Flat Roof Solar Mounting for Commercial Roofs: Ballasted, Fixed or Adhered
Updated 10 November 2026 · SEO Dons Editorial
What are the main ways to mount PV on a flat commercial roof?
Three families cover most commercial flat roofs.
| Type | What holds it down | What it asks of the roof | Manufacturer data |
|---|---|---|---|
| Ballasted | Ballast weight, friction and wind deflectors | Extra dead load and a known friction value between mat and covering | K2 lists its Dome 6 as ballasted with no roof penetration up to 3 degrees of inclination |
| Mechanically fixed | Anchors into the structure, sometimes with ballast too | Penetrations through the covering that must be sealed | K2 moves to a fixed connection above 3 degrees; Enstall's FlatFix Fusion uses ballast, anchoring or both |
| Welded or bonded to the membrane | A weld or adhesive to the waterproofing | A compatible membrane and its maker's agreement | Bauder welds its mounting units to specific Bauder membranes, with no penetrations and no additional ballast |
Flat roofs have a fall, so check each kit's pitch limit. Enstall, whose brand heritage is Esdec, gives a maximum of 7 degrees for FlatFix Fusion, with glue from 3 degrees. Bonded modules are a different product, covered in the guide to lightweight panels for weak commercial roofs.
How are wind uplift and ballast worked out?
Wind, not module weight, sets the holding-down force. BRE states that these systems must be designed and installed to withstand the maximum wind loads expected in their lifetime, and its 2014 revision of Digest 489 applies the Eurocode for wind actions, BS EN 1991-1-4, with its UK National Annex.
BSI describes that Eurocode as covering elements attached to a structure, including components and their fixings. First-generation Eurocodes remain the applicable UK standards until 30 March 2028 unless the relevant authority or the project specification says otherwise.
For scale, Enstall lists a wind load of up to 1,200 N/m2 for its landscape set-up and 950 N/m2 for portrait. Dividing 1,200 N by 9.81 gives about 122 kg on every square metre. That is illustrative arithmetic, not a design value: tilt, deflectors and roof position change the net force, which is why a site calculation sets the ballast.
MCS installations cover systems up to 50 kW. MCS 012 issue 3.0, in effect since 10 May 2025, certifies flat-roof kits up to 10 degrees, both mechanically and non-mechanically attached, and since 10 August 2026 the MCS Product Directory has shown only mounting products certified to it, bar a few bespoke extensions.
Where no certified kit suits, clause 5.5.5 of MIS 3002, updated on 18 March 2026 and mandatory for MCS installers from 18 June 2026, asks the installer to evidence completed structural and wind loading calculations for the specific mounting system. Larger arrays sit outside MCS, but the same evidence is a fair benchmark to request.
There is no universal ballast figure. It depends on wind conditions, building height, tilt, deflectors, the row's position on the roof and the friction between support and covering. Enstall says its calculator allows quick project calculations, and K2 sells a test weight for measuring the coefficient of friction on a roof, which points to sliding resistance being measured on the real surface rather than assumed.
Trays have ceilings too. K2 lists a maximum permissible ballast of 148 kg across two porters for modules 1,998 to 2,405 mm long, so a layout needing more must change or add anchors.
Ask for a ballast plan showing kilograms per support by roof zone, with the total in kg/m2. Part A of the Building Regulations requires a building to sustain and transmit combined dead, imposed and wind loads to the ground, so whether the roof can carry that total is a structural engineer's decision. The commercial PV systems guide shows where that check sits in the design sequence. Whether a ballasted array goes to building control as a full plans application or is self-certified by a registered installer is covered in the building regulations guide for commercial rooftop PV.
How do mounting systems protect the roof covering?
- Mats and base plates. K2's Dome 6 uses an EPDM protective mat, and K2 lists a polyethylene base plate for gravel roofs.
- Thermal movement. Enstall says movable connectors let the system expand thermally without damaging the covering.
- Membrane maker's position. Bauder's units are designed for specific Bauder bituminous or single-ply systems, and NFRC sells guidance note GN65, Considerations for Clients when Installing PV Panels on Commercial Roofs.
- Later repairs. Bauder notes that panels can be removed individually for membrane maintenance, so plan how rows come off and go back.
NFRC's summary is blunt: fixings penetrate the roof covering, panels add weight to the structure and the array changes how water drains across the surface. Ask the designer how each is handled for your build-up, and get the roof manufacturer's position in writing. What fire research says about the space under an array and fire spread across a roof membrane is summarised in the solar panel fires guide.
What do the Class J limits mean for a flat-roof layout?
In England, Class J does not permit PV on a flat roof if its highest part would be more than 1 metre above the highest part of the roof, excluding any chimney, or if it would be within 1 metre of the roof's external edge. K2's minimum distance to the roof edge for Dome 6 is 600 mm, so the planning limit is the tighter one.
