Installing your new window: information for you or your fitter

This guide explains how a replacement uPVC window goes in, so you know what the job involves and what your fitter will do. We supply only and don't offer fitting. The skill is in the levelling and packing, not brute force. A competent DIYer with a helper should budget half a day for a first window and read the whole guide before the old one comes out.

The window in this guide is one of our made-to-measure casement windows. You can also design and price uPVC windows online.

1. 1

   Check the window and the opening

   Before anything comes out, unwrap the new window and check it against your order and your opening: the frame should be roughly 10mm smaller than the structural opening in both directions (5mm clearance each side), exactly as our measuring guide deducts. Make sure there is a sound lintel above the opening. The frame must never carry the wall. Pick a dry day, and plan the old window out and the new one in within the same session.

2. 2

   Remove the old window

   Take the opening sashes off their hinges first, then remove the glass from any fixed sections: on uPVC pop the beads out, on timber pull the putty and sprigs. With the weight gone, saw through the old frame at the middle of the cill, head and both jambs, then lever each section inwards with a pry bar. Work steadily: the brickwork, plaster and DPC around the opening are what you're protecting. Sweep everything out and pull any old fixings.

3. 3

   Prepare the opening and dry-fit the frame

   Brush the opening clean and check the base the cill will sit on is sound, reasonably flat and dry. Now dry-fit: offer the new frame (and cill) into the opening with a helper, check the clearance is even all round and nothing fouls. It's far easier to sort a tight spot now than with a foamed, half-fixed frame. If your window came with the sashes or glass in, you can remove them to make the frame lighter. Note which unit came from where.

4. 4

   Fit and level the cill

   If you're using a projecting cill, fit the end caps and run silicone along their joints, then sit the cill in place on flat packers. Get it dead level along its length and front to back. Every course of the window above copies the cill, so time spent here pays back double. Screw the frame to the cill (or clip it, depending on profile) with a bead of silicone between the two so water can't track underneath.

5. 5

   Fix the frame: level, plumb and square

   Stand the frame on the cill, pack the jambs so it's central, and check plumb both ways and the diagonals are equal. Drill through the frame into the wall and fix with proper frame fixings: 150-250mm from every corner, no more than 600mm apart, with a packer behind every fixing point. Nip the screws up snug. Over-tightening without a packer bows the profile, and a bowed frame means sticking sashes forever. Re-check level and plumb after every fixing.

6. 6

   Glaze, then toe and heel

   Sit each glass unit on bridge packers in the bottom rebate (they straddle the drainage channel), lift the unit in with glass suckers if you have them, and centre it. Fixed panes just need the units packed square; opening sashes must be toe and heeled: packed diagonally from the bottom hinge corner to the top lock corner so they can't drop. Our toe & heel guide covers it packer by packer. Refit the beads, ends first, tapped home with a nylon mallet.

7. 7

   Foam the gap and seal outside

   Fill the fitting gap between frame and wall with low-expansion foam: a light, continuous run; high-expansion foam can bow the frame. Once cured, trim it flush. Outside, run a neat silicone bead between frame and brickwork all round, and seal under the cill. One golden rule: never silicone over the drainage slots on the bottom of the frame or cill. They're what lets rainwater back out.

8. 8

   Trim, finish and test everything

   Inside, cover the foam line with uPVC trim or make good the plaster. Peel the protective tape off the profiles (don't leave it to bake on in the sun), and clip on any handle covers and cill end trims. Then test: every opener should swing freely and pull in snug, every handle should lock without force, and trickle vents should open and close. Anything stiff now only gets worse. Adjust the packing before you clear up.