Rebated Doors
The “French” doors you usually find in the UK are not French at all, as you’d probably expect. As you close proper French double doors they fit together by one door having a rounded edge profile and the other door having a cupped edge profile. When they’re properly sized and when the wood is properly seasoned, prepared and painted so that it doesn’t expand or shrink too much, this is an excellently draught-proof arrangement. Naturally it’s getting harder and harder to get proper doors that are properly fitted.
Here in the UK, “French” double door fit together with rebates. The edges where the doors meet have L-shaped profiles. This is nearly impossible to draught-proof.
Fitting locks (deadlocks or sashlocks) to either countries’ arrangments is challenging. Normally a rebated door will have a rebated lock. The lock will either be specially shaped or will have an adapter such that the bolts and keeps (boltholes) can still be placed centrally, across the rebate.
A lazy or ignorant fitter might be tempted to fit the lock off centre so that they don’t have to deal with the step shape of the rebate. Unfortunately this means that the bolt keeps will find themselves in the thin and fragile sticking out part of the other door. And the lock case will be too close to the face of its door and easily broken out and opened.
Even when fitted properly rebated doors won’t be secured particularly well by a deadlock or sash lock. Most deadlocks and sash locks, for example, lose their BS (British Standard) rating when used between rebated double doors.
You really need to fit mortised bolts as well. Mortised means that the bolts are inside the door rather than screwed to its face. They are also known as rack bolt or star-key bolts because a star shaped key that goes into a small hole is used to wind the bolt in or out.
I agree about the French doors as they are an easy target. As French doors are being used they can easily go out of shape which could mean that the locking alignment may need adjusting. Even if you get a proper locksmith to fit the locks properly they will need adjusting on an ongoing basis to make sure that your home is adequately secured.
How often should you maintain locks on a French door?
Most locks don’t need maintenance, especially if their moving parts are made of brass, which is a wonderful alloy as it hardly oxidizes (rusts) at all, and what little there is is slippery making brass self-lubricating.
Don’t ever put WD40 in a lock. It is not a lubricant despite what their marketing people try to imply. Instead, it’s a slow acting glue! If you must lubricate a lock, use silicone spray or PTFE spray.
So the main thing is to use all the locks and bolts on a regular basis so that you’ll spot any impending trouble before it strikes.
(Sorry about the huge delay. Normally I get an email from Wordpress when a comment is posted, but not this time.)