Solar PV Roof Survey: Orientation, Pitch, Shading Analysis, Structural Load Check and Roof Condition Assessment

Summary

The site survey is the most important step in a solar PV installation. A poorly conducted survey leads to incorrect system sizing, undetected shading problems, structural issues emerging during installation, or a roof that needs repair within years of installing panels. All of these cost money and damage customer trust.

For solar PV installers, a thorough survey should take 1–2 hours at a typical domestic property. It must cover the roof structure, orientation, shading, condition, and access, and the results must feed directly into the system design.

Key Facts

• Orientation (azimuth) — the direction the roof slope faces; south (180°) is optimal in the UK; southeast (135°) and southwest (225°) lose only 5–10% of potential yield; east (90°) or west (270°) lose approximately 20–30%; north-facing roofs are unsuitable for the main array
• Pitch (tilt angle) — the angle of the roof from horizontal; UK optimal solar tilt is approximately 30–35° for a fixed array; standard UK domestic roof pitch is 30–50°; very low pitch (<15°) reduces self-cleaning of panels by rain
• Shading analysis — identify obstacles that cast shadows on the panels at any time of year; summer morning/evening shadows from chimneys, trees, and dormers are most impactful; a shadow on even one cell in a module can disproportionately reduce output of that module and connected modules in the same string
• SAP (Standard Assessment Procedure) — the UK energy efficiency methodology; solar PV yield calculations for planning and EPCs use SAP; MCS system design requires calculations equivalent to SAP
• PVGis / PV*SOL / SolarEdge Designer — digital tools for yield estimation; input orientation, pitch, and shading to estimate annual kWh output
• Dead load — static weight of the solar panel array; typical panel weight 20kg/m²; most UK domestic roofs have adequate structural capacity if in good condition
• Live load — snow and wind loads; panels must be fixed to withstand the defined wind uplift load for the location
• Rafter condition — panels are fixed to the roof structure, typically through tiles/slates via hooks or brackets into rafters; rafter condition must be checked, particularly in older properties
• Roof covering condition — a roof with failing felt, cracked tiles, or evidence of significant moss/lichen growth should be assessed and repaired before panels are installed
• Permitted development — for planning purposes, panels on a roof do not extend more than 200mm above the roof surface (in conservation areas, additional restrictions apply); the survey confirms whether the installation meets PD criteria

Quick Reference Table: UK Orientation and Yield Guide

Detailed Guidance

Orientation and Pitch Assessment

Measuring orientation: Use a compass or a mapping tool (Google Maps satellite view, what3words) to confirm the azimuth of the roof slope. Confirm on site — the front door or street may not align with south simply because the road runs north–south.

Measuring pitch: A digital angle finder / pitch gauge is the most accurate tool. Alternatively, measure the rise and run from inside the loft: pitch = arctan(rise/run). Common UK roof pitches: 30° (many post-1950s houses), 35–40° (Victorian and Edwardian), 45° (steeper traditional properties), 20–25° (1970s bungalows).

Split-orientation roofs: Some properties have usable south-facing and east/west-facing slopes. Both can be used, but separate strings or micro-inverters/power optimisers may be needed to avoid the weaker-performing slope reducing the output of the better one (see string inverter vs microinverter).

Flat roofs: Flat roofs (pitch <10°) require ballasted or penetrating mounting frames to tilt panels at the optimal angle. See solar pv on flat roofs.

Shading Analysis

Shading is the most underestimated factor in solar PV surveys. Even small shading events at critical times of day can significantly reduce system output.

Common shading sources:

• Chimneys (particularly significant as they are often on the ridge, creating shadows across the panel array in winter)
• Neighbouring buildings (particularly on east–west terraced properties in the late afternoon)
• Trees (deciduous trees lose leaves in winter, reducing winter shading; evergreen trees shade year-round; check tree growth trajectory — a young oak may shade significantly in 10 years)
• Roof features: dormers, satellite dishes, TV aerials, vent pipes, sky lights (Velux, etc.)
• Parapet walls on flat roofs

Shading assessment methods:

Horizon shading tool (e.g., SunEye, Solmetric): handheld device or app that photographs the sky hemisphere from the panel location; analyses the sun path and identifies horizon obstructions; produces a shading loss percentage for each month. Most accurate method.

Digital simulation (PVGis, PV*SOL, HelioScope): input roof geometry and shade obstacles; the software models sun path for the location and calculates shading loss. Requires accurate input of obstacle sizes and distances. Faster than site tools.

Visual assessment (quick survey method): stand at the panel location and visually assess horizon obstructions. Note any obstacles within approximately 45° of south azimuth and below 30° elevation that are present at any time of year. Flag these in the report.

Shading and string design: A shadow on one panel in a string reduces the output of the entire string (since panels are connected in series — the weakest panel limits the string). Where partial shading is unavoidable, power optimisers (panel-level MPPT) or microinverters can mitigate the loss by decoupling each panel. This is a significant design decision — flag shading during the survey so the correct inverter technology can be specified.

