NHBC Chapter 8.1, Approved Documents L & P · 21 checks · proplot.co.uk
The last look inside the walls — first fix M&E, fire stopping and cavity barriers, insulation continuity, noggins, ventilation ductwork and Part L evidence.
21 checks · Reviewed by the ProPlot site team · How we write these guides
In short
The pre-plaster inspection happens once first fix is complete and before boarding. The inspector checks electrical and plumbing first fix, notching and drilling of joists and studs, insulation and vapour control layers, cavity barriers and fire stopping, noggins and fixings for future fittings, and ventilation ductwork routes.
Pre-plaster is the last honest look at a house. After boarding, everything on this list is behind a skim coat, and the cost of correcting it multiplies. It is also the visit with the widest scope: structure, electrics, plumbing, ventilation, fire, thermal performance and the practical stuff like noggins for a wall-hung boiler all get checked in the same walk.
The inspector works room by room. First fix cables clipped and protected, backboxes at the right heights, pipework pressure tested with joints accessible, door linings plumb, window boards supported, stud partitions with the noggins actually in place. Then the compliance items: cavity barriers and fire stopping at every compartment penetration, insulation tight and continuous with no daylight at junctions, MVHR or extract ducting insulated and routed as designed.
This is also the visit that generates most of your Part L photographic evidence. The BREL report needs images of insulation, thermal bridging details and airtightness measures at the point they can still be seen. Take them as you complete each room rather than in a panic the night before the assessor asks.
When it happens
When first fix is complete and insulation is installed, immediately before plasterboard goes on. Book it so boarding can start the same week — this is the visit that most often holds up a programme.
Who inspects
Warranty provider inspector, building control, and often the SAP assessor or their appointed photographer for BREL evidence.
If it fails
Anything missed here comes out of a finished wall. A single omitted cavity barrier or an untested pipe run can mean opening a boarded and skimmed room, re-decorating, and re-inspecting.
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:
The measurable values most often argued about at the pre-plaster visit, each traced to an openly published source. Take a tape to the ones that apply before you book.
| What | Value | Type |
|---|---|---|
| 32mm waste pipe supportIndependent 32mm polypropylene discharge or waste pipes, conforming to relevant British Standards such as BS 5254 or BS EN 1451, must be adequately supported with pipe clips throughout their run to the soil stack or manifoldEnsure that 32mm waste pipes are supported correctly with pipe clips at appropriate intervals to prevent sagging or undue stress on connections. | 32 mm | Exact |
| Cable depth in a wall before extra protection is neededMeasured from the finished surface on either side of the wallWhere the depth cannot be achieved, confirm earthed capping or 30 mA RCD protection is in place and recorded. | 50 mm | Minimum |
| Cavity barriers around openings in a cavity wallIncludes windows, doors, and any duct or vent passing through the cavityPhotograph reveals and heads before the frame goes in; afterwards the barrier cannot be seen. | Continuous around the full perimeter of the opening | Minimum |
| Cavity subdivision in a roof voidMaximum distance between cavity barriers in a concealed roof spaceCheck the barrier line matches the drawing before the loft is closed up. | 20 m | Maximum |
| Clearance from combustible materialsAppliances that generate heat, such as solid fuel burners, must maintain a minimum separation distance of 75mm from any combustible materials to prevent fireMeasure the clearance between heat-producing appliances, flues, and all surrounding combustible materials to ensure it meets the minimum 75mm requirement. | 75 mm | Minimum |
| Constructional hearth thickness and extentSolid fuel appliance on a constructional hearthCheck the hearth dimensions against the appliance footprint before the fireplace is finished, and record the depth of concrete before it is screeded over. | 125 mm thick, at least 840 × 840 mm on plan, extending 500 mm in front of the appliance and 150 mm each side | Minimum |
| Doors onto a protected stairway in a three-storey houseApplies where any floor level is more than 4.5 m above ground levelCheck the ironmongery and intumescent seals at handover, and that no room door has been swapped for a standard blank. | Fire doors to every habitable room opening onto the stair | Minimum |
