French Door Locks and How to Make Double Doors Secure

French doors have two hinged leaves that meet in the middle, and their security depends heavily on how the second closing leaf is bolted to the frame. To make them as secure as a standard front door, they need a central lock approved to British Standards and additional bolts fitted at the top and bottom of the doors. This guide explains which locks and standards apply to timber, uPVC and aluminium double doors, so you know exactly what to ask a locksmith to fit. If you have sliding doors, our guide to patio door locks and security covers those instead.

Most external doors lock securely into a solid frame. French doors lock into each other. Because the two doors meet in the middle without a central pillar, the overall strength of the pair relies on the door that closes last being anchored firmly in place. If that anchor is weak, the main lock has nothing solid to anchor against.

Because the two doors meet in the middle, a single central lock is not enough on its own. The doors should be bolted vertically into the solid head and sill of the door frame.

You cannot rely on a single central lock to protect a pair of doors. Securing French doors properly means fitting hardware that holds the first door to the second, and the second door securely to the building itself. A professional locksmith will look at the material your doors are made from before deciding which extra bolts are needed to achieve this.

The first and second closing leaf

When you use French doors day to day, you usually open and close just one side. This is called the first closing leaf, or the active leaf. It holds the main lock and the handles you use to get in and out. The other side is the second closing leaf, or the inactive leaf.

The second closing leaf is normally held shut by bolts at the top and bottom. When you want to open both doors wide, you unlock the main door, open it, and then release the top and bottom bolts on the second door to swing that one open too. When you close them, the second leaf must be bolted securely into the frame first, so the first leaf has something solid to lock into.

On timber doors, the edges where the two doors meet are often rebated. This means they are shaped to overlap each other when closed. The bolt positions below are measured from the front of this rebated edge.

If the bolts holding the second closing leaf are flimsy, or if they only lock the two doors together rather than fixing into the frame, the whole setup is vulnerable. You must secure the second leaf properly to give the main lock a solid foundation.

Locks and bolts for timber French doors

Solid timber external glazed double doorsets require a specific combination of locks to meet modern security expectations. The central lock should be a five-lever mortice sashlock fitted halfway up the leading edge of the first closing leaf. A sashlock includes both a latch to hold the door shut and a key operated bolt.

The central lock alone is not enough. You also need bolts at the top and bottom to secure the doors against being forced. These can be mortice rack bolts, which are fitted inside the wood and operated by a key, or surface mounted locking throw bolts.

To secure a pair of wooden French doors properly, the hardware must be fitted in a very specific way.

• A five-lever mortice sashlock must be fitted halfway up the leading edge of the first closing door.
• The central lock needs to carry a Kitemark proving it meets the BS 3621:2017 standard.
• Mortice rack bolts or surface mounted throw bolts must go at the top and bottom of both doors.
• All top and bottom bolts must secure vertically into the solid frame of the building and not horizontally into the opposing leaf.

Police guidance says the bolts must secure into the frame, not into the opposing leaf. The Master Locksmiths Association provides clear rules on where these vertical bolts should sit. The bolts for double doors are fitted vertically to the top and bottom of the doors, between 50 mm and 100 mm from the front of the rebated edge of the door, avoiding any joints in the timber.

On very substantial doors, it could be necessary to fit them only to the top and bottom of the first closing leaf. A professional locksmith will look at the thickness of your wooden doors and advise you on the best position for the bolts.

Multipoint locks on uPVC and aluminium French doors

Modern uPVC, aluminium and composite French doors do not use traditional mortice locks or separate rack bolts. Instead, they use a multipoint locking mechanism. This is a long metal strip concealed in the edge of the door that engages multiple hooks and bolts when you lift the handle.

The mechanism is driven by a central gearbox. When you lift the handle, the gearbox pushes the hooks or dead bolts out from the door edge and into receivers on the second closing leaf. You then turn the key in a euro cylinder lock to fix everything in place. The Police Crime Prevention Handbook advises that you must always double lock UPVC and multi-point locking doors by lifting the handle, turning the key and removing the key.

For the second closing leaf, an MLA minimum is a multipoint lock with at least three locking points. These must include a central bolt and two dead or hook bolts that lock securely into the head and sill of the frame, all locked by one operation of the key. If a multipoint lock is not used on the second leaf, the main locking system must be supported by two key operated rim or mortice security bolts.

The table below shows the minimum throw requirements for bolts on the second closing leaf of double casement doors.

These figures are the minimum throw requirements from the MLA Guidelines for minimum security requirements for domestic property (Edition 19, 1 January 2025).

Cylinder and handle standards

The euro cylinder is the barrel where you insert the key on uPVC, aluminium and composite doors. Euro cylinders can be vulnerable to lock snapping, so French doors need hardware that resists it.

The cylinder should be approved to the Sold Secure SS312 Diamond standard, or at least carry a TS 007 three star Kitemark. If you have a one-star cylinder fitted, it must be paired with TS 007 two-star security handles to provide the same level of protection. Door handles should be approved to SS301 Bronze or TS 007 two star.

You can often tell just by looking at your doors whether the security hardware needs to be upgraded by a locksmith.

