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How to fit a sash jammer without locking yourself out

Updated: October 2026

How to fit a sash jammer starts with where the arm goes, before the first hole in a uPVC back door. Two arms on a single door, high and low, is the locksmith count. The arm has to clear the door when you park it, and it must not drop and trap you inside. This page is that sequence. It is not a ranking of packs. Shelf prices are on the [additional door lock comparison](/hub/door-and-window-security/additional-door-lock). Peter F. used locksmith fitting notes and the amazon.co.uk review faults in October 2026. A qualifying purchase can earn a commission at no extra cost to you.

How to fit a sash jammer starts with where the arm goes, before the first hole in a uPVC back door. Two arms on a single door, high and low, is the locksmith count.

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Sandiyago Sandiyago sash jammer, 4 pack

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8.6/10about £8.99
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zgpzlhd zgpzlhd sash jammer, 6 pack

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On this page
  1. 1.Where the arm goes
  2. 2.Steel or a jack nut
  3. 3.After it is on
  4. 4.When to stop and call a locksmith

Where the arm goes

Count the arms before you mark the frame. Master Locksmiths Association guidance is two jammers on a single door and one on a small window. Home Secure uses four on French doors or a patio set. One arm on a back door leaves the opposite corner free to flex. Mark both positions with the door shut, high and low, clear of the handle. Inward doors take the body on the frame; outward doors take it on the moving sash. Defender, Carl F Groupco and Door Window Supplies use that split. Hold the body in place, open the door a few degrees, and check that the door still clears it before the second hole. These packs are non-locking twist arms. Which type to buy is on the buying guide.

Steel or a jack nut

Drill a small pilot and see what you hit. ERA's Fab & Fix sheet uses a 3 mm pilot. If the drill meets steel, the screw goes into that steel. If the pilot finds only plastic, the same sheet opens the hole to 9 mm and uses a jack nut with an M5 machine screw at least 35 mm long. Home Secure gives the same fork. Sandiyago reviews put the supplied screws at about 30 mm, too short for some doors, and spacers that sheared. If you cannot feel steel and you do not have jack nuts, stop. The cylinder you still need is on the uPVC extra lock page.
hzuisst hzuisst French door handle lock

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hzuisst hzuisst French door handle lock
9.0/10about £19.99
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After it is on

Park the arm down, or to the side, so gravity cannot drop it across the door. Close the door, set both arms, and open them again in the dark from inside. If a stiff arm needs two hands, it does not belong on the door you would use in a fire. Stiff arms showed up inside a single Sandiyago pack. Do not give the stiff one the only exit. When you leave, the arms are off and the main lock is on. Lift the handle, then turn the key, which is the order ProtectUK gives for a multipoint door.

When to stop and call a locksmith

Call a locksmith if the frame is hollow and you are not fitting jack nuts, if the door is a fire door or an HMO exit, or if the multipoint has failed on the only way out. Cheshire Fire and Rescue expects an HMO escape door to open from inside without a removable key. A jammer you cannot clear quickly fails that test. These packs are a householder fit only when the pilot finds steel, or when you already know a jack nut. Which pack is the better-known one, and who should buy a clamp instead, is on the best additional door lock page.

Common questions

How many jammers on one door?
Two on a single door, one high and one low, is the Master Locksmiths Association guide. One is the guide for a small window. Four is the Home Secure count for French doors or a patio set. One jammer on a back door leaves the far corner unsupported. Fit the pair, then test that you can still open the door from inside.
Do I screw into the frame or the door?
Inward-opening: the body goes on the frame. Outward-opening: the body goes on the moving sash. That split is in Defender, Carl F Groupco and Door Window Supplies notes. The arm then covers the gap. If you put an inward body on the door, the arm swings the wrong way and you will drill a second set of holes to correct it. Check the door still clears the body before the second hole.
What if the drill does not hit steel?
Stop, unless you are fitting jack nuts. ERA's sheet opens a hollow result from a 3 mm pilot to 9 mm and uses a jack nut with an M5 machine screw at least 35 mm. A screw that spins in plastic will not hold an attack, or a heavy arm. If you do not have that fixing, leave the hole and call a locksmith. Do not keep driving a short screw in the hope the plastic will hold.
Are the screws in an £8.99 pack long enough?
Often they are not. Sandiyago reviews describe screws around 30 mm, which is shorter than the 35 mm machine screw ERA specifies once you are in a jack nut. Spacers in that pack also sheared. Plan on your own screws if the frame is thick, and do not rely on the spacer to make a short screw reach steel.
How do I avoid locking myself out?
Park each arm so it cannot fall across the door. Then open the door from inside with the arms set, and again with them parked, including in the dark. A stiff arm on the only exit is the one to move or remove. These packs have no key, so nobody outside can free you if the arm is across. The Master Locksmiths Association treats a non-locking jammer as an occupied-house extra.
Can I fit this on an outward-opening door?
Yes, if you put the body on the moving sash and the arm still covers the frame when it is thrown. The screws and the steel test are the same as on an inward door. A portable brace is the wrong substitute here. Brace makers limit that tool to inward doors with a gap. After the jammer is tight, the multipoint still has to lock.

Related pages

Portable or fixed additional door lock

Additional door lock

Best additional door lock for the door you have

Extra door lock UK: choose the job before the brand

UPVC door extra lock beside the main cylinder

Sources

Public UK sources used for this page in October 2026. Prices, stars and review counts are the amazon.co.uk figures in this comparison. No lab test was run.

  1. Lichfield District Council, protect your home (retrieved October 2026)
  2. West Yorkshire Police euro-cylinder note (Walton Parish Council PDF) (retrieved October 2026)
  3. The AA, door locks and home insurance (retrieved October 2026)
  4. ProtectUK, security at home (retrieved October 2026)
  5. MLA sash jammers FAQ (retrieved October 2026)
  6. DoorJammer UK FAQ (retrieved October 2026)
  7. Cheshire Fire and Rescue, HMO fastenings (retrieved October 2026)
  8. DASH HMO lock guide 2023 (retrieved October 2026)