Roof orientation and shading

Orientation sets the ceiling on what a roof can produce; shading determines how much of that ceiling you actually reach. A south-facing slope at a typical domestic pitch gives the highest annual total. East and west slopes give less in total but deliver it across a longer day, which frequently suits a household better. North-facing slopes are materially worse and are rarely a primary choice. Shading is the more damaging variable, because one obstruction can cost far more output than its shadow area suggests — and unlike aspect, it can often be mitigated.

Aspect

Think of aspect as the shape of the day as much as the size of it.

South. Highest annual yield, concentrated in the middle of the day. Excellent if you have storage or daytime consumption; less so if the house is empty from nine to five and you are exporting the peak.

South-east and south-west. A small reduction on south, with the peak shifted towards morning or afternoon respectively.

East and west. Lower annual total than south, but two peaks — one in the morning, one in the evening — that line up better with when many households actually use electricity. A split east–west array spreads generation across a wider window and reduces the size of the midday surplus.

North. Materially lower output. Occasionally worth adding where a household needs more capacity than the good slopes provide and the economics still work, but not a starting point.

Pitch

Pitch interacts with aspect. A steeper roof favours the low winter sun; a shallower one favours the high summer sun. Most domestic pitches fall in a range that works reasonably across the year, and almost nobody alters a roof for solar.

Flat roofs are the exception, because pitch becomes a design decision rather than a given. Frames can set panels at a chosen angle and orientation, with the trade-off being wind loading, ballast weight and row spacing to prevent one row shading the next.

Shading: why it costs more than it looks

Panels in a series string behave like links in a chain. Current through the string is limited by the least productive module, so shade across part of one panel can pull down the output of every panel on that string. Bypass diodes within the panel limit the damage, but they do not eliminate it.

This is why an accurate shading assessment matters more than a rough one, and why the answer to “will that chimney matter?” is not obvious by eye.

Common culprits, roughly in order of how often we find them:

• Chimneys and stacks, particularly on terraces and semis
• Trees, including ones on neighbouring land
• Neighbouring buildings and extensions
• Dormers and roof projections on the same roof
• Aerials, satellite dishes, soil vent pipes
• Rows of panels shading each other on flat-roof frames

Assessing it properly

Shade moves through the day and through the year. An obstruction that clears the array in June can sit across it for most of a December day, because the winter sun tracks low.

A proper assessment models the sun path across the year against the obstructions measured on site — not a glance at the roof on the afternoon of the survey. It produces a shading factor that feeds into the performance estimate, and it informs the array layout: sometimes the right decision is simply not to place panels in the worst-affected positions.

What mitigates shading

Layout. The cheapest mitigation. Leaving the shaded zone empty and placing panels where light is clean often beats filling the roof and accepting the loss.

Stringing. Grouping panels so that shade-affected modules sit on a separate string from clean ones limits the contagion.

Power optimisers. Module-level electronics that let each panel operate at its own optimum, so a shaded module no longer constrains its neighbours. Adds cost and adds components on the roof.

Microinverters. Conversion at each panel rather than centrally. Similar isolation benefit, different architecture and different failure profile.

Removing the obstruction. Sometimes the tree can be reduced, the aerial relocated or the dish moved. Sometimes it belongs to someone else or is protected.

None of these recover output that was never there. They stop a small problem becoming a large one.

What we do at survey

We measure the roof, record obstruction positions and heights, model shade across the year, and design the layout and stringing around what we find. Where shading is significant, we will tell you what it costs before you commit rather than after.

Any figures quoted for orientation losses are illustrative industry values, not Solar Fx figures for your roof. Actual generation, savings and payback depend on the property, roof orientation and pitch, shading, system design, your electricity usage and tariff, export rates and future energy prices. Figures shown are estimates and are not guaranteed.