Roof Substrate Decision Tree
Eight UK commercial roof types. One structural answer each.
Mounting method, survey approach, and key risk factor for every commercial roof substrate we see on solar PV projects. Click any card for the standards detail.
Download 1-Page PDFThe right structural assessment depends on the roof substrate. Trapezoidal sheet and standing seam metal accept clamp-based mounting. Built-up felt and single-ply membranes typically take ballasted systems with engineered protection layers. Asbestos cement requires HSE-licensed handling and ballasted mounting that avoids fastener penetration. Composite metal panels behave like trapezoidal in terms of mounting but require careful thermal bridging review. Concrete decks accept all mounting types subject to load capacity check. Each substrate has a different wind-uplift exposure under BS EN 1991-1-4.
Each card lists the typical buildings, the recommended mounting and survey approach, the key risk factor, and the standards anchor. Click through to the detail glossary entry for each.
Trapezoidal sheet steel
Warehouses · Distribution centres · Light industrial
- Mounting
- Penetrating self-tapping screws into crown, or non-penetrating clamps where profile permits
- Survey
- Drone roof condition assessment + on-site verification
- Key risk
- Fastener uplift utilisation, sheet end-lap watertight integrity
- Standards
- BS EN 1991-1-4 · BRE Digest 489 · Manufacturer spec
Standing seam metal
Architectural commercial · Civic · Modern industrial
- Mounting
- Non-penetrating standing seam clamps (S-5! or equivalent), preserves manufacturer warranty
- Survey
- Drone + on-site, with seam profile measurement
- Key risk
- Seam compatibility with clamp model, thermal expansion
- Standards
- BS EN 1991-1-4 · BRE Digest 489 · Roof manufacturer warranty
Built-up felt
Older industrial · Retail estate · Public sector flat roofs
- Mounting
- Ballasted with protection mat, or mechanically fixed through to deck where structure permits
- Survey
- On-site walkover essential (drone supplements but does not replace)
- Key risk
- Membrane degradation, deck deflection, ballast point-load concentration
- Standards
- BS EN 1991-1-4 · BRE Digest 489 · BS EN 1991-1-3
Single-ply membrane
Modern commercial · Logistics · Data centres
- Mounting
- Ballasted with protection layer (TPO / PVC / EPDM compatible mat), or hot-air-welded mounting feet
- Survey
- Drone + on-site to confirm membrane lap detail and fixings
- Key risk
- Membrane puncture risk, manufacturer warranty preservation
- Standards
- BS EN 1991-1-4 · BRE Digest 489 · Membrane manufacturer spec
Asbestos cement / fibre cement
Pre-2000 industrial · Older warehouses · Agricultural
- Mounting
- Non-penetrating only. Over-purlin rail systems that bridge the sheet, never fastening through
- Survey
- Drone first, then on-site under CAR 2012 / asbestos management plan
- Key risk
- Asbestos disturbance, sheet fragility, HSE licensing requirement
- Standards
- CAR 2012 · HSE Asbestos Licence · BS EN 1991-1-4
Composite / insulated metal panel
Modern industrial · Cold storage · Architectural commercial
- Mounting
- Penetrating with insulated fixing kit (avoids thermal bridge), or seam clamps where compatible
- Survey
- Drone + on-site with panel manufacturer spec verification
- Key risk
- Thermal bridging at fixings, panel core delamination
- Standards
- BS EN 1991-1-4 · Panel manufacturer spec · Building Regs Part L
Concrete slab with liquid waterproofing
Civic · Hospitals · Multi-storey commercial · Schools
- Mounting
- All options viable: ballasted, mechanically fixed, or chemical-anchored to slab subject to load check
- Survey
- On-site essential to verify slab spans, reinforcement, and condition
- Key risk
- Slab dead-load utilisation, waterproofing detail at penetrations
- Standards
- BS EN 1991-1-4 · BS EN 1992-1-1 · Building Regs Approved Doc A
Clay or concrete tile
Heritage commercial · Office conversions · Some retail
- Mounting
- Penetrating hook fixings to rafters or purlins. Tile interface must be sealed to maintain weather tightness
- Survey
- On-site walkover required to assess rafter spacing, tile condition, batten integrity
- Key risk
- Rafter spacing capacity, listed building consent on heritage sites
- Standards
- BS EN 1991-1-4 · BS 5534 · Building Regs Approved Doc A
How to read the cards
Each card lists the four engineering decisions that change with substrate: mounting method, survey approach, dominant risk factor, and the standards we anchor the assessment to. The cards are not a substitute for site-specific calculation; they show the starting point an experienced engineer would take with each substrate.
Mounting recommendations assume system size up to typical UK commercial scale (50 to 500 kWp). For larger arrays or systems on heritage / listed assets, the assessment becomes site-specific from the outset.
Need a substrate-specific structural assessment? From desk to roof, in 48 hours.