• The euro profile lock protrudes beyond the metal handle on the outside of the door.
• The lock face does not carry a TS 007 three star Kitemark or a Sold Secure Diamond rating.
• The door handles are standard rather than TS 007 two-star security handles.
• The multipoint mechanism feels stiff and puts extra pressure on the key when you turn it.

If a europrofile lock protrudes beyond the handle, consider getting an MLA member to change it to a shorter lock that is more resistant to snapping and security accredited to British Standards. Upgrading to an anti snap lock is a straightforward job that usually takes a locksmith well under an hour.

The cost to fit an anti snap lock including labour is approximately from £130, with the lock alone starting from £60 supply only. These are MLA estimates from its uPVC door security page and vary by area, so always ask a locksmith for a total price including parts and VAT before work starts. If you want to know more about how these locks work, our anti snap locks explained guide has the details.

Hinge bolts and outward opening doors

French doors often open outwards into the garden to save space inside the room. When doors open outwards, the hinges are exposed on the outside. Exposed hinges can be a weak point even when the locks are strong.

To stop this happening, double doors require two pairs of hinge bolts located as close as possible to the hinges. Hinge bolts are thick metal pins fitted to the edge of the door. When the door closes, the pins slot into reinforced holes in the door frame. The bolts help keep the door in the frame if the hinges fail.

Alternatively, you can have new hinges fitted that have these security bolts built into them integrally. A locksmith or specialist door fitter can install these for you. Hinge bolts are an inexpensive security upgrade, costing from £12 for a pack of two supply only. This is an MLA estimate for supply only, and fitting costs will be charged on top.

Sash jammers and add on devices

Sash jammers are small metal arms that screw onto the door frame and swing downwards to hold the door firmly shut. They are a popular and cheap way to add extra security to uPVC and wooden frames. While this guide covers double doors, you can read our guide to sash window locks and security if you need to secure traditional sliding windows.

The MLA says French doors take four sash jammers, two at the top and two at the bottom. The price is from around £12 for a good quality jammer excluding professional fitting. This is an MLA estimate from its sash jammers guide, and labour costs will depend on your area.

Sash jammers do have some disadvantages to consider before you have them fitted. They can wear out over time with repeated use, and not all of them can be locked from outside. This means they can only be used when the home is occupied and you are locking up for the night.

Another add on device for double doors is a Patlock. It stops the external handles working even if the cylinder lock is snapped. A Patlock is Secured by Design approved and costs from around £50.

Keeping a way out in a fire

Adding extra locks and bolts to your French doors makes them harder to force open from the outside, but it also makes them harder to open from the inside. In an emergency, you need to be able to get out of your house quickly. Fumbling for multiple keys in the dark when there is smoke in the room is highly dangerous.

Bolts that need a key to open can trap people in a fire. For this reason, knob operated mortice bolts are preferred for fire safety reasons on emergency or final exit doors. These bolts have a small thumbturn on the inside instead of a keyhole, meaning anyone can open them instantly without searching for a key.

You must always give yourself a means of escape in a fire. If your top and bottom bolts do require a key, or if you lock the central multipoint mechanism from the inside with a key, you must keep that key very close to the door at night. It should be hidden from view of the glass, but easy for anyone inside to reach immediately if they need to escape.

Common questions

Do both French doors need locks?

The first closing leaf holds the main lock and handle, while the second closing leaf must be secured by bolts at the top and bottom. If the second leaf is not bolted firmly into the solid head and sill of the door frame, the main lock has nothing solid to anchor against.

How much does it cost to upgrade French door locks?

Prices vary based on your location, the time of day and the standard of the lock. The MLA price estimates earlier in this guide give a starting point for each item. Always ask a local locksmith for a fixed price that includes parts, labour and VAT before they start work.

What standard should French door locks meet?

For solid timber doors, the central sashlock should meet the BS 3621 standard. For uPVC, aluminium and composite doors, the euro cylinder should meet the Sold Secure SS312 Diamond standard or have a TS 007 three star Kitemark.

Can a locksmith fit sash jammers to French doors?

Yes, a locksmith can fit sash jammers to give you extra security from the inside. French doors take four jammers, two at the top and two at the bottom. Bear in mind that because they often cannot be locked from the outside, they are mainly used when you are inside the house.

What a locksmith can check and what it costs

Before you hire a locksmith to work on your double doors, you need to know exactly what to ask them. Knowing the right questions ensures you get the correct hardware fitted and stops you paying for unnecessary work.

Use this checklist when you speak to a locksmith on the phone:

• Tell them whether your doors are made of timber, uPVC, aluminium or composite materials so they bring the right parts.
• Ask for a total price that includes the call-out, the labour, the parts and the VAT.
• Check whether they plan to fit a BS 3621 sashlock for timber doors, or a TS 007 three-star cylinder for uPVC doors.
• Confirm that any top and bottom bolts will secure vertically into the door frame and not horizontally into the other door.
• Ask if they charge a higher rate for evenings or weekends, and wait for normal working hours if the job is not an emergency.
• Check they have a real local address and ask to see their ID when they arrive.

Upgrading the security on your French doors is a sensible step that can bring peace of mind. If your doors feel flimsy, or if your euro cylinder sticks out past the handle, it is time to have the hardware checked. You can find a locksmith near you to inspect your doors, supply the correct British Standard locks and fit the necessary bolts to keep your home secure.