A bad window install usually fails at the seal, the frame, or the cill, not the glass itself
You have just paid several thousand pounds for new windows and you want to be sure the job was done right. The most common snags found by Trading Standards and FENSA are draughts from failed perimeter seals and water ingress at the cill junction, not cracked panes. A snagging checklist focuses on visible gaps, operational stiffness, and condensation between panes, all signs of a failed install. The homeowner’s goal is to identify defects within the first 28 days (typical snagging period) so the installer must return under warranty. This article gives you a room-by-room checklist you can print and use immediately.
Before you start, what to check on the paperwork and the product label
Before you inspect a single window, check the paperwork. Verify the installer is FENSA-registered or CERTASS-registered by using the search tool on the GOV.UK competent person scheme page (GOV.UK, Competent Person Schemes for building control). If the installer is not on either register, the installation is not legally compliant and your local authority will not accept it.
Confirm the glass carries a British Standard Kitemark (BS EN 1279 or BS EN 12150) and the spacer bar is warm-edge (stainless steel or plastic, not aluminium). Check the energy label: a replacement double-glazed unit should have a U-value of 1.2 W/m²K or lower (triple glazing: 0.8 W/m²K) (DESNZ, Building Regulations Approved Document L (2026 update)). Ensure the installation certificate (FENSA or equivalent) has been lodged with your local authority, you should receive a copy within 14 days. Without it, selling your home later becomes difficult.
The frame and cill check, where water and air get in
Run a credit card along the perimeter seal between the frame and the wall. If it slides in more than 2 mm, the sealant bead is too thin or missing entirely. This is the single most common snag reported to FENSA. Next, pour a bucket of water over the cill (outside) and watch inside for drips at the cill junction or the frame corners, water ingress is a FENSA-notifiable defect that can rot the surrounding timber or masonry.
Check the cill slope: it must angle down and away from the frame (at least 5 degrees) to shed water. A flat or inward-sloping cill will cause standing water and eventual rot. Look for gaps larger than 3 mm between the frame and the masonry, these must be filled with expanding foam and then sealed with a flexible mastic. Any gap here is a direct route for draughts and damp.
The glass and glazing bead check, condensation and visible defects
Hold a spirit level vertically against the glass. Any bow or twist of more than 3 mm across a 1.2 m pane indicates the pane was not installed level. Check for condensation between the panes, this means the inner seal has failed and the unit must be replaced, not just resealed (GGF technical note). Internal condensation is a manufacturing defect, not something you can fix with a wipe.
Inspect the glazing beads (the strips holding the glass in). They must be mitred neatly with no gaps wider than 1 mm, and they should be the same colour as the frame, mismatched beads are a cosmetic snag but worth noting. Run a bare hand around the inside edge of the glass at night. If you feel a draught, the gasket (rubber seal) is not compressing properly. That draught will cost you money on your heating bill every winter.
Quick numbers, the three measurements you need to take
| Measurement | Pass threshold | How to test | Source |
|---|---|---|---|
| Perimeter gap (frame to wall) | ≤ 2 mm | Credit card test | FENSA installation standard |
| Frame twist (diagonal difference) | ≤ 3 mm over 1.2 m | Spirit level on glass | BS 7412 (PVC-U windows) |
| U-value (on label) | ≤ 1.2 W/m²K (double) | Read product label | Approved Document L (DESNZ) |
The opening and locking check, stiffness is a red flag
Open and close each window fully three times. Any sticking, scraping, or need to lift the handle to close means the hinges are misaligned. With the window closed, try to lift the sash, if it moves up more than 2 mm, the friction hinges are not adjusted correctly. This will cause the window to sag over time and eventually fail to seal.
Check the lock: the handle should turn smoothly to 90 degrees and the locking points should engage fully (you should see them push into the keepers). A stiff handle or a lock that does not fully engage is a security risk. Test the trickle vent (if fitted): it must open and close freely and the mesh must be intact. A broken trickle vent fails Building Regulations Part F (Building Regulations Approved Document F (2026 update)).
How to verify your installer and what to do if you find a snag
Confirm the installer holds a current FENSA or CERTASS certificate by searching at gov.uk/check-competent-person (GOV.UK, Check a competent person). If you find a snag, send a written snagging list (with photos and measurements) to the installer by email within 7 days of completion, keep proof of delivery. The installer must fix all snags under the Consumer Rights Act 2015 within a reasonable time (usually 14–28 days) at no extra cost (GOV.UK, Consumer Rights Act 2015 guidance).
If the installer refuses, escalate to FENSA or CERTASS for dispute resolution, or to Trading Standards if the defect is structural (e.g., water ingress into the wall). How to claim for faulty windows under the Consumer Rights Act FENSA certificate and building regulations compliance explained Do not let a snag slide, a small gap today becomes a rotting frame in three years.
Frequently Asked Questions
A window snagging checklist is a room-by-room guide to spot defects in a new window install. FENSA and Trading Standards recommend checking perimeter seals, cill junctions, and operational stiffness within 28 days of installation.
Check the installer is FENSA or CERTASS registered on GOV.UK, then run a credit card along the frame seal – no more than 2 mm gap. Pour water on the cill to test for water ingress, and confirm the glass U-value is 1.2 W/m²K or lower (DESNZ, Building Regulations 2026).
Common window snags include draughts from failed perimeter seals, water ingress at the cill junction, and condensation between panes. FENSA reports that sealant gaps over 2 mm are the most frequent issue.
You typically have 28 days from installation to snag new windows. This period is standard under the installer warranty and allows you to request free repairs for defects like draughts or water ingress.
Yes, a FENSA or equivalent certificate is required for legally compliant window installations in the UK. Without it, your local authority will not accept the work, and selling your home becomes difficult (GOV.UK, Competent Person Schemes).
