Asbestos cement roof, 16 kWp, and a grant deadline

The roof is presumed to contain asbestos until proven otherwise by a refurbishment and demolition asbestos survey - the 1978 date and the sheet type make that presumption reasonable. No work starts until the survey report is in hand and the findings are built into the method statement.

Required before access:

• Refurbishment/demolition asbestos survey; if asbestos is confirmed, the work is notifiable non-licensed work, requiring the right RPE (FFP3), disposable coverings, no dry brushing, no power tools on the sheets, bagging and disposal as hazardous waste, and a register of those exposed.
• A site-specific risk assessment and method statement covering fragile roof access: proprietary edge protection or netting, roof ladders and crawling boards spanning the purlins, fall-arrest harness with anchor points, rescue plan, and never walking directly on the sheets.
• Confirmation that operatives have the competence and training for work at height and asbestos awareness.
• Emergency arrangements, exclusion zone at ground level, and a check on the weather/wind forecast.

Fixing method: drilling the crowns is unacceptable - it releases fibres and destroys the sheet's strength and weatherproofing. Use non-penetrative mounting: standing-seam or rail systems clamped to the purlins with over-sheet straps, or a manufacturer's system specifically approved for fibre cement sheet, with the load taken to the purlins rather than the sheet. If penetration cannot be avoided, the sheets must be removed and replaced by a competent contractor, which changes both cost and programme.

16.2 kWp on a three-phase supply: 16.2 / 3 = 5.4 kW per phase, and 5400 / 230 = 23.5 A per phase.

G98 only covers installations with a rated AC current of 16 A or less per phase (3.68 kW single-phase equivalent per phase). 23.5 A per phase is well outside that, so this is a G99 application, not a G98 notification. Depending on the inverter's declared output and the DNO's own thresholds, a 16 kW three-phase installation will usually go through the DNO's fast-track or standard application process, requiring a completed application form, single-line diagram, inverter G99 type-test certificate and often a protection setting record.

Consequences the customer must be told about immediately:

• No energising, and often no connection work that assumes approval, until the DNO accepts the application. Turnaround is commonly several weeks and can be extended if the local network needs reinforcement.
• The grant deadline and the start date are therefore both at risk, and the risk sits with the customer, not with an assumption that the DNO will say yes.
• The 22 kW existing peak load is helpful - most generation will be self-consumed, which supports the application - but it does not remove the need to apply.

Recommendation: submit the G99 application before ordering equipment, state the restriction in writing, and do not let the grant timetable drive an unlawful connection.

The mounting system cannot be selected from a photograph of the roof covering. The following must be established:

• Purlins: spacing, section size, condition and fixing centres, since the clamps or straps transfer the load to them, not to the sheets. Corrugated fibre cement has almost no point-load capacity.
• Sheet condition and thickness: whether the sheets are sound enough to be spanned and whether any are to be replaced while access is available.
• Structural capacity to BS EN 1991 (dead, snow and wind loads including the increased uplift at ridge and eaves), and a check by a competent structural engineer where the building is agricultural and older, or where the purlin data is unknown.
• Wind uplift calculation for the site's exposure and the array's position on the roof, to size the number of fixings and any ballast-free design.
• Access, walkway and maintenance routes, plus safe cable routes that do not rely on the sheets for support.

If any of these cannot be confirmed, the mounting design is not complete and the order should not be placed.