Solar Panels on Roofs: Structural Loading, Roofer Responsibilities & Waterproofing

Summary

Solar photovoltaic (PV) installation on existing UK homes has accelerated dramatically since the introduction of the Smart Export Guarantee (SEG) and the broader energy cost crisis. Millions of UK homes now have rooftop PV, and the roofing trade has had to adapt — roofers are frequently called to fix PV installations that were poorly waterproofed at the penetration points, and to carry out re-roofing work on roofs that already have solar panels.

The interfaces between the roofing contractor and the PV installer are critical points where responsibility must be clearly understood. The PV installer is responsible for the panel, mounting rails, electrical connections, and inverter. The roofer is responsible for ensuring the roof remains watertight at all points where the mounting hardware penetrates or bears on the tile/slate covering. In practice, many PV installations are done without a qualified roofer's involvement at all — which is why solar-related roof leaks are a growing segment of roofing repair work.

Key Facts

• Structural loading — Typical crystalline silicon PV panel: 10–15kg/m². Including mounting rails and hardware: 15–25kg/m². For a 4kWp system (approximately 12m²): total additional load of 200–300kg. Most UK domestic roofs can accommodate this without modification — but always verify
• Rafter size check — BS 5268-3 (superseded) or Approved Document A span tables: confirm existing rafter size and centres can carry the additional panel load. Get the PV installer to confirm their system's load data
• Tile hook vs through-roof fix — Two common mounting systems: (a) Tile hooks that hook over the tile beneath — no roof penetration but relies on tile structure; (b) Through-roof fixings that screw through tile and into rafter — penetrations must be sealed with lead soaker or proprietary flashing kit
• Through-roof penetration sealing — Proprietary sealing kits (Ejot, BEP, Unirac) or custom lead soakers are required at every through-roof fixing. A sealant-only fix is NOT adequate
• Lead soaker for penetration — Custom-made or proprietary lead soaker: Code 4 minimum (1.32mm). Formed to the profile of the tile with a minimum 100mm upstand behind the fixing and 150mm overlap over the tiles below
• Planning — Solar panels on a domestic dwelling are usually permitted development (GPDO Class A, Part 14) provided: no panel is installed above the roofline, no panel projects more than 200mm above the roof plane, and the installation is not on a listed building or in certain Article 4 areas. Check with LPA for conservation areas
• MCS certification — Panels installed by an MCS-certified contractor are eligible for SEG payments. Non-certified installations cannot access SEG. Roofers should not make electrical connections
• Fire risk — Solar panels on roofs create a confined fire compartment below them. Emergency services guidance (LABC and NFRC) has been produced on fire fighting in solar-equipped buildings. Hot spots in panels are a fire risk — MCS installation and regular cleaning required

Quick Reference Table

Detailed Guidance

Structural Assessment

Before any PV installation, the structural capacity of the roof must be confirmed. This is primarily the PV installer's responsibility (MCS requirement), but roofers should be aware of the indicators of concern:

• Rafters undersized for span (compare to Approved Document A span tables)
• Rafters showing deflection under existing load (bowed rafters visible from the loft)
• Evidence of woodworm, rot, or previous repair to rafters
• Rafter spacing greater than 600mm (may require intermediate strutting or blocking at panel locations)
• Very heavy existing tiles (old concrete tiles can be 50–55kg/m²) — adding panels to an already-heavy roof may approach rafter capacity

If there is any concern, instruct the customer to obtain a structural engineer's sign-off before the PV installation proceeds.

Roofer Involvement in PV Installation

A roofer may be involved in a PV installation in several ways:

During new PV installation:

• Assess and advise on roof condition before installation
• Remove and replace tiles around fixing points if required by the installer
• Fabricate and install custom lead soakers where the installer specifies through-roof fixings
• Oversee that the installer's fixings do not compromise the batten, felt, or tile around the fixing points
• Check for any cracked or broken tiles caused by the installation and replace

Callout for existing leaking PV installation:

• Identify the source of the leak (usually at a penetration point or tile crack adjacent to a fixing)
• Remedy: remove adjacent tiles, install or replace lead soaker, relay tiles
• Document clearly: the cause of the leak was the PV installation, not the roofing contractor's work (important for liability)

