In-Roof Solar Mounting Buying Guide

What in-roof solar actually is

A conventional on-roof solar array sits about 40–60mm above your tiles on stainless roof hooks and aluminium rails. An in-roof (or roof-integrated) array does something quite different: the tiles directly beneath the panels are removed, and a weathertight tray of aluminium flashings, waterproof strips and pre-compressed foam takes their place. The panels then slot into that tray so their top surface is flush with the surrounding tile line — often near-flush with slate, and slightly proud of thick concrete tiles.

The panels themselves become part of the roof covering. Rainwater runs off the panel glass into a bottom flashing designed like a valley gutter, over the tiles below, and away into the eaves. Wind can't lift the leading edge the way it can with a proud-mounted panel, birds can't nest underneath, and the roofline reads as a single continuous surface from the ground.

In-roof vs on-roof: which fits your job

Neither system is inherently better. In-roof genuinely earns its place on new-builds, full re-roofs, planning-sensitive properties and anywhere the finished look matters. On-roof stays cheaper and simpler on straightforward retrofits where the existing tiles are sound. The three factors that usually decide it are:

• Timing. Is the roof already being disturbed anyway? If yes, in-roof adds little labour on top.
• Aesthetic weight. Conservation areas, Article 4 directions and design-led architects usually push toward in-roof.
• Roof condition. Tiles that need replacing in the next 5–10 years should probably be replaced now, under the array — making in-roof the sensible move.

For on-roof projects, see our pitched-roof mounting buying guide covering Fastensol, Renusol and Renusol HL kits with a live kit builder for hooks, rails and clamps.

Roof suitability: pitch and tile type

In-roof systems ask more of the roof beneath them than on-roof does, because the flashings and waterproof strips assume specific tile geometry. Before speccing any brand, check pitch, tile profile and the condition of the battens and underlay.

Roof pitch

All three systems in this guide are designed for pitched slate and tile roofs. The published limits are:

Below 12°, rainwater doesn't shed reliably off the panel glass and manufacturers withdraw the seal warranty. Viridian's certification starts at 20°, so shallower pitches (12°–19°) narrow the choice to GSE or S4. Above 50°, tile support and safe installer access become the practical limits for GSE; Viridian is certified up to 60°, and S4's frame system is warranted up to 70° for gable and mansard roof faces.

Tile compatibility

All three systems work with the main UK tile families. Deeply profiled tiles (Roman, Double Roman, deep pantile) need a tilt filet — a tapered timber batten under the bottom flashing — to lift the flashing edge to tile crown height and keep water from tracking back under the array.

Batten section and roof structure

The battens beneath an in-roof array carry both the mounting frame load and the wind uplift the whole array transfers into the roof structure. Under Eurocode 1 and BS 5534, the minimum acceptable batten section is 25 × 50mm, and manufacturers recommend 27 × 100mm for maximum load spreading. The support batten directly under any flexible waterproofing strip must be a maximum 18mm thick, so the strip lies flat without a step.

On any roof more than 15–20 years old, budget for stripping and replacing the battens under the array footprint plus one extra row all round. Old wire nails and split battens are the single most common cause of failed uplift tests.

The three systems compared

We stock three in-roof systems, each with a genuinely different philosophy. S4 is a universal modular frame that fits almost any 60-cell or 66-cell panel. GSE is a proven half-frame system with a huge component ecosystem and the deepest UK certification history. Viridian Clearline Fusion is a complete panel-plus-flashing system where you're really buying an integrated roof product rather than mounting hardware for third-party panels.

Kit calculator

Pick a brand, tell us how the array is laid out, and we'll assemble a starting bill of materials with live pricing from the Aizo store. The result is a good working estimate — treat it as a specification starting point rather than a final purchase list, since real projects need a site survey to finalise flashing overlaps, roof-edge treatments and wind-zone screw counts.

Compatible solar panels

S4 and GSE accept most 60-cell and 66-cell monocrystalline panels around 1720–1780mm × 1130–1140mm. Viridian Clearline Fusion only accepts Viridian's own PV16 panels because the flashings are engineered around the PV16 profile. Below are the panels most commonly specified in the arrays we quote.

