Solar Surveys Ltd: Created page with "= Desktop structural roof loading assessment = A desktop structural roof loading assessment is a structural appraisal of a roof's capacity to carry a proposed additional load, c..."

2026-08-17T14:52:18Z

Created page with "= Desktop structural roof loading assessment = A desktop structural roof loading assessment is a structural appraisal of a roof's capacity to carry a proposed additional load, c..."

New page

= Desktop structural roof loading assessment =

A desktop structural roof loading assessment is a structural appraisal of a roof's capacity to carry a proposed additional load, carried out from existing information rather than from a site visit. In the context of rooftop solar photovoltaic (PV), it is used to establish whether a roof is likely to be able to support a proposed array before committing to a full site survey or to the installation itself.

= What it is, and what it is not =

A desktop assessment is a genuine engineering appraisal, undertaken and signed by an engineer, and based on calculation rather than opinion. It is not a site survey, and it is not a substitute for one where the structure is unknown, undocumented or in questionable condition.

The distinction matters because the two answer different questions. A desktop assessment asks whether the structure, as recorded, has the capacity for the proposed array. A site survey additionally establishes whether the structure as built matches the record, and what condition it is actually in.

= Information required =

A desktop assessment depends on the quality of the information available. Typically this includes original structural drawings, calculations or as-built records; member sizes, spans and spacings for the primary frame, purlins, rafters or joists; the roof build-up and covering type; building geometry, height, and site location for the derivation of wind and snow actions; and details of the proposed array, meaning module type and weight, mounting system, tilt, layout and, for ballasted systems, the ballast schedule.

Because the assessment does not depend on site access, its programme is governed almost entirely by the completeness of the information supplied. Where the required data is provided in full at the outset, the assessment can be issued quickly, and a benchmark of 48 hours from receipt of complete information is achievable. Where information is missing, the exercise stalls at the gap rather than proceeding on assumption, which is why the requirements above are stated as requirements rather than preferences.

Where records are incomplete and the work must nonetheless proceed, assumptions must be stated explicitly and the assessment qualified accordingly.

= Loads considered =

The assessment considers the permanent load of the array, mounting system and any ballast, together with wind actions derived from BS EN 1991-1-4, snow actions from BS EN 1991-1-3, and the relevant combinations under BS EN 1990. Imposed and maintenance access loads are also considered.

= Typical outcomes =

A desktop assessment generally concludes in one of several ways. Adequate: the structure has sufficient capacity for the array as proposed. Adequate with modification: the array can proceed if the layout, tilt or ballast arrangement is revised. Strengthening required: specified structural works are needed before installation. Inconclusive pending survey: the available information is insufficient, and a site survey is required to resolve it.

The last outcome is a legitimate and common result rather than a failure of the exercise. Identifying that the record is inadequate is itself a useful finding.

= Where it fits in a project =

A desktop assessment is normally undertaken early, at feasibility or during procurement, where it is used to screen a roof or a portfolio of roofs before survey costs are committed. On a portfolio, it allows sites to be triaged so that survey effort is concentrated where the structural question is genuinely open.

= Regulatory context =

BRE Digest 489, revised 2014, identifies that where a retrofit solar system increases the applied loading on the roof by more than 15%, Approved Document A requires the structural integrity of the roof structure and the supporting structure to be assessed. This applies to building works carried out in England, and in Wales in some circumstances, with equivalent provisions in Scotland and Northern Ireland. A desktop assessment can be the means of carrying out that assessment where the structural record is adequate to support it. It does not discharge the requirement where the record is not adequate, and it cannot satisfy cases where the applicable standards call for verification informed by the structure as built.

= Limitations =

The principal limitation is that the assessment relies on the record being accurate. Buildings are altered, structures are modified, and members corrode or are damaged in service. A desktop assessment cannot detect any of this. Where the consequence of an error is significant, or where the building's history is uncertain, the assessment should be confirmed by survey.

= Related articles on Designing Buildings =

Structural assessment for rooftop solar PV · Ballasted solar PV on flat roofs · Structural survey · Condition survey · Solar photovoltaics · Eurocodes

= External references =

BS EN 1990, Eurocode: Basis of structural design.

BS EN 1991-1-3 and BS EN 1991-1-4, Eurocode 1: Actions on structures.

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

[https://www.solarsurveys.co.uk/desktop-reports Solar Surveys], desktop structural roof loading reports for commercial rooftop solar — an example of the process described in this article, including the information required and the form of the output.

Desktop Structural Roof Loading Assessment - Revision history

Solar Surveys Ltd: Created page with "= Desktop structural roof loading assessment = A desktop structural roof loading assessment is a structural appraisal of a roof's capacity to carry a proposed additional load, c..."