How do I check new windows and doors have been fitted properly?

A window installation is one of the few household jobs where the defects are easy to see and hard to get fixed once the balance has been paid. The list below takes an hour and it is the hour that decides whether small problems are corrected as part of the job or become your problem next winter.

Step by step

1. Inspect in daylight and before the balance is paid.Arrange the final inspection while the installers are still on site if you can. A defect list agreed on the day is dealt with; the same list emailed a fortnight later becomes a negotiation.
2. Operate everything, several times.Every sash, every lock, every handle, every restrictor. Look for anything that needs a shove. A window that is marginal on a mild fitting day will be unusable in a cold snap.
3. Do the paper test on the seals.Close a strip of paper in the sash and pull. It should grip with a consistent resistance at every point around the frame. Somewhere it pulls out easily is somewhere the seal is not compressing.
4. Check the frames are actually square and plumb.A long level on every jamb, in two directions, and across the head. A frame packed out of plumb produces sashes that will never sit right and hardware that will fail early.
5. Look at the outside sealant closely.It should be continuous, tooled to a neat concave, adhering to both the frame and the masonry, and with no gaps at corners or behind cills. Gaps here are the commonest source of water ingress after a new installation.
6. Check the drainage.Find the weep slots in the bottom of each outer frame and check they are present, clear and not sealed over. Pour a little water into the bottom of a frame and watch it drain out.
7. Check the glass specification you paid for.Look for the etched safety mark in doors, side panels and low-level glazing. Look at the spacer bar for the manufacturer and date. If you specified laminated, obscure or a particular pattern, verify it now.
8. Check the ventilation.Trickle vents where specified, and confirm they open. If the old windows had vents and the new ones do not, ask why in writing — replacing ventilated windows with unventilated ones causes condensation and may not comply.
9. Check inside for making-good.Plaster reveals, window boards, sealant lines, architraves and any damage to decoration. Agree explicitly who is making good what before the work starts, because this is the most common dispute on a window job.
10. Confirm the paperwork.Ask when the building regulations certificate will arrive and from which scheme, get the guarantee in writing with its length and what it covers, and get the insurance-backed guarantee document. Withhold a meaningful final payment until the snagging list is closed.
11. Recheck after the first heavy rain and the first cold snap.Water ingress and stiffness show up under conditions that do not exist on a dry installation day. Report anything within the guarantee period promptly and in writing.

Common mistakes

• Paying the balance before inspecting — It removes the only practical leverage you have. A retained final payment gets a snagging list closed in days rather than months.
• Accepting 'that will settle' about a stiff window — Windows do not bed in. A stiff sash on day one is a frame packed out of square or a hardware setting that is wrong, and it will get worse in cold weather.
• Not agreeing making-good in advance — Reveals, window boards and decoration are where installers' and customers' expectations diverge. Put it in the order in writing.

Checklist

• Every opening sash operates smoothly and locks with light pressure
• Every handle sits square and returns properly
• Gaps around each sash are even top, bottom and both sides
• Seals compress evenly all round — check with a strip of paper
• Frames are plumb and level, checked with a level, not by eye
• Outside sealant is continuous, tooled and adhering to both surfaces
• Inside making-good is complete and the plaster reveals are sound
• Drainage slots in the outer frame are present and clear
• Trickle vents fitted where specified, and they open and close
• Safety glass marks present in doors, side panels and low-level glazing
• Sealed units are clean, unscratched and free of internal debris
• Cills project far enough and have a working drip
• All packers and shipping blocks removed, and no fixing screws left proud
• Old frames, glass and debris taken away
• Certificate, guarantee and any insurance-backed guarantee confirmed in writing