Installation considerations

Roof preparation

Strip the tiles across the array footprint plus one extra row all round — two extra rows for slate and plain tile installations, because the shorter overlap of these coverings needs more clearance for the flashings. Inspect the underlay: any tears, ballooning or brittle sections under the array need to be replaced now, because you won't get back to them without stripping the array. Replace any battens that split, are attached with wire nails, or show rot around the fixings.

For a rectangular array, S4's own field-sizing formula is a useful sanity check on tile-strip width regardless of brand: field width equals module width × panel count, plus the inter-module gaps, plus 400mm total (200mm clear on each side for the lateral flashings); field height equals module height × row count plus the inter-module gaps plus 150mm for the waterproofing strip at tile level.

Screw fastening into battens

Whichever system you're fixing, the screw-spacing rules are the same timber engineering regardless of brand. Under Eurocode 5 (EC5) and BS 5268, screws into softwood battens (pine) need a minimum centre-to-centre spacing of 3× the screw diameter to avoid splitting; hardwood battens (oak, beech and other dense timbers) need 4× the diameter, or pre-drilling, plus a minimum edge distance of 2× the diameter (so a 6mm screw needs at least 12mm clearance from the batten edge). Never overtighten into dry timber, and use screws with an anti-splitting tip.

DC cable routing under an in-roof array

Route all DC cables beneath the frames so no cable passes through the water-shedding surface. Each system provides a defined cable pathway:

• GSE Landscape / Portrait — each half-frame has a central hole for micro-inverter fixing and cable pass-through, and moulded channels beneath the frame walls that guide the DC trunk cleanly between panels.
• S4 RooFit — the manual documents two supported routing options: cables passed under the frame assembly between battens, or over the top of the frame to reach the connection point — giving more flexibility in siting the optimiser or micro-inverter than a single fixed channel.
• Viridian Clearline Fusion — the ArcBox connector enclosure on each panel houses the MC4 connections inside a KIWA fire-tested container, so any DC arc is contained within the roof void rather than in open air.

Cables must be secured every ~600mm with UV-stable clips or ties so they can't chafe on tile edges, batten fixings or frame corners. Never route a cable over the top of a flashing or through the seal line where a clamp could pinch it. Support MC4 connectors so panel weight doesn't hang on the connector body over time — a small drip loop below each connection prevents water tracking into the plug.

Micro-inverters and DC optimisers in in-roof arrays

Every in-roof installation shares one hard constraint: once the tiles and perimeter foam are in place, servicing any electronics under the array means lifting panels. The choice of module-level power electronics (MLPE) — Enphase microinverters, SolarEdge optimisers, Tigo optimisers — should therefore be made with future access in mind, not just electrical performance.

Enphase microinverters (IQ7A, IQ8, IQ8P, IQ8M)

Enphase units are bolted to the panel frame via the supplied mounting bracket, or to a dedicated batten through the central frame hole on GSE Landscape. The Q-cable trunk daisy-chains under the array to the field wiring termination. For in-roof:

• Position each microinverter at the head of the panel (top edge) rather than the foot, so the connector is nearer the ridge cable exit and easier to reach if a panel above the failure is lifted.
• Leave the Q-cable trunk with a service loop of at least 200mm at each drop, so an inverter can be swapped without pulling the trunk under tension.
• Use Enphase's own IQ Cable rather than third-party trunking — the connector interface is proprietary and the warranty depends on it.
• The Envoy / IQ Gateway lives indoors and doesn't factor into the roof-side install.

