In roof solar panels cost 30-60% more to install than an equivalent on-roof system on a typical retrofit, according to installer cost data published in 2025 (The British Solar Blog, 2025). That premium is the single biggest reason most UK quotes still default to rail-mounted, on-roof arrays, even as demand for a flush, tile-level finish keeps growing among homeowners doing loft conversions or full re-roofs.
For installers, "in roof solar" versus "on-roof" isn't really an aesthetics argument. It's a quoting decision that touches structural loading, MCS 012 mounting compliance, BS 5534 roofing obligations, and how many extra hours a two-person crew needs on the roof. This guide compares both systems from that angle, with the numbers a UK solar installer actually needs before pricing a job.
Key Takeaways
- In-roof solar panel kits typically add 30-60% to install cost on a retrofit, but can undercut on-roof once a full re-roof or new build is already scheduled.
- In-roof panels run 5-10% less efficient than on-roof panels due to reduced rear ventilation.
- From August 2025, flat roof mounting systems must carry MCS 012 certification, with no fallback design clause.
- BS 5534 still governs any tile or slate work disturbed by in-roof solar panel trays, including underlay penetration sealing.
- UK solar installations passed the two million mark in March 2026, and the resulting demand pressure makes install-time efficiency a genuine competitive edge.
What Is the Difference Between In-Roof and On-Roof Solar Panels?
In-roof solar replaces a section of tiles or slates so the panels sit flush with the roof plane, functioning as the weatherproof covering itself. On-roof solar uses rails fixed to the rafters through the existing tiles, with panels mounted on top. Both generate electricity identically; the difference is purely mechanical, in how the array meets the roof structure (Sunsave, 2026).
An on-roof array leaves the original roof covering intact and adds hardware above it, so existing waterproofing stays untouched. An in-roof solar panel kit removes that covering across the array footprint and substitutes purpose-built trays, flashing and the panel itself as the new weather barrier. That distinction is why in-roof work is fundamentally a roofing job with an electrical component bolted on, not the reverse.
In-Roof Solar Panels vs On-Roof: Cost, Installation Time and Structural Impact
On a straight retrofit, in-roof systems cost more and take longer to fit because a roofer has to strip tiles, batten out the array area and dress bespoke flashing before any panel goes on. On-roof stays faster and cheaper because the crew works over the existing covering rather than replacing part of it (The British Solar Blog, 2025). The table below sets out how the two compare across the variables that actually shape a quote.
- Typical cost vs on-roof (retrofit): In-roof solar: 30-60% premium, more on complex roofs; On-roof solar: Baseline
- Typical cost (new build / full re-roof): In-roof solar: Can be cost-neutral or cheaper, tiles saved offset panel cost; On-roof solar: Baseline plus full roof covering cost
- Installation time: In-roof solar: Longer; requires roofer plus solar installer, tile removal and flashing; On-roof solar: Shorter; rails fixed over existing covering
- Panel weight: In-roof solar: Roughly half, around 10kg per panel; On-roof solar: Around 20kg per panel
- Warranty: In-roof solar: Around 15 years typical; On-roof solar: Around 25 years typical
- Efficiency: In-roof solar: 5-10% lower due to reduced rear ventilation; On-roof solar: Baseline, benefits from airflow beneath
- Aesthetics: In-roof solar: Flush, low-profile, favoured in conservation areas; On-roof solar: Visible rail and frame, standard profile
- Structural/roofing implication: In-roof solar: Roof covering penetrated, BS 5534 underlay rules apply directly; On-roof solar: Existing covering largely undisturbed, fixings still need BS 5534-compliant sealing
How Does MCS 012 Affect In-Roof Solar Panel Trays and Mounting Kits?
MCS 012 is the standard that certifies solar mounting systems, and it now covers both mechanically fixed and ballasted products following the release of Issue 3.0 (Alternergy, 2025). From 10 August 2025 flat roof mounting systems must hold MCS 012 certification outright, with the previous Clause 5.5.5 design workaround withdrawn entirely (MCS, 2025).
Pitched-roof in-roof solar panel trays sit slightly differently under the standard, since they're assessed as part of a roof-integrated system rather than a bolt-on frame, but installers should still confirm the specific tray and flashing kit carries current MCS certification before quoting. A field observation worth flagging here: several installers report that manufacturer datasheets for older in-roof kits still reference the previous MCS 012 issue, which is a red flag worth chasing up with the supplier before the job goes on the schedule. Getting this wrong risks an MCS compliance failure that blocks the customer's Smart Export Guarantee application later. For a fuller walkthrough of what MCS certification actually requires on a job file, see Reonic's guide to MCS compliance.
What Does BS 5534 Require for In-Roof Solar Panel Installations?
BS 5534:2014+A2:2018 is the code of practice for slating and tiling on pitched roofs, and it applies directly whenever an in-roof solar panel kit disturbs the existing covering. The standard's core requirement is mechanical fixing rather than reliance on mortar, plus specific rules for perimeter zones under wind loading (Marley, 2025).
Where an in-roof mounting tray penetrates the underlay, BS 5534 sections 4.9 and 6.2 require that the penetration not compromise the underlay's performance, meaning it needs sealing with a purpose-designed product rather than generic mastic (SSQ Group, 2025). Installers who treat in-roof jobs as a solar task first and a roofing task second tend to be the ones who get callbacks over leaks eighteen months later. Reonic's separate breakdown of BS 5534 for solar installers goes through the fixing tables and eaves/verge detailing in more depth.
Is In-Roof Solar Panels UK Adoption Growing, or Still a Niche?
