Yes, 12845 doesn't make any recommendation in this respect except the one kurnal mentioned. Look up in any large sprinklered out-of-town retail outlet and you'll see sprinklers at around 10m above the goods that are no more than 2 to 2.5m above the floor. They will be slow to respond but once they go off they will do their job.
The droplets falling directly towards the fire may evaporate in mid air before they reach the fire but remember that the job of the sprinklers is to wet everything around the fire so that it won't catch fire. It is not to put the fire out (though that is often a beneficial side effect). The droplets that are wetting the ceiling and the walls and the doors and all the goods around the fire will get to their targets and will restrict the spread of fire.
As kurnal has discussed, the delay in actuation is not of critical important because during that time delay the fire is growing vertically within the very space that is causing your concerns and its lateral growth will be restricted because of the absence of a ceiling immediately above it.
Yes, high sprinklers can be an issue and you're right to have concerns but I guess you're talking about relatively small spaces (as compared to a warehouse, say) and, as such, you can be fairly confident that the temperature will build up at ceiling level fairly rapidly. Also, you shouldn't get the problem that some warehouses with high sprinklers might experience where sprinklers some distance away from the fire can actuate.
Regarding the sidewall sprinklers that were mentioned, these cannot offer you a feasible solution. I suppose the suggestion is to have them lower down the walls in the vicinity of the fire load but they are designed to be at high level (albeit on a side wall) just the same as other heads - they won't go off if they're at low level, not unless the fire is directly below them or until the smoke layer has descended right down to their level.
Stu
