It depends on what you want the detection to do. That might sound stupid, but there are broadly two main categories of fire detection 'L' (Life safety) systems which aim to raise the alarm to warn occupants in enough time that they will be able to move to a place of safety 'P' (Property protection) which - as it says on the tin- is for building/asset protection Generally, P systems have more coverage/heads than L as L systems mostly need to maintain coverage for escape routes. But some L systems may require riser coverage in certain circumstances where the alarm needs to be raised as early as possible (Sleeping risks, vulnerable occupants with slow escape times etc). You need advice from a competent person who has sight of the building plans, understands the premises' use and knows occupancy details. It's not wise to take advice 'blind' over the net

Agree with Jonty, one head at the top as hot air rises. If you have a sprinkler system in the building, then there should be a sprinkler head in the top of the riser shaft. Peter