The 4% shading rule (MCS guidance): MCS guidance suggests that annual shading losses above approximately 4% of potential generation are significant enough to require calculation and customer communication. Below 4%, the loss is often within the normal variation of yield estimates.

Structural Assessment

Rafter identification: Panels are fixed to rafters via brackets or hooks that pass through the roof covering. Rafters must be located, and their condition assessed. In modern houses, rafter spacing is typically 400mm or 600mm centres. Older houses may have wider or irregular spacing.

Use a rafter finder (magnet or sensor) or open the loft hatch and measure directly. The rafter size and condition in the loft is a good indicator of overall structural capacity.

Load check: A typical mono/poly crystalline panel array (18–20 panels, ~450Wp each) adds approximately 18–20kg/m² dead load. For a roof in good structural condition, this is within the design capacity of most UK domestic roofs (which are typically designed for 0.75–1.5 kN/m² live load). However, if the rafters are undersized, notched, or previously damaged, a structural engineer's assessment should be recommended.

Wind uplift: Panels act as sails and must resist uplift forces from wind. The panel mounting system must be designed and fixed to withstand the design wind load for the site (based on BS EN 1991-1-4 / BS 6399 wind speed for the location). Most MCS-compliant mounting systems have been structurally assessed by the manufacturer. Use the manufacturer's design guide to confirm appropriate fixing frequency and hook/bracket specification.

Roof covering condition: Assess from inside the loft and from a ladder at the eaves:

• Tiles: cracked, slipped, or missing tiles must be replaced before installation; a substantial number of defective tiles indicates the roof covering is near end of life
• Felt (sarking): check in the loft; failed felt (torn, perished) does not directly prevent solar installation but indicates the roof may need attention within years; flag to the customer
• Battens: timber battens carry the tiles; check for rot, cracking, or deterioration at the eaves; failed battens will require strip and relay of the relevant section
• Ridge and hip tiles: assess condition; repointing may be needed before installation
• Lead flashings: assess condition around chimneys, skylights, and valleys; failing lead should be renewed before panels are installed over the adjacent area
• Moss and lichen: heavy moss growth retains moisture and accelerates tile and batten deterioration; significant moss should be cleared before installation; biocide treatment recommended

Reroofing recommendation: If the roof is expected to need major work within 5–10 years, advise the customer to reroof first. Removing and reinstalling 20 solar panels to access the roof typically costs £800–£1,500 in additional labour — comparable to what they might save by delaying the reroofing.

Survey Documentation

A professional solar PV survey report should include:

• Roof orientation and pitch (measured values, not estimated)
• Panel placement diagram (annotated aerial image or sketch)
• Shading assessment results (tool used; estimated annual shading loss %)
• Roof condition notes with photos
• Structural observations
• System design parameters (panel count, kWp, inverter specification, estimated annual yield in kWh)
• Any conditions recommended before installation (reroofing, tree trimming, aerial relocation)

This report forms the basis of the customer proposal and the MCS design documentation.

Frequently Asked Questions

Can I install solar panels on a roof with a Velux skylight?

Yes, but the panels must maintain the required clearance from the Velux frame (typically 300–500mm to allow access for Velux maintenance and to avoid debris/moss accumulation). The Velux area may result in a smaller array. Factor this into the panel layout during the survey.

My customer has a concrete interlocking tile roof at 15° pitch. Will solar work?

15° pitch is workable but suboptimal (optimal UK angle is 30–35°). The yield loss relative to optimal is approximately 5–8%. More significantly, at very low pitch, panels do not self-clean effectively in rain, which can reduce long-term performance. If the customer accepts these limitations, installation is feasible. Consider a higher-efficiency panel to partially compensate for the low pitch angle.

Should I use roof hooks or continuous rail for fixing?

Both are standard options. Roof hooks (individual hook points under tiles, projecting through the tile for a bracket) are the most common UK domestic approach — they are faster to install on standard interlocking or plain tiles. Continuous rail (aluminium rail running along multiple rafter centres) provides more fixing points and is preferred for slate roofs or on roofs where rafter spacing is irregular. Confirm the mounting system is MCS 012-compliant and has a structural calculation available.

Regulations & Standards

• MCS 012 — Solar PV Product Standard; installation and system design requirements

• MCS MIS 3002 — MCS Installation Standard for solar PV (technical requirements for roof mounting, structural loading, and survey)

• BS EN 1991-1-4 (BS 6399-2) — Wind loads on buildings; relevant to panel mounting structural design

• Building Regulations Approved Document A — structural loading; relevant to roof structural capacity assessment

• MCS MIS 3002 — installation standard for solar PV

• PVGIS — EU Solar Radiation Database — yield estimation tool by location, pitch, and orientation

• SolarEdge Designer — detailed shading and system design tool

• string inverter vs microinverter — inverter selection for shaded or split-orientation roofs

• solar pv system sizing — kWp and kWh yield calculations based on survey data

• scaffold and roof access solar — roof access and fixing into different tile types

• solar pv planning permission — permitted development criteria for roof-mounted panels