasted south-facing system can make the difference between clearance and refusal.

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