How to fit a window restrictor

Video by Paddy’s DIY. This guide draws on "How to fit a window restrictor / child safety catch on a uPVC window" from Paddy’s DIY, a UK channel specialising in windows, doors, and locks. The video is clear on positioning the bracket correctly and on testing the release mechanism — two things that are easy to get subtly wrong first time.

1. Choose the right restrictor type

There are two main types for uPVC casement windows. A restrictor arm is a hinged metal arm that locks between the sash and the frame, limiting how far the window can open. A cable restrictor uses a short steel cable on a locking mechanism to do the same job. Both work well; cable restrictors are slightly cheaper and less visible.

For top-hung windows, an arm restrictor is easier to fit and more secure. For side-hung casements, either works. Check the product is rated for child safety — it should state compliance with BS EN 13126-8 or similar. Some cheaper catches are not designed for safety use.

2. Decide on the opening limit

The standard guidance is to restrict the opening to no more than 100 mm. That is enough for ventilation but too narrow for a child to fall through. Some restrictors have multiple locking positions — fix them for 100 mm unless you have a specific reason to set more.

If the window is used as a means of escape in a fire, the restrictor must have a key-operated or adult-only override. This is a legal requirement in certain situations — worth checking if you are fitting restrictors in a rental property.

3. Mark the position on the frame

Open the window to exactly 100 mm and hold the restrictor arm or cable bracket against the frame. The body of the restrictor typically sits on the window frame (the fixed part); the pin or cable end attaches to the sash. Check the manufacturer’s diagram for your specific model, as fitting positions vary.

Use a pencil to mark the screw hole positions. A square or small engineer’s rule helps keep the bracket perpendicular to the frame edge.

4. Drill pilot holes

Use a drill bit slightly smaller than the supplied self-tapping screws — usually a 2.5 mm bit for the screws that come with most uPVC restrictors. Drill slowly and do not go through the wall of the uPVC frame. These screws bite into the plastic, so you only need a shallow pilot.

Clean the swarf away with a brush. A blob of leftover plastic swarf under the bracket will stop it sitting flat.

5. Fix the bracket to the frame

Offer up the bracket and drive the screws in by hand first to make sure they are threading correctly, then tighten with a screwdriver. Do not over-tighten — uPVC is relatively soft and you can crack the frame if you go at it too hard. Snug is enough.

Fit the second half of the restrictor to the sash in the same way, following the manufacturer’s instructions for your specific model.

6. Test the restriction

Open the window fully and then let it engage the restrictor. It should stop firmly at the preset limit and not flex or shift. Push on the sash firmly from outside (or ask someone to) — it should hold without the bracket moving or the screws pulling out.

If the screws are pulling through the plastic, you have hit a hollow section of the profile. Reposition the bracket slightly and try again, or use longer screws with a larger head to spread the load.

7. Test the release mechanism

Most restrictors allow adults to release the limit and open the window fully — for cleaning, or for escape in an emergency. Test the release now: it should take a deliberate action (pressing a button, turning a key, or pinching a lever) that a young child could not manage by accident.

If the release is too easy to operate, the restrictor is not doing its job. Return it and choose a more secure model.

8. Fit restrictors to all at-risk windows

Any window above ground level that a child could reach is a candidate. Do the bedroom windows first, then landings and playrooms. It is worth going round the whole house rather than fitting one or two and stopping — the ones you miss are the ones that matter.

When to call a handyman

Call Richard if the frames are old or fragile, if you need restrictors on a large number of windows quickly, or if the property is a rental and you need written evidence that the job has been done to standard. Fitting a batch of restrictors across a house is a straightforward afternoon’s work for a handyman.