Structural surveys: when your roof cannot take it
Most roofs carry an array without difficulty. The ones that do not are identifiable in advance, and MCS publishes the list of warning signs — five specific conditions that mean a qualified structural engineer should be consulted rather than a calculation applied.
Most roofs take an array without difficulty. The exceptions are predictable.
MCS’s installation standard requires the contractor to ensure the roof structure is checked by a suitably competent person, and to consult a qualified structural engineer where the structure is unusual “or there is any doubt whatsoever”.
It then lists what counts as unusual — five conditions you can check yourself before anyone quotes.
The five triggers
MIS 3002 gives this list of what makes a roof structure unusual, meaning a qualified structural engineer should be consulted rather than a standard calculation applied:
| Trigger | What it means in practice |
|---|---|
| Not a standard roof type | Construction not covered by the standard’s own methodologies — unusual trusses, historic framing, hybrid structures |
| Signs of structural distress | Deflection, sagging ridge or purlins, cracked or split timbers, movement at the wall plate |
| Post-construction modification | Removal of timbers, notching, or a change of roof covering to a heavier material |
| Shallow pitch | Below 30° to the horizontal |
| Snow build-up potential | Dormers, valleys, parapets — anywhere snow can accumulate rather than shed |
Three of those are visible from a loft with a torch, and two from the ground. If any apply to your roof, the correct answer to “has a structural engineer looked at this?” is yes.
“Post-construction modification” is the trigger that catches the most houses, and almost nobody thinks of it.
A loft conversion removes or alters roof timbers by design. So does a large rooflight, a dormer, or a re-roof from slate to concrete tile — the last of which adds substantially more weight than an array will.
If any of that has been done to your house, say so at survey. The installer cannot see it from a photograph.
What the standard requires of an installer
Two obligations worth knowing, because they give you something specific to ask for.
The structure must be checked. MIS 3002 requires the MCS contractor to ensure the roof structure is checked by a suitably competent person to confirm it can withstand the loads imposed by the system.
Both weight and wind must be calculated. Where a non-MCS-certified mounting system is used, the contractor must be able to provide evidence that a structural assessment and a wind loading calculation have been completed for the specific mounting system — based on calculations from the manufacturer, a third-party structural engineer, or appropriate software.
The standard also notes that such calculations “do not constitute compliance alone” and must be directly linked to the design and installation of the system. A generic manufacturer datasheet is not an assessment of your roof.
Alongside those, the standard requires the contractor to be able to evidence how the weathertightness of the roof has been maintained, and that the customer has been informed they should undertake an updated fire risk assessment to confirm the roof’s fire performance has been maintained.
Those are all things you can ask to see.
Weight, and why it is usually not the problem
A domestic array adds roughly 12 to 20 kg per square metre including mounting. For context, that is broadly comparable to adding a layer of tiles — real, but well inside what a sound modern roof carries.
Which is why the risk concentrates in specific places rather than being general:
Older roofs, particularly pre-war structures with irregular or undersized timbers and no design calculations behind them.
Roofs already carrying heavy coverings. A stone slate or concrete tile roof is already near its design load; a slate roof re-covered in concrete tile at some point in its life is a particular concern, and this is the “change of roof covering to a heavier material” trigger.
Altered structures. As above — the load path may no longer be what the original design assumed.
Point loads. How the array’s weight is delivered into the structure matters as much as the total. A mounting system that concentrates load onto few fixings behaves differently from one that spreads it.
Wind matters more than people expect
On an exposed site, wind uplift can be the governing case rather than dead weight. An array is a large surface, and the pressures at roof edges and corners are considerably higher than in the middle of a slope.
The consequences run both ways: the fixings must hold the array down, and the roof structure must resist the uplift transmitted into it. This is why the standard requires a wind loading calculation for the specific mounting system rather than a generic figure — and why a design that is fine in a sheltered suburb may not be on a coastal or hilltop site.
Ask for the wind calculation specifically. It is the one most likely to have been skipped.
Sequencing: do the roof first
If the roof covering is near the end of its life, this decision comes before the solar decision.
An array installed over a covering that needs replacing in five years means paying to remove the array, re-roof, and refit it — often with scaffolding twice. That cost can approach the value of the generation you gained in the meantime.
Signs the roof should come first: widespread slipped or cracked tiles, failed underfelt visible from the loft, repeated leaks, or a covering at the end of its typical service life.
The opportunity: if you are re-roofing anyway, an in-roof system becomes viable and the scaffolding is already there.
What to ask for, in writing
- Who carried out the structural check, and what are their qualifications?
- Was any of the five unusual-structure triggers present? If so, was a structural engineer consulted?
- The wind loading calculation for the specific mounting system on this roof.
- How weathertightness is maintained — method, and whether existing roof warranties survive.
- Confirmation about the fire risk assessment, which the standard requires you to be told about.
- The condition of the covering, and whether it has years left in it.
An installer doing this properly will have all of it. One who treats the questions as obstructive has told you something useful.
If the answer is no
A roof that cannot take a full array is not the end of it:
- A smaller array, or a lighter mounting system
- A different plane — an outbuilding or garage roof, which is still Class A permitted development and often structurally simpler
- Strengthening works, costed against the value of the generation
- Ground mounting, if the plot allows it
- Re-roofing first, and installing as part of that work
The one option that is not available is proceeding anyway. Overloading a roof is a material alteration under the building regulations and a genuine safety risk, and it is your house.
Sources
Contains public sector information licensed under the Open Government Licence v3.0.
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