Penetration Sealing Methods

Lead soaker around through-roof fixing:

1. Calculate the soaker size: upstand height behind the fixing (minimum 75mm), width (fixing OD + 75mm each side), and downstand over tiles below (minimum 150mm)
2. Form the soaker from Code 4 lead sheet. Upstand must be dressed against the rafter/tile and sealed at the sides with turned-in edges (not open at the side)
3. Drill the fixing hole through the soaker, tile, batten, and rafter
4. Insert stainless steel fixing bolt through soaker and waterproof washer
5. Seal around the bolt head with appropriate sealant (EPDM-compatible or bituminous sealant)
6. Replace tiles below the soaker, overlapping the soaker's downstand by minimum 100mm

Proprietary sealing kits: Several manufacturers (Ejot, Sika, Wirquin) supply proprietary through-roof fixing kits with pre-formed EPDM gaskets. These are quicker to install than custom lead soakers and are tested and approved for various tile profiles. They must be specific to the tile profile — a gasket for a flat concrete tile will not seal correctly on a deep profiled tile.

Tile hook systems (no penetration): Some mounting systems use a hook that slides under the existing tile and hooks over the tile batten. No roof penetration is made, but the mounting rail bears on the tile structure. The hook must be a certified product for the specific tile type — using an incorrect hook on a tile profile it is not designed for risks tile cracking or lifting.

Re-Roofing Under Existing Solar Panels

Re-roofing work on a property with existing PV requires careful coordination:

1. The PV installer (or an MCS-qualified engineer) must de-energise and remove the panels before the roofer begins stripping. Never attempt to move a live PV panel.
2. Document the existing fixing positions and panel layout before removal.
3. Complete the re-roof (new felt, battens, tiles) and confirm the new surface is weathertight.
4. The PV installer reinstalls the panels using new or existing fixings — all penetrations re-sealed.
5. The roofer confirms the final waterproofing at all penetration points.

Quote the de-installation and re-installation as a separate line in the contract, and clarify that this is the PV installer's scope — not the roofer's.

Frequently Asked Questions

Is planning permission needed for solar panels?

For most domestic properties in England, installing solar panels is permitted development under GPDO 2015 Part 14 Class A, provided: (a) the panels are on a dwelling, (b) no panel protrudes more than 200mm above the roof plane, (c) no panel is above the highest point of the roof, and (d) the property is not a listed building. Conservation areas may have additional restrictions — check with the LPA before assuming PD applies.

Can a roofer install solar panels themselves?

No — not the electrical aspects. The electrical connection of PV panels to the inverter and mains supply must be carried out by a person registered on an appropriate scheme (Part P competent person scheme, specifically the MCS scheme for solar PV). A roofer with an MCS certification can carry out the full installation; without it, the electrical work must be a separate contractor.

What are the common leak locations on solar panel installations?

The most common: (1) Through-roof fixings that were sealed with mastic only (no lead soaker or EPDM gasket) — the mastic fails within 3–5 years and becomes an easy water pathway; (2) Cracked tiles adjacent to fixings that were drilled or tightened without adequate care; (3) Tile hooks that were installed on an incompatible tile profile, causing uplift and a gap at the overlap.

Regulations & Standards

• MCS 001 — MCS Installation Standard for photovoltaic systems (covers structural loading requirements)

• BS EN 62446-1 — Grid connected PV systems: documentation and commissioning

• GPDO 2015 Part 14 Class A — Permitted development for solar panels on dwellings

• BS 5534:2014 — Re-fixing tiles around panel installations

• BS EN 12588 — Lead sheet for flashings and soakers

• NFRC Solar PV and Roofing Guidance — Roofer responsibilities and best practice for solar roof installations

• MCS Installation Standard MCS 001 — Solar PV installation standard including structural assessment

• Planning Portal — Solar Panels PD Rules — Permitted development guidance for domestic solar

• re roofing — Re-roofing under existing panels

• solar pv — PV electrical system design and installation

• leadwork — Lead soaker and flashing specification

• planning permission — Permitted development rules generally