In-roof vs on-roof solar

On-roof mounting fixes rails to the rafters through the existing covering and sits the panels above it. In-roof mounting removes the covering across the array area and builds the panels into the roof plane on a tray or flashing system, so the array replaces the tiles rather than sitting on them. On-roof is quicker, less expensive and easier to service. In-roof looks flush and integrated, weathers as part of the roof, and is often preferred where appearance carries weight. Neither is universally correct — the roof, the property and what you care about decide it.

How on-roof works

Tile hooks or equivalent fixings are set into the rafters, the covering is relieved so it beds back down properly, and rails span between the fixings. Panels clamp to the rails.

The existing roof covering continues to do the weathering. The array sits above it with an air gap, which helps keep cell temperatures down and makes individual panels straightforward to remove for service.

The critical detail is the fixing. Fixings go into rafters, not battens, and the covering has to be relieved and re-laid properly so it sits flat and sheds water. Most of the on-roof failures we get called to inspect are one of those two things done badly.

How in-roof works

The covering is stripped across the array footprint. A tray, deck or flashing system is installed onto battens or a suitable substrate, and panels drop into it. Flashings and soakers weather the perimeter into the surrounding tiles or slates, and the array becomes part of the roof surface.

The result sits within the roof plane. From ground level a well-executed in-roof array reads as a recessed panel of glass rather than an addition.

Because the tray system is now doing the weathering across that area, the detailing has to be right. This is skilled roofing work as much as solar work.

Comparing them honestly

Appearance. In-roof wins clearly. Flush, no visible rails or clamps, no shadow line above the tiles. On-roof with all-black panels and black clamps is tidy but still evidently mounted on top.

Cost. On-roof is less expensive. In-roof involves stripping covering, more materials, more labour and more time on the roof.

Weathering risk. On-roof leaves the existing covering intact and doing its job. In-roof replaces it across the array, so the quality of the detailing carries more weight.

Temperature. On-roof has better airflow behind the panels. In-roof systems manage this by design, but the physics favours a ventilated gap.

Servicing. On-roof panels come off easily. In-roof access is more involved, particularly for a panel in the middle of an array.

Suitability. In-roof needs a reasonably regular, uninterrupted roof area. Cut-up roofs with hips, valleys and dormers are harder to detail and sometimes unsuitable.

When in-roof is the obvious choice

Three situations recur.

New build or a full re-roof. The covering is coming off anyway, so much of the cost differential disappears. If you are re-roofing and considering solar within a few years, doing them together is nearly always the better decision.

Planning-sensitive properties. Roof solar on residential buildings may be permitted development subject to limits and conditions, but listed buildings, conservation areas and World Heritage Sites carry additional restrictions and may require consent. Where a flush appearance is the difference between a scheme that is acceptable and one that is not, in-roof is worth the premium.

Where appearance is simply the priority. Some owners care a great deal about how the front of their house looks. That is a legitimate reason on its own.

When on-roof is the sensible choice

Most of the time, frankly. A sound existing covering, a rear or side slope, a household focused on generation and cost rather than kerb appeal — on-roof does the job with less disturbance to the building and less to go wrong.

It is also the right answer where the roof covering is unusual or fragile, where the array is broken into several small blocks, or where you want to keep the option of reconfiguring the array later.

What we assess

At survey we look at covering type and condition, rafter spacing and structure, the regularity of the array area, the property’s planning status and how visible the roof is. Then we tell you which mounting approach we would recommend and why — including where the cheaper option is the better one.

Where a roof is due re-covering within a few years, we will say so. Fitting an array onto a covering that needs replacing shortly afterwards means paying to remove and refit it.