The SevenDay guide on how to install a new uPVC Window

Q – How to Install a new uPVC Window

It is often assumed that changing your windows is a simple job, however there are many risks and complications that may occur along the way. Therefore, always seek expert advice or ask your SevenDay team for advice if you looking to engage in some DIY at your premise and are in any way unsure. 

The following is a guide only and removal can differ depending upon the property type or age and style of window.

A step-by-step guide to fitting a uPVC window into a prepared opening – see the SevenDay guide on how to remove an existing window.

Read this first

Fitting a uPVC window is a manageable job for a competent DIYer or tradesperson, but a few points matter for safety, weatherproofing, and staying within the rules:

Replacement windows in England and Wales must comply with Building Regulations (Part L for energy efficiency, Part F for ventilation, and on new dwellings Part Q for security , and Part B for any fire-escape requirement (although on replacement you can not make the fire escape from upstairs windows worse than before). The work should be either certified by a FENSA or CERTASS registered installer, or you notify Building Control and have it inspected. If you fit it yourself, the compliance and certificate are your responsibility.

• Habitable rooms have minimum ventilation requirements — trickle vents are generally required on replacement windows. Check whether the old window had them and match or improve on it.
• Bedrooms and habitable upstairs rooms often need at least one window that meets fire-escape (egress) sizing. Don’t reduce an opening below that without checking.
• Glass and frames are heavy and awkward. Work with a second person, especially above the ground floor, and use proper access equipment — never makeshift platforms.

This guide assumes the opening is already clear, sound, and correctly sized, with a good lintel above and a sound sill below. If the lintel or surrounding structure is in any doubt, get it checked before fitting.

Tools and materials you will need

Have everything ready before you lift the frame into place:

• Tape measure, spirit level (ideally 1.2m), and a pencil
• Cordless drill/driver and a hammer drill with masonry bits
• Window fixing screws (frame fixings) or fixing lugs, plus wall plugs to suit the substrate
• Packers / glazing shims in assorted thicknesses
• Low-expansion (gun-grade) expanding foam
• Exterior-grade low-modulus silicone sealant and a sealant gun
• Utility knife, putty knife / bolster, and a rubber mallet
• Safety glasses, gloves, Kevlar sleeves and a dust mask
• Cloths and solvent cleaner for finishing

Step-by-step installation

1. Check the opening and the frame. Measure the opening width and height in three places each. The frame should be roughly 10mm smaller than the opening on each dimension, leaving a 5mm gap around the perimeter for packing and tolerance.
2. Check the opening is square and the sill is level. Note any low or high spots — you will correct these with packers, not by forcing the frame.
3. Remove the glazed units from the frame (unclip the beads and lift the glass out) so you are lifting an empty, lighter frame. Keep the beads and glass labelled so you know which opening they fit in and safe.
4. Prepare the frame – If fixing through the frame, pre-drill the fixing holes through the frame at the same spacing while it is on the bench — much easier than overhead. Ensure they are drilled at points where there is no damage to brickwork of the house so the fixing will be secure when installed.
5. Apply a continuous bead of silicone along the sill where the frame will sit, fit the cill end caps and any expansion gaskets the system requires. [tip] ensure you do not silicone over any small drainage holes that will be made in the underneath of the main frame.
6. Offer up and pack the frame
   • Lift the frame into the opening and sit it on packers at the sill so it is centred, with an even gap all round.
   • Pack under the sill at the fixing points and at the ends so the frame bears evenly and is not resting on the bead of silicone alone.
   • Level the sill first, then plumb the jambs, then check the head. Use packers at each fixing point to hold the frame true. Always pack at fixing points so tightening a screw can’t bow the frame.
   • Check the frame is not twisted (in ‘wind’) by sighting across it, and check the diagonals are equal so it is square.
7. Fix the frame
   • With the frame held true, mark the fixing positions onto the masonry through the pre-drilled holes or lugs. 
   • Drill into the masonry, insert wall plugs, and drive the fixings home. If the bricks are of sound standard you may not need wall plugs or lugs, but a quality masonry screw direct into the brick will suffice — ensure fixings are screwed firm, but not so tight that the frame bows. Re-check level and plumb as you go, adjusting packers if needed.
   • Work from the sill upward, alternating sides so the frame stays balanced and true.
   • Once fixed, check the frame is still square and that an opening sash if removed, is refitted temporarily, opens and closes cleanly with an even gap all round.
8. Re-glaze and refit sashes
   • Refit the sealed glazed units onto their packers (location and bridge packers) so the weight of the glass squares the sash — this is what keeps a casement operating smoothly. Follow the toe-and-heel packing the system specifies.
   • Clip the glazing beads back on, long edges first then short, tapping home gently with a rubber mallet. [tip] do not use a standard hammer as the bead can be easily dented/damaged.
   • Rehang any opening sashes that were removed for installation and adjust the hinges so they sit central in the frame with an even gap.
9. Insulate and seal
   • Fill the perimeter gap with low-expansion foam from the inside, working around steadily. Don’t over-fill — expanding foam can bow a frame if packed too tight. Let it cure, then trim flush once set.
   • Apply a neat, continuous bead of low-modulus exterior silicone around the outside where the frame meets the masonry (including underneath the cill) tooling it smooth. Seal the inside perimeter too. Often silicone finishes can look unsightly and ruin the appearance of a newly installed window. An alternative is to finish with a trim beading to match the frame, with this you cut the bead to length, offer up to ensure they fit correctly and then silicone behind where the trim will fix. If a quality silicone is used then this will act like glue and the trim will be held secure in place. [tip] The normal is to have a square bead across the top of the window, with quad beads down each side, this eliminates the need for precise mitred corners of the beading.
   • Do not seal over the drainage slots on the outside of the frame — these let water escape and must stay clear.
10. Finish and check
    • Remove the protective tape from the frame promptly — leaving it on in sunlight makes it bond and hard to remove.
    • Clean the glass and frame, operate every sash and lock to confirm smooth running, and check the trickle vents work.
    • Make good the internal reveals (plaster, trim, or a uPVC reveal liner) and the external render or pointing as required.

Common mistakes to avoid

• Fixing the frame without packers at the fixing points, so tightening the screws bows or twists it.
• Over-tightening fixings or over-filling with foam, both of which bow the frame and stop sashes closing evenly.
• Skipping the toe-and-heel glass packing, which leaves casements dropping and binding.
• Sealing over the drainage slots, which traps water inside the frame.
• Forgetting trickle vents where the regulations or the room’s ventilation require them.
• Relying on foam alone to hold the window in — foam insulates and seals, mechanical fixings carry the load.

This guide covers fitting a standard uPVC casement into a sound, correctly sized opening. Bay windows, structural openings, coupled or large multi-light frames, and anything load-bearing are more involved and are best fitted by, or checked with, a professional installer. For Building Regulations sign-off, a FENSA or CERTASS registered installer can self-certify the work.