This is where I first learned how to fit double glazed windows with my parents. Below I’ll guide you through the full DIY process so you can do it too.
All of the windows shown here were purchased from Roundbrand Ltd double glazing. This article is not sponsored; I’m simply proud to mention a family friend who supplied our windows.
Removing the Old Window
Before fitting the new double glazed unit, remove the existing window and any wooden beading or glazing panes. In our case my dad removed the old glass and beading safely off camera. If the frame is fixed with screws or nails, carefully cut or pry away the old frame sections with a handsaw and crowbar. Work slowly to avoid damaging the surrounding wall.
Cut the External uPVC Window Sill
With the opening cleared, measure the width of the aperture accurately. Transfer that measurement to the uPVC sill—this piece faces outward. A handsaw or fine-tooth saw will give a neat finish; have someone hold the sill steady while you cut to avoid splintering. Accurate cutting ensures the sill sits flush with the wall and the new frame.
Sealing the UPVC Window Sill
Apply a bead of clear silicone along the inner lips of the sill and across the centre before you fit the frame. This provides a weatherproof seal and helps the sill grip the window frame during initial positioning. Use a continuous, neat bead to avoid gaps under the frame.
Screwing the UPVC Window to the Sill
Place the window frame onto the siliconed lip and hold it in position. Predrill three pilot holes through the frame into the sill and insert screws, taking care not to penetrate all the way through the sill. These initial fixings secure the sill to the frame and stabilise the unit for the next steps.
Fitting the Double Glazing
With the sill attached, slot the complete window unit into the opening. Use plastic or timber packers to level the frame and keep it square in the aperture. Check alignment at multiple points—top, middle and bottom—and have someone stand outside to confirm the frame sits flush with the external wall. Proper packing prevents distortion and ensures smooth operation of the sashes.
Screwing the UPVC Window to the Brick Wall
Once the frame is level, mark and predrill through the window frame into the brickwork. We used masonry screws (often called Spax screws) so no separate wall plugs were required. Drive screws evenly along the frame, but avoid over-tightening—overtightening can deform the uPVC. Check the window opens freely after the first few fixings to ensure nothing is binding.
Practical Tips for Fitting Double Glazed Windows
Work progressively around the frame rather than fixing one side completely first—this prevents twisting or warping. After a few screws, open and close the window to test for smooth operation; if it’s stiff, adjust the packers or loosen a screw and recheck. Avoid drilling into the sill from above; instead fix through the frame into the wall at the required intervals.
Removing and Replacing the UPVC Beading
Removing the internal beading to fit the glass can be fiddly and needs patience. Tap gently along the join with a flat filling knife and a double-glazing hammer to loosen the beading, working a little at a time. Once removed, press the glass pane into the frame firmly so it seats correctly.
After seating the glass, replace the beading slowly and tap it back into place with a plastic mallet to avoid damage. Take care not to force any section—go incrementally around the frame until the beading sits flush and secure.
Sealing Around the Window
Fill any gaps between the frame and the wall with expanding foam to provide insulation and prevent drafts. Trim the excess foam once it cures, then finish externally and internally with a neat bead of silicone to weatherproof the joint. For aesthetic matching with brickwork, some installers recommend using coloured silicone (for example, brown against red brick), but that choice is a matter of personal preference and appearance.
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