A competent DIYer or any general builder can fit a made-to-measure uPVC window in 2–4 hours per unit. Here's the honest process, with the things nobody tells you.
Read this first: if the opening is above ground floor, if the lintel looks suspect, or if the existing window is a structural member (bay, bow, arched reveal), get a builder involved. Window fitting itself is straightforward — dealing with surprises inside the wall is not.
Allow 2 to 4 hours per window on your first one. After a few you'll be down to 90 minutes. Never start a job late in the afternoon — if foam or silicone doesn't cure before overnight temperatures drop, you'll be redoing parts of it in the morning.
Don't fit in heavy rain — foam won't cure properly if the opening is wet. Don't fit below 5°C — silicone doesn't skin. Ideal is a dry day between 10°C and 20°C.
Replacement windows in England and Wales must comply with Building Regulations Approved Documents L (thermal), F (ventilation) and N (safety glazing). Our windows are specified to meet all three. Compliance is documented either via:
| Tool / Material | Purpose |
|---|---|
| Spirit level (600mm+) | Checking plumb and level on all four sides |
| Pry bar / crowbar | Removing old frame once cuts are made |
| Reciprocating saw OR oscillating multi-tool | Cutting through old frame corners |
| Cordless drill + masonry bits | Drilling fixings into brickwork |
| Silicone gun + neutral-cure sealant | Weather-sealing internal and external joints |
| Expanding foam gun + gun-grade foam | Insulating and locking the frame into the opening |
| Stanley knife | Trimming cured foam |
| Plastic packers / shims | Raising frame to level in 1–4mm increments |
| Frame fixings (7.5 x 92mm) | Mechanically fixing frame to wall |
| Dust sheets + safety glasses | Obvious reasons |
Move furniture 2 metres clear of the window. Lay dust sheets — window removal creates fine masonry dust that gets everywhere. Open a window in an adjacent room to avoid pressure issues when the old window is pulled out.
Open each sash to 90°. On side hung sashes, the hinge pin usually lifts out once the sash is at 90° — check your particular hinge type. On top hung, unscrew the two hinge plates from the frame side. Set sashes aside in a safe place; you can recycle the glass locally.
Using a reciprocating saw or multi-tool, cut diagonally across each of the four corners of the outer frame — through the frame material, not into the wall. This breaks the frame into four pieces.
Starting at the bottom, use the pry bar against the masonry (not the brickwork face) and lever each section free. Older windows may be foam-fixed, screw-fixed or wedge-fixed — expect to encounter whatever was most convenient in 1985.
Remove debris, old sealant, crumbling mortar. Check:
If the opening is out of square by more than 10mm, you'll need to pack or chase masonry before the new frame goes in.
Lift the frame into the opening without sashes fitted. Check the reveal is even all the way round (roughly 5mm each side for packing). Mark the frame's position with a pencil.
Insert plastic packers at the two bottom corners first. Raise the frame until the bottom sill edge reads dead level on a spirit level. Then check plumb — tilt front-to-back adjustment is done with packers behind the frame at the jambs.
Golden rule: level first, then plumb, then check square (diagonals should be within 3mm). Only when all three pass do you move on.
Drill through the outer frame into the brickwork at the following points:
Use 7.5 x 92mm uPVC frame screws with caps. Snug but don't overtighten — uPVC distorts under clamp load. The screw is there for shear resistance, not compressive clamp.
Shake the foam can well. Apply in a single continuous bead around the full perimeter of the frame (internal side is fine). Apply roughly half the volume you think — it expands 2–3x. Leave to cure 2 hours minimum before trimming.
Low-expansion gun-grade foam is important. High-expansion can bow the frame inwards and make sashes bind.
Once foam is cured, rehang the sashes on their hinges. Operate each sash through its full range — if any stiffness, check the keeps on the frame are aligned. Small adjustment screws on the keeps allow 2–3mm correction.
With Stanley knife, trim cured foam flush with the frame and the wall on both inside and outside. Apply a bead of neutral-cure silicone to the external joint first (weather seal), tooling off for a clean finish. Repeat on the internal joint.
Neutral-cure silicone is non-negotiable on uPVC — acetoxy-cure (vinegar smell) attacks the frame plasticiser over time.
Operate every sash. Engage the multi-point lock on each — listen for a clean "click" on each point. Open the trickle vent. Check no draughts around the frame perimeter. If any sash drops at the latch when closed, adjust the hinge screws (usually a 4mm Allen key — consult your specific hardware).
File your certificates:
These are required for future property sales and insurance purposes.
Usually the frame is slightly out of plumb or square. Check diagonals are within 3mm. Check hinge tension — most modern hinges have a small adjustment screw allowing 2–3mm correction in three axes.
Hinge-side sag. Adjust the hinge screws to raise the free edge. If adjustment maxes out, the hinge may be worn or the sash itself may be distorted.
Foam didn't fully fill the void. Drill a 3mm hole at the midpoint of the gap and re-foam through it. Retrim and reseal.
Silicone bead broken or not fully adhered. Strip back and redo — dry surface, appropriate primer if recommended by sealant manufacturer.
Get a professional glazier or builder if:
Your order ships with a one-page site-specific fitting reference sheet. Any questions during fitting, call us on 0800 861 1450.
Back to Configurator →This guide is provided for general information. It does not constitute professional installation advice. Always work safely, follow manufacturer hardware instructions, and check Building Regulations requirements for your specific installation with your Local Authority Building Control. We accept no liability for installation carried out by customers or third parties.