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6.3 Commentary

6.3.1 General
The addition of a lock or latch to a fire-resisting doorset can reduce
the fire performance of the doorset.

Locks and latches come in a range of sizes, methods of fixing
(rim/mortise) and are manufactured using a range of materials, all of which can affect the fire performance of the door set.

6.3.1.1
Removal of significant sections of the door. The fitting of a lock or a
latch (particularly a mortice device) entails removing wood from the fire-resisting timber door so reducing the material which provides fire protection and insulation.

The reduction in the amount of wood in the stile or rail can also
impair the stability of the door, leaving it less able to resist distortion caused by the heat of a fully developed fire.

6.3.1.2 Voids in the Structure of the Door
All mortising in fire-resisting timber doors should be performed as accurately as possible to avoid creating unnecessary voids.
Clearance holes for keys, cylinders and spindles should be no larger than the sizes specified on the manufacturer’s fitting instructions.
If the latch or lock calls for the fitting of some form of intumescent
protection and/or use of covered escutcheons, these must be used to ensure that the fire resistance of the door is no less than when
tested. (Refer to BS 8214: 16.4 for guidance on preparation and fitting).

Steel doors are normally of hollow construction and voids for mortice locks do not generally create a risk of integrity failure. However
hollow steel doors should be constructed with a lock reinforcing support to ensure that the lock case:

Lock reinforcing support
needed in steel doors

Section 6 - Locks & Latches