I applied and paid for Building Control notification of my solar installs back in November 2023.
Having already installed and commissioned my system, I was a little concerned that I would receive a telling off from the Building Control officer when he eventually came around to inspect the work in that I should have had them round before starting.
I booked a visit for Wednesday morning and in the days leading up to this visit I also decided to have a go at some structural design calculations to see if they would be complaining about the strength of my various roof structures.
The Timber Beam Calculator has a breakdown of all the various calculations based on roof type, along with some demo worked examples, so I decided to build a spreadsheet to model the calculations.
The worked examples assume a dead load of around 0.5kN/m2 of roofing structure (excluding the joists), this theoretical roof will be something like roofing felt, plywood, joists, plasterboard and skim.
The joists are omitted because the values associated with them are calculated separately based on actual size and grade of timber and then added into the calculations.
I have two different roof compositions, one for the ‘Garage’ roof and one for the ‘Office’ roof.
The Office roof is built as follows:
- EDPM
- GRP (old fibreglass covering)
- 18mm OSB 3
- 18mm marine plywood
- 50mm Kingspan insulation
- 18mm marine plywood
- 63 x 220mm joists on 400mm centres, max clear span 2000mm
- 9.5mm plasterboard and skim
I then added in the load for the solar panels and associated framing to give a total dead load (excluding the joists) of 0.77kN/m2
The Garage roof is slightly different:
- EDPM
- GRP (old fibreglass covering)
- 18mm OSB 3
- 18mm marine plywood
- 150mm Kingspan insulation
- 18mm plywood
- 50 x 150mm joists on 300mm centres, max clear span 2600mm
- 12.5mm plasterboard and green glue
- 12.5mm plasterboard and skim
I then added in the load for the solar panels and associated framing to give a total dead load (excluding the joists) of 1.03kN/m2
This then allowed me run all of the various calculations for each of long, medium and short term loading for bending, shear and deflection.
Fortunately, both roof structures passed, even with the additional solar load, the closest worry was the short term load deflection on the Garage roof which came out as 58.2% but still well under the allowable 100% figure.
Office design summary
Garage design summary
When the building control inspector finally arrived, I immediately confessed that this was not really a pre-install visit as I had already completed the installation and commissioned it.
Turns out he was absolutely fine with this and even happier after I provided him with a full explanation of the installation and my qualifications as an electrician and Enphase certified installer.
I explained the roof structures of both flat roof areas, along with their histories concerning the different layers and said I would supply him with my calculations that he would in turn pass on to his structural engineer.
I also explained about how this was an all AC installation and that the only DC element was between each panel and its microinverter at around 50V.
I then explained all of the various isolators and cable types used in the installation and he was more than happy with the design and explanations.
He inspected the flat roof areas as well as the inside of the pre-fab garage and was satisfied that everything was more than capable of meeting the building control requirements.
I pointed out the extra load spreaders and feet that I had installed to further reduce the point loading under the ballasted areas.
In all a very pleasant experience and hopefully I should receive my Part A notification from the council stating that all is good from a structural stand point in due course.