Also known as French windows, they are intrinsically less secure than single leaf doors, which may reflect on their comparative lack of popularity as compared with the modern patio door. However, this need not be the case provided that suitable security measures are taken.
Both doors should be fitted with mortice rack bolts (bolts fitted within the door and operated internally by a threaded key). They should always be fitted at 90º to the grain of the wood, reducing the likelihood of the wood splitting if subjected to pressure. Alternatively, use surface-mounted locking bolts (push to lock, key to open). Whichever type you use, fit top and bottom to provide rigidity.
If the style of door is capable of it, a mortice sash lock can be fitted for extra security (rebate sets may also be required).
As most French Doors are outward opening, the addition of hinge bolts is also recommended. They should be located 100-150mm (4-6″) below the top hinge and similarly above the bottom.
Note: It would be acceptable, for insurance purposes, for key operated bolts only to be fitted top and bottom of both doors.
Consider replacing ordinary or toughened glass panels with laminated glass – two pieces of glass bonded together with a sheet of laminate – as they offer much greater resistance to attack.
UPVC/PVCU French doors are generally unsuitable for retro-fit security devices. Not only is the material not strong enough to support devices fitted with steel screws unless secured into the internal metal framework, but such changes to the original design may invalidate an existing warranty or possibly damage the integral locking assembly. If in doubt, consult the installer/manufacturer. Modern designs will usually incorporate deadlock shoot bolts or a multi-point locking system, both throwing a number of bolts from the door into the frame. Under these circumstances there will not normally be any need for additional devices.