After much research I opted to use
Sikaflex 292i High Strength Marine Adhesive to bond the solar panel in place.
Sikaflex 292i is a super strong Marine Construction Adhesive that's resistant to seawater, is suitable for structural joints that are subject to Extreme Dynamic Stresses. This adhesive also provides joint flexibility when applied at the correct thickness.
- High mechanical loadings
- Vibration dampening
- Wide adhesion range
- Hydrolysis resistant
- Wide adhesion range
- Solvent free
- Elastic
According to the Sikaflex website it is available in black and white. Unfortunately I could only find the white version in stock in the UK so I used this under the panel to do the main high strength bonding and then used Sikaflex 29
1i in black to seal the front edge, the rear edge and the ends of the transverse beads so the white did not show. Sikaflex 291i is also a marine adhesive but with lower sheer strength.
You can find application information and correct usage of the adhesive on the Sikaflex website. Note the correct way to cut the nozzle to ensure a thick parallel sided bead of adhesive is applied.
I used Scotchbrite to key the roof paint finish and Isopropyl Alcohol (IPA) to clean the surfaces before application of the adhesive.
4mm slices of 8mm soft silicon pipe were embedded in the adhesive to ensure a 4mm air gap was created under the panel to maintain ventilation and drainage. (It is too easy to press the panel down too far if spacers are not used).
The thicker adhesive beads also ensure there is allowable movement due to expansion and contraction due to temperature changes. The use of soft silicon spacers prevented any 'hard spots'.
The adhesive beads layout is similar to that described by Photonic Universe and PV Logic but I did not leave any gap on the front edge:
I used 1 tube of each and there was adhesive left over of both.
Please note that I cannot provide any official guidance or approval that this is the best adhesive to use for this application. It is just what I decided to use for myself after research.
Hope this helps.