The question, in installers' own words
In January 2023, an installer in Dorset posted this on ElectriciansForums:
"As part of mcs you need to consider roof loads for basic roofs like a truss roof you don't need a structural engineer and they give you figures for the calculations. Am I correct in thinking I can just use easy pv to do this calculation if I put in correct figures. Or would they expect me to have a more in depth knowledge on this subject."
Later that year, in the same thread, another installer replied:
"I have a solar company and my MCS visit is in September. I never knew I needed to understand roof loads, so I will have to learn."
Two working installers. One with an assessment booked. Neither certain what is being asked of them, and both willing to say so in public, which is more than most of the industry manages.
They are not outliers. Two years earlier, a thread titled "Solar Sole traders/small firms and the MCS" opened with the same worry from the other direction — that the requirement was a barrier to entry. Its author wrote that "it appears as though you need a structural engineer to assess the weight carrying capability of the structure… If so then i think this is a shame as it means only the big boys can do this work."
An experienced installer, posting as BXB, replied:
"The structural survey requirements have always seemed a bit vague to me. Some installers ignore them, some carry them out on every job… for smaller domestic jobs you can have the survey done remotely (desktop survey) using photographs and measurements."
Read that first sentence again. Some ignore them, some do them every time. When practice spreads that far across a small industry, the spread is itself the evidence: the requirement is not being read the same way twice.
The shape of the obligation
We can describe the shape of what MCS asks. We are not going to quote a clause number at you, because we have not verified which clause of MIS 3002 carries it, and a wrong clause reference in a compliance post is worse than no reference at all.
The shape has three parts.
Somebody competent has to satisfy themselves that the structure will carry the array — the modules, the mounting system, and whatever the wind and the weather do to them once they are up there.
That assessment has to be recorded. Not remembered, not reasoned through on the ladder and forgotten. Written down, attached to the job.
An assessor will ask to see it. That is what an assessment visit is: someone opens your job files and looks for the evidence that the thinking happened.
Nothing in that shape says a chartered engineer must do the thinking. Nothing in it says one must not. That is precisely the gap the Dorset installer fell into, and asking a forum could not close it.
One caveat. MCS is rolling out a redeveloped Installer Scheme throughout 2026 and into 2027, and says the technical requirements "aren't changing… but they have been restructured and streamlined to be more focussed as 'technical truths'". Restructured is not unchanged in the place you last looked. If you learned this material from an older version, the wording you remember may no longer live where you left it.
What "desktop survey" actually refers to
BXB's description is the most useful sentence in either thread: for smaller domestic jobs, the survey can be done remotely, from photographs and measurements.
That is one experienced installer describing what he sees done. It is not an MCS rule, and we are not going to dress it up as one. What it tells you is that remote assessment is not a fringe shortcut invented by firms cutting corners — it is ordinary practice on ordinary domestic roofs.
What it does not tell you is where the edge of "smaller domestic" sits, or whether last Tuesday's photographs would satisfy next spring's assessor.
What a defensible record looks like
Set aside what the standard requires — we have not sourced that, and will not guess. Ask the narrower question: if an assessor opened this job file, what would they look for.
| In the record | Why it is there |
|---|---|
| Rafter or truss dimensions and spacing | Establishes what is carrying the load. Measured on the day, not inferred from the age of the house. |
| The roof covering — tile or slate, type, and what is already up there | The existing dead load is what you are adding to. An in-roof system removes some of it. |
| Array mass and mounting method | Module count and module weight, plus rails, hooks or feet and their spacing. On-roof and in-roof are not the same load case. |
| Photographs, dated | The loft space, the rafters, the roof plane. A tape in shot wherever you took a measurement. |
| Who assessed it, when, and what they concluded | A name, a date, and one plain sentence stating the conclusion. |
Notice what is absent from that table. No kN/m². No dead load figure, no imposed load figure, no snow or wind method. We have not sourced MCS-published load figures, so we are not going to reproduce any. Be wary of any page that does, without telling you where the numbers came from.
The Dorset installer wrote that "they give you figures for the calculations". He may well be right. We have not verified it, so we are not repeating it as fact — and nor should you, on the strength of a blog post.
Half that table exists as data before anyone climbs a ladder. Module count, module weight, layout, mounting method — that is the design, known the moment the design is finished. The gap is that it stays in the design tool and gets reconstructed from memory months later, when the assessor asks. That is the part of this we think about at Solar Engine: what the design already knows should arrive in the record as data. It will not tell you whether the roof takes the load. Nothing short of an assessment will.
Where the engineer threshold sits
We do not know. We are not going to guess, and we would rather say that plainly than give you a clean-sounding rule you could be marked down against.
Anyone who hands you a threshold — systems over X kWp need an engineer, trussed roofs never do — without pointing at the clause that says so has invented it. That includes us. This is a compliance question with one right answer for your scheme and your scope, and there are exactly two places to get it.
Read MIS 3002 yourself, and then put the threshold question to your certification body in writing. Not to a forum, not to a supplier, and not to a blog. Your certification body is the organisation that issued your certificate and sends the assessor who will judge the record. They are the only party whose answer is worth anything, because theirs is the answer you will be held to.
There is a further reason to settle it now. NICEIC, describing the new risk-based surveillance model, sets out an "Enhanced assessment frequency" tier: annual assessment plus "a minimum of five sites per technology". If you land in that tier, the number of job files someone opens goes up sharply. A gap in your structural record stops being something you might get away with and becomes something that gets found.
What to do this week
Pull up MIS 3002 and read the structural section end to end, not the summary someone sent you.
Send one email to your certification body: where does the structural engineer threshold sit for the work you actually do — domestic pitched roofs, trussed roofs, whatever your scope is. Get the answer in writing and file it with your scheme paperwork. A written answer from your certifier is itself part of the record.
Then take your last three installs and see whether you could produce, today, the rafter dimensions, the covering, the array mass, the mounting method, the photographs, the date and the assessor's name. If you cannot, the problem is not that you did the assessment badly. It is that you did it in your head.
Fix the template, not the memory.