| Ductwork Fixing and Condensate ManagementMVHR ductwork must be securely fixed at appropriate intervals to prevent sagging, typically no more than 750mm apart, or as per manufacturer guidelinesVerify ductwork is securely fixed at correct intervals, horizontal runs fall correctly for drainage, and penetrations slope outwards; check for isolation from surfaces. | 750 mm | Maximum |
| Electrical accessory spacingElectrical socket outlets and switches should be installed with a minimum spacing of 100mm between their centres to ensure adequate access and compliance with wiring regulationsVisually inspect spacing between adjacent electrical accessories to ensure it meets the 100mm minimum centre-to-centre distance. | 100 mm | Minimum |
| Exceptions to combustible material clearanceExceptions to the standard 75mm clearance from combustible materials are permitted under specific conditionsFor any reduced clearance, verify the presence of a 25mm non-combustible shield or check manufacturer's instructions for gas appliances specifically permitting it. | 75 mm | Exact |
| Fire barrier at the party wall and roof junctionGaps at truss positions and at the tile battens closed with a fire-resisting materialInspect from inside the loft with a light before the plot is handed over. | Continuous to the underside of the roof covering | Minimum |
| Fire resistance of a compartment floor between flatsIncludes the ceiling line, service voids and any penetrations through itInspect above the ceiling before it is boarded — downlights, ducts and pipework all break the line. | 60 minutes | Minimum |
| Fire resistance of a wall separating dwellingsApplies to the wall and to its junctions with floors, roofs and external wallsCheck the junction detail at the roof line and at every floor, not just the wall build-up. | 60 minutes | Minimum |
| Fire stopping at a service penetrationSealed with a system tested for that substrate, aperture size and service typeRecord the product, batch and installer for each penetration; unlabelled foam is not evidence. | Reinstate the full fire resistance of the element | Minimum |
| Fixing Plywood to BattensPlywood sheets fixed to battens require 10-gauge nails or screws, placed at 150mm centres along perimeters and 300mm centres on intermediate supportsConfirm that plywood is fixed with the specified gauge nails/screws, at correct centres and edge distances, to battens. | 150 mm | Minimum |
| Flue liner requirements for new buildsFlue liners must be selected based on the expected flue gas temperature; for temperatures unlikely to exceed 250°C, requirements for gas flues apply, while for higher or unknown temperatures, solid fuel flue pipe standards are usedVerify that rigid flue liners are installed and are appropriate for the anticipated flue gas temperatures of the connected appliance. | 250 °C | Exact |
| Gradient of a branch discharge pipeMeasured along the branch from the appliance to the stackCheck with a level at first fix — a flat branch will not be found again until the occupier reports it. | 18 to 90 mm per metre for a 75 mm or 100 mm branch | Range |
| Headroom over a stairStairs and landings within a dwellingCheck headroom under the trimmer and under any bulkhead before plasterboard, when a correction is still cheap. | 2000 mm, reducing to 1900 mm at the centre of a loft conversion stair and 1800 mm at the side | Minimum |
| Maximum length of a WC branch to the stackFor a 100 mm branch serving a single WC, with no more than 90 mm/m fallMeasure the actual routed length, not the straight line on the drawing. | 6 m | Maximum |
| Maximum pipe diameter through a compartment wall or floorNominal internal diameter for a non-fire-rated pipe; larger pipes need a tested sleeve, collar or non-combustible materialCheck the pipe type and diameter at every separating wall or floor crossing, and log the collar product where one is used. | 40 mm | Maximum |
| Maximum unbroken cavity before a barrier is neededConcealed cavity in a wall or roof of a dwellingCheck barriers at the head and edges of cavities, around openings, and where a cavity crosses a compartment line — the run length is the easiest one to miss. | 20 m | Maximum |
| Minimum Hot Water Storage CapacityDomestic hot water storage systems must provide a minimum capacity to adequately supply hot water at a target temperature, typically 60°CVerify the installed hot water cylinder volume matches the design requirements for the dwelling size and anticipated usage. | 60 °C | Minimum |
| Permitted notches and holes in floor joistsSolid timber floor joists — engineered joists follow the manufacturer's zones insteadWalk the floor with first fix before decking. Any notch mid-span, any hole off the centreline, and any cut in an engineered joist web outside the printed zones is a fail. | Notches: max depth 0.125 x joist depth, only between 0.07 and 0.25 of the span. Holes: max diameter 0.25 x joist depth, on the centreline, between 0.25 and 0.4 of the span, at least 3 diameters apart | Range |
