Hi Martin,
Civvy makes a good point. Do the rooms on the inside of these windows communicate with the primary escape route inside the building and, if so, is it through fire resisting doors? If there are fire doors between these rooms and the main staircase then a good deal of your problem simply doesn't exist; the theory being that a fire in any of these rooms (that adjoin the external route) will not take out the primary route (because of the fire doors) so everyone can just use this primary route and the external doesn't matter. People will only use the rear escape route when the fire is somewhere else in the building and affecting access to or use of the main staircase (in which case, it cannot be in one of the rooms adjacent to the side walkway).
There are a couple of issues with this simple response though. If the separation between the primary escape staircase and the rooms is a single line of fire resistance (e.g. a single fire door) then this line of fire resistance has critical importance in the evacuation strategy and possibly has too much dependence placed upon it to maintain safe means of escape throughout the building. This is not a terminal problem as the single line of fire resistance may be able to be augmented by an L3 fire alarm system (which the building may already have). Early response to an evacuation signal will drastically and very significantly reduce the possibility of both routes being lost to a single fire. If there is a known history of these fire doors being wedged open then this problem will have to be addressed (perhaps, for example, by magnetic hold-open devices).
Another issue which must be considered is the possible presence of people in the back yard when a fire occurs in one of the rooms that adjoin the escape route down the side. These people might be inclined to use the side route in preference to any other and should be afforded protection when they do so. They may have an alternative route through the building, separated from the fire room by fire resistance, and this would be fine, but they may not. The travel distance you give is not great and if automatic detection is located in the rooms adjoining the side escape route then it would be very difficult to imagine a fire in one of these rooms growing to such a size, before people can safely get past, that it will make it difficult for people to make their way down the external path that is outside the room of origin. Make sure the alarm is audible outside.
I think some simple fire separation and automatic smoke detection can provide you with a solution.
Note that if the route is more than 1800mm wide, we wouldn't worry about it at all as people would be deemed to be far enough away at 1800mm so that a fire would not be capable of stopping them from using the walkway - an old and possibly out of date view - the situation is much more likely to be made safe by the installation of appropriate AFD.
Stu
