NHBC Chapters 6.1 and 6.7, Approved Documents F, K, L, Q and B · 24 checks · proplot.co.uk
Cavity trays and closers, DPC and sill detail, fixings and packers, perimeter sealing and airtightness, glazing specification, security and escape requirements.
24 checks · Reviewed by the ProPlot site team · How we write these guides
In short
The windows and external doors inspection checks the opening detail before and after the unit goes in: cavity closers and trays, DPC returns, sill support and end dams, correct fixings and packers, a continuous perimeter seal inside and out, and the right glazing, security and escape specification for each opening.
The window itself is rarely the problem. Almost every window-related defect is an opening detail: a missing cavity tray, a DPC that stops short, an unsealed perimeter, or a cill with no end dams. Water then arrives in the reveal months later and the window gets the blame.
This stage is also where the airtightness strategy is won or lost. The perimeter gap around every opening in the building envelope is either sealed with a proper tape or sealant system before plastering, or it is not — and once the reveal is skimmed you cannot tell, but the pressure test can.
Finally, openings carry more regulation than anything else in the wall: escape provision, glazing safety, restrictors at low cill heights, ventilation provision, security certification and thermal performance all land in the same aperture. Check the schedule per opening rather than assuming the supplier got it right.
When it happens
As each plot is glazed, following the superstructure inspection. The opening detail must be checked before the unit is fitted and the perimeter seal before plastering.
Who inspects
Site manager and the installation contractor, with the warranty inspector reviewing the detail at the superstructure and pre-plaster visits.
If it fails
A missing tray or unsealed perimeter means water ingress or a failed pressure test, and the correction involves removing finishes, sometimes brickwork, around a unit already installed.
Concealed work cannot be re-photographed later. Take these before the element is covered and file them against the plot.
A Reportable Item (RI) is a defect the inspector records against the plot. NHBC weigh each one by its Damage Potential, and the score feeds your site's Construction Quality Review. These are the ones raised most often here:
0 of 24 ticked — saved on this device
Check cavity trays are installed above every opening, with end dams and correctly lapped with the DPC (NHBC 6.1)
NHBC 6.1Verify weep holes are provided above the tray at the correct spacing and are clear of mortar (NHBC 6.1)
NHBC 6.1Confirm vertical DPCs return into the reveals and are continuous from cill to head (NHBC 6.1)
NHBC 6.1Check cavity closers are the specified insulated type, fitted tight with no gaps at the corners (AD L)
AD LVerify the cill is fully supported, laid to fall, with end dams or stop ends as detailed (NHBC 6.7)
NHBC 6.7Confirm the lintel is the correct type and bearing for the opening, with any insulation in place (NHBC 6.1)
NHBC 6.1Check the unit is plumb, level, square and centred in the opening within the permitted tolerance (NHBC 6.7)
NHBC 6.7Verify fixing type, positions and spacings follow the manufacturer’s instructions (NHBC 6.7)
NHBC 6.7Confirm packers are used at every fixing point and at load-bearing positions so the frame is not distorted (NHBC 6.7)
NHBC 6.7Check the frame has not been twisted or bowed by over-tightened fixings — operate every sash (NHBC 6.7)
NHBC 6.7Verify structural openings match the schedule so the tolerance gap is even on all sides (NHBC 6.7)
NHBC 6.7Confirm the perimeter gap is insulated and sealed internally with a continuous airtightness system (AD L)
AD LCheck the external perimeter seal is continuous, of even width and applied to clean, dry substrate (NHBC 6.7)
NHBC 6.7Verify no daylight is visible around the frame from inside before the reveal is finished (AD L)
AD LConfirm the reveal insulation and closer detail avoids a thermal bridge at the frame junction (AD L)
AD LCheck safety glazing is fitted in critical locations and is permanently marked as such (AD K)
AD KVerify glazing units, coatings and spacer type match the specification used in the SAP calculation (AD L)
AD LConfirm escape windows achieve the required clear openable area and cill height as installed (AD B)
AD BCheck restrictors are fitted where the cill height requires fall protection (AD K)
AD KVerify trickle vents are fitted where the ventilation strategy relies on them, and are operable (AD F)
AD FConfirm easily accessible doors and windows meet the security standard and certification is on file (AD Q)
AD QCheck external door thresholds are level access where required, and weathered correctly (AD M)
AD MVerify door sets are fitted with the specified locking, hinges and hardware, all operating freely (AD Q)
AD QConfirm the threshold detail maintains the DPC and drainage line without bridging the cavity (NHBC 6.7)
NHBC 6.7The unit will be fine. It is the hole it sits in and the six weeks it spends on a live site that decide whether the customer is happy. No inspector will fail the visit on these — they are the difference between a plot that passes and a plot that hands over clean.
Photograph every opening before the unit goes in
The tray, the dams, the DPC and the closer are all visible for about an hour. That photograph is the only proof they exist.
When: Immediately before installation.
Leave the protective film on until the final clean, but check the expiry
Film protects the frame from mortar and scaffold damage, but left too long in sunlight it bonds and becomes a job in itself to remove.
When: From delivery to final clean.
Test the perimeter seal by eye from inside and outside before plastering
Any daylight visible around the frame is a hole in the airtightness line and a cold bridge in the finished house.
When: Before the reveal is boarded or plastered.
Operate every opening light and lock before signing the plot off
Distortion from over-tightened fixings is common and only shows itself when the sash is used. It is cheap to correct while the fitter is still on site.
When: At installation, and again at pre-handover.
Measure the clear openable area of escape windows on the installed unit
The scheduled size and the achieved size differ once the frame, sash and hardware are in. Building control measures the achieved figure.
When: On the first plot of each house type.
Fit corner and cill protection where a route passes an opening
Damaged frames and scratched glass at handover are almost always caused by traffic during second fix.
When: As soon as the unit is installed.
1 inspection points that apply at the windows and external doors stage, each written in our own words from openly published guidance, with how to check it on site and what counts as acceptable. Use the checklist above on the day and this register when you need to settle a detail.
Cavity tray missing or without end dams
Water tracks along the tray and discharges into the reveal instead of out through the weeps. Usually shows as damp at the head of the window in the first winter.
Reportable Item; correcting it means removing brickwork above the opening.
Frame fixed without packers behind the fixing points
The frame pulls out of square as the screws are tightened, the sash binds, and the seal compression is uneven.
Perimeter never sealed internally
The external mastic looks fine, but the internal airtightness line is open. It shows up as a failed pressure test and cold reveals.
Fails the Part L airtightness target.
Escape window short of the required clear opening
The scheduled size was right but the installed frame, sash and hardware reduce the clear area below the requirement.
Blocks the completion certificate until the unit is changed.
Safety glazing missing in a critical location
Side panels next to doors and low-level glazing supplied as standard units with no permanent mark.
Requires replacement glazing, not a repair.
ProPlot's audit tool includes these windows and external doors checks as interactive helpers — tick items off, attach GPS-stamped photos, and share the report with your inspector in one tap. Free while in beta.
The key stage inspection sequence on a typical UK housing plot.
Excavation & foundations
21 checks →
Drainage
20 checks →
Basement / lower ground
28 checks →
Ground Floor & Slab
19 checks →
Superstructure
22 checks →
Timber Frame Erection
21 checks →
Roof
22 checks →
Insulation & Airtightness
23 checks →
First Fix
28 checks →
Fire Stopping
22 checks →
Pre-Plaster
21 checks →
Second Fix
31 checks →
External Works & Pre-Handover
23 checks →
Pre-handover
22 checks →
General Quality
20 checks →
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