GUIDE 12 – DIY Windows Installation Guide

Fitting your own windows is going to require, time, patience and some hard work. Preparation is the key, so make sure you get the initial measurements right (you can never measure too many times). Get to know all the names of the different parts of the windows, what they do and where they go. Before you do anything, sit down with a cup of tea and read our handy guide through a few times, until you know exactly what you’re doing.

If this is your first project, then we advise you to start with something easy. Try a small downstairs window, but once you gain experience and confidence you’ll be ready to tackle more challenging windows.

Follow our guide carefully to help you through each step of the process.

uPVC Window Fitting Guide


You Will Need:

  • Drill
  • Chisel
  • Screwdriver
  • Spirit Level
  • Door and frame sealant
  • Saw
  • Frame Fixings
  • 8mm Drill Bit



  1. Make sure there is a lintel above any window before you remove it, as uPVC windows are not designed to be load bearing.
  2. Check size and style of window matches your order.
  3. Clean site around window, including removal of ornaments and curtains etc.


Removal of Existing Window:

  1. First remove openers (parts of window that open) by using a screwdriver or nail bar to remove all opening sashes.
  2. Carefully crack and remove the glass from the inside, starting at the top corner.
  3. On the inside of the window use a sharp blade to cut through plaster seal or grout if tiled around the frame, to minimise damage.
  4. Saw through the uprights (mullions) of the frame. ALL cuts should be made at an angle so that the frame can be pulled apart easily. Don’t cut all the way through or you might damage the plaster or masonry.
  5. Use a chisel to get in behind the frame and lever inward at point of cut. Now you can cut all the way through.
  6. Pull at the frame and wiggle until it breaks free.
  7. Use the same process for the sill and head timber, using a chisel to lever it away from the wall before cutting through at an angle.
  8. Remove any debris around the opening, including protrusions of old cement.


Fitting Your New Window:

  1. If fitting a sill, position onto the brickwork, ensuring the up stand fits against the plaster line of the jambs. If necessary, trim the horns (part of exterior sill that extends past width of window, ideally by 50mm at each end).
  2. Use the plastic packers to level the sill with around 5mm clearance between it and the brickwork.
  3. Use fixing bolts to secure the sill to the brickwork and check that it’s level. Do not over tighten the bolts.
  4. Use silicone seal along the back edge of the sill up stand.
  5. Secure the end caps using superglue or silicone.
  6. Remove all glazing beads and mark to ensure they go back in their original position.
  7. Put the window in position, placing base against the sill up stand, fitting snugly against the bead of silicone.
  8. Clean off excess silicone.
  9. Use a spirit level to ensure the window is vertical and up against the plaster line, if appropriate.
  10. Wedge the window into position using plastic packers, taking care not to over-pack and bend framework.
  11. Secure the bottom of the window to the sill using screws.
  12. Drill holes through the frame into the brickwork, ready to insert the frame fixers.
  13. Tap in fixers and screw up, taking care not to over tighten and warp the frame.
  14. Measure window diagonals to check they are square.
  15. Before positioning the glass, insert glazing bridges in the frame recess.
  16. Position the glass unit, using glazing packers to ensure it is central within the opening and gently square up as necessary.
  17. Fit the top glazing bead first, followed by the bottom and then the sides.
  18. Carefully gun a bead of silicone between the outside masonry and window frame, wiping off excess.
  19. Inside, place a bead of silicone between plaster and window.
  20. Remove protective tape from the window and clean where necessary.
  21. Leave window for a couple of hours before opening, allowing seals to set.



  • Do make sure all your measurements are accurate.
  • Handle windows with care. Don’t knock the edges or rest them on concrete, instead use wood blocks or something soft.
  • Do take your time at each stage to avoid costly mistakes.
  • Don’t over pack or over tighten screws, as this could bend the framework.
  • If your window needs a wedge gasket, don’t cut it too short – an easy mistake to make.
  • Don’t run sealant along the outside gap between the sill and frame, as this will block the concealed drainage mechanism.
  • Do push sealant forward, rather than pulling back to ensure it pushes into any gaps.
  • Do countersink fixing holes to ensure bolts don’t obstruct glass.
  • Do stand back and admire your handywork.