SolarEdge DC optimisers (P-series and S-series)

P-series optimisers (P370, P401, P505, P801, P850) clip to the panel frame using SolarEdge's integrated mounting bracket, then output onto the DC string that runs back to a SolarEdge inverter. S-series optimisers (S440, S500B, S650B, S1200B) are physically similar but with higher power ratings and a slightly different form factor. For in-roof:

• Fix each optimiser to the panel it serves before the panel is fitted into the frame — retrofitting after the fact is close to impossible under an in-roof array.
• The DC string runs beneath the frames as one continuous MC4-terminated cable pair. Route it along the array's short axis if possible so total in-array cable length is minimised.
• For SolarEdge-ready panels (Aiko, Trina and others offer factory-fitted S-series options), the optimiser is built into the panel junction box. This is by far the cleanest option for in-roof: no external hardware to fit, no bracket to fail, and the panel is the only unit to replace if the optimiser dies.
• Group SafeDC status is only established once the inverter is connected — during commissioning, treat every cable in the array as live regardless of optimiser output.

Tigo TS4 optimisers (TS4-A-O, TS4-A-F, TS4-A-S, TS4-A-M)

Tigo units either clip to the panel frame with the standard TS4 bracket or (on TS4-A-F flex versions) plug in-line into the DC circuit via MC4 tails. They work with any string inverter that supports RSD (rapid shutdown) or with Tigo's own Cloud Connect Advanced gateway. For in-roof:

• The flex (F) versions with MC4 tails are the easiest to retrofit — you can leave the option open by pre-fitting Tigo compatible MC4 pigtails on each panel at install time even if you're not fitting the optimiser now.
• Bracket-mounted TS4 units clip to the panel frame the same way SolarEdge P-series do, so the same "fit before you drop the panel in" advice applies.
• Optional module-level monitoring requires the Cloud Connect Advanced (CCA) gateway indoors.

Plan a service panel

Whichever MLPE you specify, designate one panel in the array as the service panel: usually the one closest to the array's cable exit point at the corner where the lateral flashing meets the roof. Note it on the O&M drawings, and make sure the perimeter foam under that panel is a slightly softer grade so it can be lifted and re-seated during a service visit without cutting into the flashing seal. On a 12-panel array, budgeting 30 minutes to expose that one panel is realistic; on a full 4×4 or larger, plan for a panel-lift crew of two and a half day if the failure is buried in the middle of the array.

Cable exit and drip loops

The DC (or AC, for microinverters) trunk exits the array at one bottom corner, typically where the lateral flashing meets the drip flashing or Flexalu strip. Take the cable through a purpose-made weatherproof grommet or under a lead-flash upstand — never through a hole punched in the flashing itself. Leave a downward drip loop of 100–150mm before the cable enters the roof void so any surface water can't wick along the cable jacket into the loft.

Sealing the perimeter

Once the flashings and panels are in, run pre-compressed foam (GSE ART106373, £19.70 for 5m, or S4 ART2662, £14.95 for 5m) around all four sides of the array, 20mm back from the flashing edge. The foam must reach the bottom connection to prevent water or debris getting in from below. Under curved tiles, use a foam strip thick enough to fill the tile crown gap — typically 40mm on Roman pantiles. On S4 arrays specifically, the pre-compressed seal at the top edge runs along the base of the Height Riser and is mandatory even when the riser is fitted — the riser corrects the tray's geometry, it doesn't replace the seal.

Tile support

On interlocking and curved tile roofs, the row of tiles immediately above the top flashing needs a tile support hook to stop the leading edge sagging over time and pulling the waterproofing strip down with it. GSE supply purpose-made tile supports with fixing hooks (see the top-connection section in the GSE Landscape Evolution manual). On slate roofs, the natural stiffness of the slate makes tile supports unnecessary.

Snow and wind loads

The design load on any UK roof array is set by the site's wind zone (1–5 on the UK BS EN 1991-1-4 map), altitude above sea level, ridge height, terrain category (country vs urban), distance from the coast, roof pitch and the panel's position on the roof face (centre, edge, corner). Snow load is calculated separately under the same standard. Manufacturers publish worst-case tables — GSE's is on page 9 of their Landscape Evolution manual — showing the maximum load per corner/edge/centre position at each pitch and ridge height combination.

Red zones on those tables mean installation is forbidden. Don't try to argue around it. Move the array or split it across two roof faces instead.

Compliance and certification

Everything we stock is CE-marked and MCS012-tested. For grant-funded work, SEG payments, and any project where a building control officer or planning consent needs sign-off, the relevant certifications are:

Frequently asked questions