I’m after some advice on where to place a smoke alarm in my garage.  There is never any car there, the risks are two fridges & odd DIY jobs.  I realise the alarm should be central to the ceiling, but the ceiling is not uniformly flat, due to 6 wooden joists running lengthways across the 5m ceiling.  Above the joists is boarding.

So I have a choice of placing the alarm either on the underside of the central protruding joists, or on the wooden boarding up above/between them.  The gap between each joist is about 400mm.  The protrusion of each joist is about 130mm below the boards.

I’m conscious of needing to be at the highest point, yet avoiding ‘voids’, which to some extent the division of the ceiling boarding with joists creates.  Where would be best?

Full guidance here: https://www.aico.co.uk/technical-support/alarm-siting/