How to Install uPVC Windows

New to installing uPVC Windows and wondering how to do it? Don’t worry.

uPVC windows are a desirable addition to any home and offer a wealth of benefits.

However, incorrect installation can lead to myriad problems, from decreased energy efficiency, to damp and the many issues associated with that complaint.

This guide on installing uPVC Windows should help you get to grips with installing uPVC windows correctly to ensure the homeowner enjoys the maximum benefits from their investment. 

Remember: Always follow manufacturer instructions. The steps below are. a guide only.

Tools Required 

As with any task, the appropriate tools should be used to ensure a satisfactory finish. Before you start installing uPVC windows, you’ll need to make sure you have the following to hand: 

  • Chisel 
  • Stanley knife 
  • Nail bar 
  • Spirit level
  • Drill and bits
  • Screwdriver 
  • Saw 
  • Frame fixings 
  • Door and frame sealant 
  • Metric tape measure
  • Light hammer 
  • Pencil 
  • Gloves 
  • Goggles


Before you start, make sure you’ve fully prepared as mistakes are difficult to rectify later in the process. First, check that each window opening has a lintel above as uPVC windows are not designed to be load-bearing. You’ll also want to check that the windows that have been delivered are the ones you ordered and then clear the space around each window, removing ornaments and drapes etc. before beginning. 

Remove existing window

Cover the surrounding area with sheets before beginning. 

Using a screwdriver, remove the opening part of the window. A nail bar will be required to remove sash style windows. Then crack the glass from the frame, starting at the top left corner. Be sure to wear gloves and goggles for this to avoid injury. 

Using a Stanley knife, cut through the plaster seal around the window frame then use a crosscut saw to cut through horizontal and vertical frame members. Finally, use your nail bar to pry the frame away from the plaster line. 

Fitting the new windows 

Place the windowsill onto the brickwork. The uprights should sit flush against the plaster line of the jambs. If required, cut the sill horns to fit around the brickwork. 

Using plastic packers, level the sill with 5mm clearance from the brickwork then secure the sill to the brickwork using 8x100mm fixing bolts. These should be positioned approximately 150mm from each end and then at 600mm intervals. Check the level as you go and be sure not to overtighten the fixing bolts. 

Place a bead of silicone along the back edge of the upstand and then use superglue to secure the end caps into position. If the sill horns have been trimmed, the end caps will need trimming to match. 

Next, remove the glazing beads from the frame of the window, taking care to mark them so they can easily be replaced. 

Now move the new window into place, taking care that the base is snug against the sill upstand, ensuring a tight seal against the silicone bead, wiping away any excess silicone. 

Using your level, check the window is vertically level and then wedge into place with plastic packers, ensuring the frame isn’t bent from overpacking. 

Be sure that the window vents are open as this will allow access to the outer frame jambs, then secure the bottom of the window to the sill, using 8x40mm screws 150mm from each corner and then at approximately 600mm intervals thereafter. 

Using 8x100mm bolts, secure window jambs to the surrounding brickwork. Bolts should again be placed 150mm from each corner and then at 600mm intervals. Make sure all screw heads are standing correctly in order to avoid problems when placing the glass. 

Shut and lock sashes and then check the outer edges for squareness before moving on to inserting the glass. 

Installing the glass

Before installing the glass, insert 25x100x15 glazing brides into the recess, using a small amount of silicone to keep the packer secure for the next step. 

Place a 28x100x5 glazing bride onto the bottom glazing bridges. Then place a glass sealed unit into the aperture, ensuring it rests squarely on the glazing packers and push as far back as it will go into the rebate without using too much force. 

Using only hand pressure, place additional 2mm glass packers where required to square the pane then carefully lock and unlock the windows to ensure no binding occurs within the locking system and that there is no movement between the glass and frame. 

Clip the glazing beads back into position, starting at the top, then bottom and then sides then gently place a bead of silicone between the masonry and the window edge, wiping off any excess. 

Finally, place a thin bead of acrylic between the plasterwork and the window and allow one hour to skin over before removing any protective tape and cleaning necessary areas with warm, soapy water. 

At MLI Building products, we supply a wide range of uPVC double glazed windows to the trade and DIY enthusiasts. Speak to a member of our team today to discuss your requirements.