In this day and age, it seems that we are a nation of DIYer’s; always looking to cut costs and do jobs ourselves. However, sometimes we may not know the right way to go about it and we usually end up creating more of a mess than we initially started.

Installing a window may seem like an easy task, when in reality it can be a very complicated process that requires expert knowledge and tools to complete to a high standard.

Here at Cosyhomes Windows, we have put together a ‘how to guide’ on the window installation process and explain how we can assist to ensure the job is finished to the highest standard.

Before You Start

It is extremely important that before you start you make sure that you have the right size, style and handing that you ordered. You will also want to check the opening size against the new window pane. The new window should be approximately 10mm smaller in both width and height than the opening.

Take Out Existing Windows

Once you have moved all of your ornaments, photos, blinds or curtains out of the way and laid down dustsheets, you can start to remove the existing window:

  • You will need to remove all opening sashes with a screwdriver before you can remove the glass.
  • Removing the glass is usually done by carefully cracking the glass and removing the shards. You will need to ensure that you have heavy duty gloves and goggles for this part of the process and always start from the top corner, discarding the shards of glass into a plastic bin as you go.
  • Once all of the shards have been removed you will need to use a stanley knife to break the plaster seal that can be found around the window frame before using a crosscut saw to cut through the remaining mullion (vertical) and transom (horizontal) frame parts to remove them.
  • It is important that during the frame removal process that you do not damage the plaster, brickwork and damp proofing of your property. Once you have cut through the frame it should be possible, by using a nail bar, to remove it away from the plaster.
  • Any remaining debris can be removed using a masonry chisel.

Fitting The New Window Frame

Once the old window has been removed and the area has been cleared of debris, it is time to install the new window:

  • The sill will need to be fitted so that it has around 5mm clearance between itself and the brickwork. Once you are happy that it is level (by using plastic packers) you can secure it using 8x 100mm fixing bolts 150mm from each end and at 600mm centres after that. You should not overtighten the bolts.
  • Now it is time to run a bead of silicone along the back edge and secure the end caps into position using either silicone or superglue.
  • You should remove the glazing beads from the window frame, marking their position up on the frame so that they can be returned to the correct position.
  • Now it is time to pop the window frame into position. The base should fit snugly against the sill, ensuring that a tight seal is created with the silicone. You should make sure that you clean off any excess silicone that you find at that time. Once in place you will want to make sure that it is level.
  • The window frame will need to be wedged into position using plastic packers, but you shouldn’t over pack as this can bend the framework.
  • Open the vents of the window frame so that you can access the outer frame jambs. The bottom of the window can now be secured to the sill from each internal corner. Once it is secured to the sill, the window jambs should be secured into the brickwork that surrounds the window.
  • Once completed, you should check that the screw heads are not standing proud as this can cause an issue with the positioning of the glass.
  • Now you can close and lock all of the sashes, checking the outer frame to ensure a square fit.


The last stage of the process is glazing. Glazing requires a gentle touch to ensure that the new glass does not crack or break during installation.

  • Before you fit in the glass you will need to secure glazing bridges into the recess of the frame. Once you have done this you can position glazing packer, using silicone onto the glazing bridges, starting from the bottom first. If you have side hung sashes then you will want to secure an additional packer onto the bridge opposite the hinge side of the frame.
  • Now is the time to add the glass into the opening. You will need to ensure that it is central and resting squarely on the packers. The glass should then be pushed in gently as far as it will go into the opening.
  • You should position 2mm glass packers to square up where needed.
  • Test the sashes by gently unlocking and opening them. This is to check that there is free movement between the glass and frame and to adjust if required.
  • Replace the glazing beads into their marked original positions. You should always place the top first, then the bottom before finally replacing those on the side.
  • To secure the window frame to the outside masonry, you will need to gun some silicone between the two. This should be left for 1 hour.
  • Once you are happy with the window and that everything is dry, you should then remove all of the protective tape, clean and leave to dry.

Ask The Experts At Cosyhomes Windows

As you can see, installing a new window into your property is a complicated process that requires professional grade tools and experience. You also have to understand and translate heavily technical regulations from Local Authority and seek approval from Building Control to change any window or door. Whilst you still may want to give it a go yourself, you may find that it is a better idea to get in touch with a professional window installation company that can install quality windows into your home. This is where we can help. Give us a call now on 0208 397 4040 or contact us via our online form and one of our team will get back to you shortly.