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How to fit a letter box or cat flap

Date posted: 17 August 2015

The method you use to fit a letterbox or a cat flap is very similar, so in this guide we offer you some tips to help you get either job right first time.

Whichever task you are carrying out, it is fairly straightforward for most DIY amateurs – but approaching the work properly and using the right tools will ensure you do not make expensive mistakes.

For both of these tasks, a powered jigsaw will make the cutting much easier. Alternatively, however, you could use a simple, manual pad saw – this is a long, narrow saw that is used for cutting small features in wood and other materials, so it is ideal for fitting letterboxes and cat flaps.

How do I fit a letterbox?

Measuring up:

Since letterboxes can be bought in a variety of different sizes, you will need to measure up first:

On the outside of the door, measure the depth (top to bottom) of the door's crossrail. Make a pencil mark at the mid-point in two places about 3.5cm apart. Draw a straight line to join up the two marks. Next, measure the width of your door's cross rail and again find the centre point, drawing a short vertical line across the one you created earlier. The centre of your letterbox should sit over the cross where the lines meet. Mark the position of the fixing bolts on each side.


  • First, drill clearance holes ready for the fixing bolts, making them a little bit bigger than the diameter of the bolt shank (usually about 6mm).
  • Using a pencil, draw the slot area where the opening is required (usually rectangular), making it a little bit larger than the hinged part of your new letterbox.
  • Drill a hole at each corner of the slot using a drill that is big enough to make a hole into which you can place the blade of the power jigsaw (or other tool) -- usually about 8mm.
  • Next, place the blade of the power jigsaw (or pad saw, if that is what you choose to use) into the first corner hole and cut carefully to remove the rectangle of door.
  • Now you have created the opening use a narrow chisel and mallet to cut recesses for the hinge pin.
  • Clear out the corners of the opening, smoothing edges using sanding paper.
  • Finally, fit the letterbox into position with the nuts supplied. If the bolts are too long (i.e. longer than your door is thick) you can simply shorten them using a hacksaw.

How do I fit a cat flap?

The majority of cat flaps available feature a straightforward locking mechanism, although there are more elaborate models available that have a remote controlled door triggered by a special collar worn by your cat(s). All cat flaps should be shipped complete with a fitting template and detailed fixing instructions, so read these thoroughly before you get started.

  • Take your cat flap template and stick it onto the door in the position you require the cat flap to be, taking into account its height and position from your cat's perspective.
  • Check it is level using a spirit level.
  • Drill through the four points as is shown on the template provided, making sure they are large enough to insert the blade of a jigsaw (usually about 8mm).
  • Remove the template from the door and use a pencil to join up the four holes into the square or rectangle shape where the cat flap will go.
  • Cut through the door along your pencil lines using a power jigsaw, then push out the central piece and sand the edges using medium-grade sand paper.
  • Next, hold the cat flap in place, lined up with the opening, check it is level, then drill clearance holes ready for the screws.
  • Grip both sides in position through the open flap and screw them together, fixing each screw through the clearance holes.
  • Test out your new cat flap to make sure the door swings freely and works as intended - then show it to your cat!

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