The planning guide works through the height geometry and the prior-approval step for arrays over 50 kW. This is the English order, so check the devolved planning rules separately for Scotland, Wales and Northern Ireland.
South-facing or east-west: what does row spacing do to yield?
Tilted south-facing rows shade the row behind, so spacing grows with tilt. East-west rows sit back to back and need little gap. PVGIS yields for Birmingham are set against illustrative geometry below.
| Layout | kWh per kWp a year (PVGIS) | Row pitch, m | Capacity per m2 of roof (south 15 degrees = 100) | Energy per m2 of roof (south 15 degrees = 100) |
|---|---|---|---|---|
| South, 10 degrees | 875 | 3.8 | 118 | 115 |
| South, 15 degrees | 904 | 4.4 | 100 | 100 |
| South, 30 degrees | 960 | 6.3 | 71 | 75 |
| East-west, 10 degrees | 800 | 2.5 per module row | 177 | 156 |
Method: 2.3 m modules, rows spaced so one does not shade the next with the sun 15 degrees above the horizon, east-west pairs with a 0.5 m gap, and no setbacks, walkways or plant. A designer works from the real horizon and a shading model. Yields are PVGIS 5.3 with 14% losses; east-west averages an east and a west plane. A 5 degree south tilt, not in the table, gives 842 kWh per kWp a year in PVGIS for the same point; on the same method its rows sit about 3.0 m apart, which scores about 146 for capacity and 136 for energy per m2 of roof against south 15 degrees at 100.
East-west gives up about 17% per kWp against a south 35 degree plane, which PVGIS puts at 968 kWh/kWp here, yet fits about 1.8 times the capacity of 15 degree south rows on the same roof. It suits roofs where area is the constraint. Where the grid connection or the site's demand is the limit, fewer, better-oriented modules can be the better trade. The yield by region guide covers 13 locations, and shading losses covers what spacing does not. Whether optimisers add anything on an east-west layout is covered in the guide to optimisers on commercial roofs.
What should a mounting design pack show?
- The wind calculation to BS EN 1991-1-4 and Digest 489, with site inputs stated.
- A ballast and anchor plan by roof zone, with totals in kg/m2 for the structural engineer.
- The friction value for the actual mat and covering pairing.
- The roof manufacturer's written position on the method.
- Edge and corner set-backs, and a Class J height check against the real highest roof element.
- How rows come off for repairs, and how fire and insurer requirements are met, as in the RC62 guide.
Metal roofs use different fixings, covered in solar panels on metal roofs, and bifacial modules raise their own spacing questions. Roof details can be sent through the site's enquiry form.
Sources
- GPDO 2015, Schedule 2, Part 14, Class J, legislation.gov.uk, checked 22 October 2026.
- Building Regulations 2010, Schedule 1 and Energy Act 2004, section 82, legislation.gov.uk, checked 22 October 2026.
- Digest 489, wind loads on roof-mounted PV and solar thermal systems (2014), BRE Group, checked 22 October 2026.
- UK National Annex to Eurocode 1, wind actions, BSI, checked 22 October 2026.
- MCS: update to MIS 3002 (18 March 2026), flat roof mounting update (1 May 2025), grace period ends (10 August 2026) and becoming certified, checked 22 October 2026.
- Manufacturers: K2 Dome 6, Enstall FlatFix Fusion, Enstall about page, BauderSOLAR and BauderSOLAR PV system, checked 22 October 2026.
- NFRC: GN65 and end-of-life roofs case study (26 March 2026), checked 22 October 2026.
- PVGIS 5.3, European Commission Joint Research Centre, checked 22 October 2026.
Common questions
Can a ballasted system go on any flat roof?
No. Manufacturers state limits. K2 gives ballast-only mounting up to 3 degrees of inclination and a fixed connection above, and Enstall gives a maximum roof pitch of 7 degrees. K2 also names the coverings it suits, so the roof manufacturer and a structural engineer still need to confirm the combination.
Is a flat-roof mounting kit MCS certified?
Some are. MCS 012 issue 3.0 covers flat-roof kits up to 10 degrees, and MCS certification itself applies to installations up to 50 kW. A larger commercial array is not an MCS installation, so ask the supplier for equivalent wind, fire and weathering evidence instead.
Does a ballasted array need planning permission?
In England, Class J permits it if the 1 metre height and 1 metre edge limits are met and the building is not listed or on a scheduled monument. Arrays over 50 kW need a prior-approval determination before work starts. The planning guide on this site sets out the steps.
Who confirms the roof can carry the ballast?
A structural engineer. The Building Regulations require a building to carry combined dead, imposed and wind loads safely, and a mounting supplier provides the loads, not the sign-off. The roof covering manufacturer separately confirms whether the mounting method suits the membrane.