Home Improvement

How To Fit UPVC Windows In 9 Steps

Written by  John Davies
Last updated: August 8, 2023

A UPVC window is the perfect budget-friendly and environmentally-friendly alternative to aluminium and timber windows. It is fire-resistant, well-insulated, sound-proof, weather-hardy, low maintenance and most importantly, secure. However, this is only true if you install it properly.

Therefore, even though this is an easy-to-follow guide on UPVC window installation, a little DIY experience is still beneficial. It will reduce mistakes that could damage materials, waste time and money or result in a poorly-installed UPVC window.

So, here is how to install a UPVC window in just 9 steps.


  • The materials: The window frame, sill, glass panes, glazing bridges (aka glazing bridges) and the glazing beads
  • Fittings: The screws that come with the package or other UPVC-recommended fittings
  • Sealant: Silicon, super glue or a recommended UPVC adhesive
  • Plastic packers
  • Safety equipment: eye goggles, gloves and boots
  • Measuring tools: measuring tape, markers, pencil, spirit level
  • Cutting and scraping tools: a crosscut saw and a utility knife, a flat pry bar, spatula or chisel
  • Screwdriver (electric and manual)
  • Power drill
  • Rubber mallet
  • Cement: in case the brickwork needs repair
  • Tarpaulin, dustsheets or plastic bags 

Now that we have covered the supplies, here is how to fit a UPVC window in 9 steps

Step 1: Assess the situation

Before going to the supply store, you must collect two crucial pieces of information. These are:

  • The required size for the UPVC window frame: Measure all four sides (left, right, up and down) of the rough opening. Then measure them one or two more times to ensure the figures are correct. After confirmation, remove 10mm (0.39 inches) from the height and weight. This will give you the ideal size for the UPVC window frame. The 10mm deductions, 5 mm on all four sides, will serve as the expansion gap.
  • The condition of the rough opening: A damaged rough opening will hurt the structural integrity of your newly UPVC window. So, inspect the brickwork and check for damages or anything that could use repair. 

Step 2: Prepare the site

You need to remove window blinds, curtains, paintings, picture frames, furniture, and other items in the area. They will get in the way and this could cause accidents that can endanger lives and property. So move objects away from the site. 

Then lay tarpaulin, dustsheets or plastic bags on the ground to contain waste distribution. You can also cover objects you can’t or don’t want to move.

Step 3: Remove the existing window

If there isn’t an existing window, you can move on to step 4. If there is, follow this procedure.

  • Remove the window panes: This will make the frame lighter and easier to remove. Everything is held together by screws and sealants. So all you have to do is unscrew screws and break seals. Do it in that order. You need a screwdriver for the former and a utility knife, flat pry bar or spatula for the latter. Remove the upper panes first, starting at one of the top corners.
  • Remove the frame: Follow the procedure you used for the window panes; simply unscrew screws and break seals. Then use the mallet and pry bar (or a crowbar) to pry the frame from the rough opening. During this process, you may have to do some cutting. That’s what the crosscut saw is for.
  • Clear the site: Remove the removed frame, glass etc. You can dispose right away or simply move them out of the way. 

During the removal, avoid damaging the brickwork. However, if there are damages, fix them in the next step before fitting the new UPVC window. As for the window frame and glass, restraints are only needed if you still intend to use them. If that’s not the case, you don’t have to worry about damaging them during the removal. 

Step 4: Prepare the rough opening

Remember, a damaged rough opening will compromise the structural integrity of the window you are about to install. So, use this step to bring that opening back in shape. 

Repair the damages you noticed in steps 1 and 3. Then clear dirt and debris (wood, glass etc.) from the surface of the opening. Finally, make sure that the surface is level on all four sides.

Step 5: Prepare the UPVC window frame

Here is how to prepare the window frame for fitting.

  • Confirm the dimensions of the window frame: Remember, it should be 10mm smaller than the width and height of the rough opening.
  • Fit the window sill: Screw the sill to the bottom of the window frame. Then determine the required size for the window sill, mark the cutting points, cut to size and cap the ends. Finally, seal the joints between the window frame, the sill and the caps. The sealant will keep water from getting into those joints and damaging the UPVC. You can use silicon, super glue or a recommended UPVC adhesive.

Note: When using screws on UPVC, tighten firmly but not too hard. You don’t want to crack the frame. 

Step 6: Fit the UPVC window frame

Depending on the weight of the UPVC window frame, you may need extra pair of hands for this step. Lift the window frame into the opening and do the following.

  • Set the window frame: Adjust the frame until it sits level and central in the window opening. Remember to maintain a 5mm clearance space on all four sides. To accomplish this, add plastic packers. Then use the spirit level to confirm that the window frame is level. Adjust until you get the desired result, a window frame sitting level and central in the opening. 
  • Add the screws: You need at least two screws on all four sides, with one screw at each end. Measure 600mm from one end and 150mm from the centre. Mark the spot. That is where the screw should be. So drill a hole into the frame (don’t drill into the wall) at that spot. Then drive a screw through the hole into the brickwork to hold the window frame in place. Repeat until to fit all the screws. 
  • Add sealant: Break off protruding packers, then add sealant between the UPVC window frame and the brickwork.

Step 7: Fix the glazing bridges

For most window panes, the glazing bridges must be at the top and bottom or left and right. So, check the manual to confirm the directions for your UPVC window. Then fit the glazing bridges as instructed. You will need to secure them with sealant. 

Step 8: Fit the glass and glazing beads

Now that the glazing bridges are in place, it is time to fit the glass panes. The process is simple. First, clean the surfaces where you will attach the panes. Then find a pane that fits each position in the window frame, install it, and then add the glazing beads that fit in that position. 

When fitting a pane of glass, put the lower part in position first. It’s safer and easier this way.  As for the beads, use the rubber mallet to hammer them into place. Be careful and gentle to avoid damaging the glass.

Step 9: Test and finish

Your UPVC window is ready. You just need to run tests, add finishing touches, and clean up. To test your newly-installed UPVC window, open, close, lock and unlock it multiple times until you are satisfied with functionality and structural integrity.

If there are issues, like improper fits or protruding screws, now is the time to fix them. You should also check the sealant to make sure that it’s enough. If it’s not, add more. If it’s too much, use the utility knife or spatula to scrap off excess sealant.

When satisfied with how your new UPVC window looks and functions, you can finally do some cleanup and round up the project. Then get rid of the waste.


DIY installations are a great way to save money. Professional installers charge £150 to £300 per day as labour costs to fit UPVC windows. So depending on the project, you can save a lot of money by installing a UPVC window by yourself.

However, this is only true if you get it right. A poor installation could make you spend more money than it would take to hire a professional. At the very least, it will leave you with a dissatisfying and unsafe UPVC window. 

We write this guide to prevent that. Hopefully, now you have all you need to fit a UPVC window by yourself.

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