In-roof and other building-integrated PV products remain a genuine niche against the broader UK market, with BIPV annual installations estimated at only 40-60 MW by 2026 against a rapidly growing national solar fleet (Coule Energy, 2025). The broader market context matters here too: certified UK solar installations hit a record 203,125 in 2025 alone, taking the total past 1.85 million, with new-build homes now accounting for 35% of that annual figure (MCS, 2025). Total UK solar installations passed 2,003,000 by the end of March 2026 (pv magazine, 2026).
That new-build growth is where in-roof solar UK demand is concentrated, because a developer fitting solar as part of a scheduled roof covering doesn't pay twice for the same square metre of roof. On a retrofit, in-roof stays a specialist request rather than a default, driven mostly by conservation area rules or a client who's already having the roof redone for other reasons.
When Does In Roof Solar Beat On-Roof on Aesthetics and Planning Grounds?
In-roof wins the planning argument in conservation areas, listed buildings and anywhere a client wants zero visible rail or frame line. Roof-mounted panels of either type generally fall under permitted development provided they don't protrude more than 200mm from the roof surface, but a flush in-roof kit removes the visual objection entirely rather than relying on the protrusion rule (Capture Energy, 2025).
Article 4 Directions in some conservation areas withdraw permitted development rights altogether, which is where in-roof solar panels become a genuinely useful sales argument rather than just a preference. Installers quoting in these postcodes should check for an Article 4 Direction before assuming permitted development applies to either system, since the local authority may require full planning consent regardless of mounting type.
In-Roof Solar Panel Kit Selection: What Should Installers Check Before Quoting?
The right in-roof solar panel kit depends on roof covering type, since flashing kits differ for flat concrete tiles, plain tiles, slate refits and new-build slate, and mixing the wrong kit with the wrong covering is the fastest route to a leak claim. Suppliers sell dedicated trays, flashing and fixings by covering type, and the kit choice should be locked before scaffolding goes up, not during the job.
A survey of the existing roof structure matters more here than on an on-roof job, because the installer is removing part of the weatherproof layer rather than adding to it. Checking batten condition, rafter spacing and any prior repairs before quoting avoids the expensive surprise of discovering rotten timber once tiles are off. Reonic's notes on photovoltaic system maintenance strategy cover how mounting choice feeds into longer-term maintenance planning, which is worth factoring into the initial quote conversation with the client.
Is In-Roof Solar Worth the Retrofit Premium for Most UK Homeowners?
For most retrofit customers, no. The 30-60% cost premium combined with a 5-10% efficiency loss and a shorter roughly 15-year warranty makes on-roof the more sensible default when budget and payback period are the priority (Sunsave, 2026).
The calculation flips when a full re-roof or new build is already happening. In that scenario the client is paying for roof covering either way, and in-roof solar can absorb part of that cost rather than adding to it on top of a separate on-roof spend. Installers should frame the choice around the client's roof timeline rather than aesthetics alone, since that's the variable that actually decides whether in-roof pencils out.
Weatherproofing and Long-Term Maintenance: In-Roof vs On-Roof
On-roof systems keep the original weatherproof layer intact and add a smaller number of roof penetrations for rail fixings, which BS 5534-compliant sealing handles reliably over decades. In-roof systems replace a larger section of the weatherproof layer with trays and flashing, so long-term performance depends entirely on flashing quality and installer competence rather than an established tile system (Marley, 2025).
Maintenance visits on in-roof arrays should include a flashing and seal check that isn't typically needed on an on-roof job, since any degradation there risks water ingress through what is now the primary roof covering rather than a secondary fixing point. Reonic's guide to solar panel maintenance sets out a practical inspection schedule that installers can adapt for in-roof-specific checks, and the MCS installation certificate process is worth revisiting for any job where the mounting kit itself has changed since the original design was signed off.
Beyond the technical case, in-roof jobs are also where quoting mistakes cost the most, since a missed structural check or an outdated MCS 012 reference on a tray kit turns a profitable job into a warranty dispute. Installers running proposal and job-tracking software, such as Reonic's platform for solar and heat pump installers, tend to catch these gaps earlier because the compliance checklist sits inside the same workflow as the quote itself, rather than being a separate step someone forgets.
FAQ
Is in-roof solar cheaper than on-roof in the UK?
Not usually on a retrofit. In-roof typically costs 30-60% more than on-roof once you account for tile removal, bespoke flashing and the extra labour of a roofer alongside the solar installer (The British Solar Blog, 2025). It can be cost-neutral or cheaper only when bundled into a scheduled re-roof or new build.
Do in-roof solar panels need MCS 012 certification?
Flat roof mounting systems must hold MCS 012 certification from 10 August 2025, with no design workaround remaining (MCS, 2025). Pitched in-roof kits are assessed under the same broader standard, so installers should confirm current certification on the specific tray and flashing kit before quoting.
Does BS 5534 apply to in-roof solar panel installations?
Yes. Any in-roof solar panel kit that disturbs tiles, slates or the underlay falls under BS 5534:2014+A2:2018, which requires mechanical fixing and proper sealing of underlay penetrations rather than reliance on mortar or generic mastic (SSQ Group, 2025).
How much longer does an in-roof solar panel installation take than on-roof?
There's no single published figure, but in-roof work reliably takes longer because it involves tile or slate removal, batten preparation and custom flashing before a panel goes on, work that on-roof installation skips entirely by mounting rails over the existing covering (Sunsave, 2026).
Are in-roof solar panels less efficient than on-roof panels?
Yes, typically by 5-10%, because in-roof panels sit flush against the roof deck with far less airflow underneath than on-roof panels, which run cooler thanks to the standard rail gap (Sunsave, 2026). That efficiency gap is worth factoring into any yield estimate given to the client.
