Guidance

The “Guide to the Installation of Photovoltaic Systems” states that structural calculations should be undertaken by a suitably competent person, and that this should include a site inspection.

It also states that where the roof is unusual in any way, such as, for example:

• There are signs of structural distress
• There are signs of post-construction modification such as removal or notching of timbers, or a change of roof covering
• The roof pitch is particularly shallow (< 30°)
• The roof design has increased potential for snow build-up (e.g. dormers, valleys, parapets). The type of construction is not detailed in the MCS guide
• or if there is any doubt whatsoever, then a qualified structural engineer should be consulted.

The tools provided here simplify the calculations that are recommended in the MCS guide. However, you take full responsibility for the calculations and should ensure you are competent to carry them out.

For installations on flat roofs, the MCS guide states that special consideration shall be given to the load of the PV system and any associated ballast, and reiterates that structural calculations shall be carried out by a suitably competent person.

Self Roof Appraisal

The Self Roof Appraisal is a structural roof-loading check for a PV installation. It estimates the dead (weight), snow and wind loads on each roof and compares them against permitted values, following the MCS / Guide to the Installation of Photovoltaic Systems method (BRE Digest 489, Building Regs Approved Document A, BS EN 1991‑1‑3 UK NA, TRADA span tables and BS 5268‑2).

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What it is — and is not

This tool simplifies the calculations recommended in the MCS guide. It does not calculate the strength of the roof. If a roof is in poor condition, was badly built, or is unusual in any way, a qualified structural engineer should still be consulted. You take full responsibility for the figures and should be competent to carry them out.

Where to find it

Project → Roof Appraisal → New Self Appraisal. Saved appraisals are listed back on the Desktop Appraisal Manager under Self Appraisals.

The wizard

The appraisal is a 4-stage wizard:

The number of roofs is dictated by the proposal — there is no manual add/remove.

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Proposal step

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Site step

Most of these are determined automatically from the project's location and can be overridden.

Site card

Terrain & topography card

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Roof step (per roof)

1. Roof structure type

Pick the roof's construction. The type sets the permitted loads, whether a timber span check applies, the default wind shape, and whether the roof can be calculated at all.

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Consult-an-engineer triggers

A roof is flagged "Consult a structural engineer" (and not calculated) if it is a Hip/valley, Asymmetric, or Flat roof — ballasted type, or if a pitched (non-flat) roof has a pitch below 30° or above 45° — the range covered by this method. This mirrors MIS 3002: shallow pitches and ballasted flat roofs need an engineer.

2. Roof & covering

3. Loads & array

Rafter / joist (TRADA span) — cut & mechanically-fixed flat only

Wind fixings — pitched roofs only (hidden for flat)

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What is calculated

For each calculable roof, four (or five) checks are run and shown in the Summary.

1. Weight loading — 15% increase (Approved Document A)

The array loading is (panel count × panel weight + mounting × area) × 9.81 / 1000 / area.

Increase % = array loading ÷ (covering dead load + imposed load)

Pass if the increase is < 15% — "not considered a significant change" under The Building Regulations 2010, Approved Document A.

2. Permitted dead load

Total dead = array loading + covering dead load. Pass if Total dead < Permitted dead load.

3. Snow loading

sₖ = zone/10 + 0.20 + (altitude − 100)/525     (BS EN 1991-1-3 UK NA, Eq NA.1)
snow on roof = μ × sₖ                           (μ = shape coefficient by pitch)

Combined imposed = array loading + snow. Pass if Combined imposed < Permitted imposed load.

4. Wind loading — BRE Digest 489

F = qₚ × A_ref × Cp,net × Cₐ × Cₜ × 1.35
Cₐ = 1 + 0.2 × (altitude − 100) / 100

• qₚ — peak velocity pressure (wind zone × building height × terrain × distance to sea).
• Cp,net — net pressure coefficient (roof shape × pitch).
• Cₐ — altitude factor. Cₜ — topography factor. 1.35 — safety factor.

The edge uplift per hook is compared against:

• BS 5268 pull-out = pull-out per mm × 1.25 × fixing length in timber (per fixing), and
• the MCS012 cert uplift (per hook).

Pass if both are satisfied.

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Flat roofs

Flat roofs are not hook-fixed, so the wind hook/pull-out check is not run for a mechanically-fixed flat roof — the results show a note to use the mounting manufacturer's wind/fixing calculation instead. A ballasted flat roof is not calculated at all: it requires a structural engineer (structure + ballast loading).

5. Rafter / joist span (TRADA) — cut & mechanically-fixed flat

The maximum permitted unsupported span is looked up from the TRADA span tables (pitch band × dead-load band × spacing × breadth × depth). Pass if permitted span > actual unsupported span.

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Results

The Summary shows each roof's tables with Pass / Fail / n/a badges and an overall result:

• all roofs pass — every calculated roof passes.
• one or more roofs fail — review the failing check.
• one or more roofs require a structural engineer — a consult roof (hip/asymmetric or out-of-range pitch) is present.

Calculate & Save validates that every roof's required fields are complete, then stores the appraisal (inputs + results) against the project. It appears in the Self Appraisals list on the Desktop Appraisal Manager.

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Verify everything

Auto-filled values (altitude, wind/snow zone, distance to sea) and the standard defaults are a starting point — verify every figure against MIS 3002 and the actual roof before relying on the result.