| Plasterboard fixing and fire resistancePlasterboard must be fixed to light steel studs with self-drilling, self-tapping screws at specified centres, typically a maximum of 300mmCheck plasterboard thickness, screw spacing, and ensure fire-rated boards are used in required locations as per design specifications. | 300 mm | Maximum |
| Protection of gas pipework through timberGas pipework that penetrates timber studs or joists must be protected by steel plates, typically 1mm thick, to prevent accidental damage from screws or nails during subsequent fit-outEnsure gas pipes passing through timber members are protected by steel plates and sealed. | 1 mm | Exact |
| Safe zones for cables buried in a wallApplies to cables concealed in a wall or partition at a depth of less than 50 mmPhotograph every wall at first fix before boarding — this is the only record the occupier will ever have. | Within 150 mm of the top of the wall or of a corner, or vertically/horizontally in line with the accessory | Maximum |
| Screed Cover for Buried PipesA minimum of 25mm screed cover is required over pipes and insulating material, and where pipes cross, a duct may be necessary to maintain this coverConfirm that the minimum 25mm screed cover is maintained over all pipes, and verify that the design team has approved pipe locations in suspended concrete floors. | 25 mm | Minimum |
| Separation of flues from combustible materialsFlues must be adequately separated from combustible materials to prevent fire hazards, typically requiring a minimum clearance of 25mmConfirm minimum 25mm clearance between flue pipe outer surface and any combustible materials, and verify non-combustible sleeving where flues pass through combustible structures. | 25 mm | Minimum |
| Smoke alarm siting distancesCeiling-mounted mains-powered alarms on circulation routesMark alarm positions against the room layout at first fix, before ceilings are boarded and the cable drops are fixed. | Within 7.5 m of the door to every habitable room, within 3 m of every bedroom door, and at least 300 mm from any wall or light fitting | Maximum |
| Stair rise and going relationshipStaircases must be dimensioned such that the relationship between the rise (R) and the going (G) falls within prescribed limits for safety and comfort, typically expressed as 2R + G between 550mm and 700mmVerify the 2R + G calculation for the staircase and check that treads are level and the strings are securely fixed at the correct angle. | 550 mm | Exact |
0 of 21 ticked — saved on this device
Check all first fix electrical is complete, clipped and tested (insulation resistance test available) (BS 7671)
BS 7671Confirm electrical consumer unit position and height meets BS 7671 requirements (BS 7671)
BS 7671Verify cables in walls are within safe zones or mechanically protected (BS 7671)
BS 7671Verify first fix plumbing is complete, pressure tested and all joints accessible for inspection (NHBC 8.1)
NHBC 8.1Verify heating system pipework is correctly clipped, insulated and tested (AD L1A)
AD L1AConfirm MVHR / extract ventilation ductwork is correctly routed, supported and insulated (AD F)
AD FConfirm soil and vent pipe connections are secure with fire collars at floor penetrations (AD B)
AD BCheck bath / shower framing is level, secure and has adequate waterproofing detail (NHBC 8.3)
NHBC 8.3Check cavity barriers and fire stopping are complete at all required locations (AD B)
AD BConfirm all penetrations through party walls and fire compartment walls are correctly sealed (AD B)
AD BVerify all pipe boxing is adequately fire-stopped where passing through fire compartments (AD B)
AD BCheck thermal board / PIR insulation is secure with no gaps, staggered joints and taped (AD L1A)
AD L1ACheck window reveals have insulation returns and cavity closers correctly positioned (AD L1A)
AD L1ACheck all loft insulation is correctly laid to specified depth with no gaps at eaves (AD L1A)
AD L1AConfirm Part L evidence photos taken — insulation, thermal bridging details, airtightness (BREL)
BRELConfirm all door linings are plumb, level and at correct height with adequate fixings (NHBC 8.2)
NHBC 8.2Inspect window boards — fitted level, supported and with drip groove to external face (NHBC 6.7)
NHBC 6.7Verify noggins fitted for all wall-mounted items (radiators, boiler, vanity units, TV brackets) (NHBC 8.2)
NHBC 8.2Verify mesh tape applied on all dissimilar material junctions to prevent plaster cracking (NHBC 8.2)
NHBC 8.2Verify staircase is securely fixed with correct balustrade height (900mm stairs, 1100mm landing) (AD K)
AD KCheck all structural openings have correct padstones, lintels and adequate bearing (AD A)
AD APre-plaster is the point where care pays back most. These are the items that decide whether second fix runs smoothly or turns into a search party. No inspector will fail the visit on these — they are the difference between a plot that passes and a plot that hands over clean.
Label every water feed at both ends
Hot, cold, heating flow and return, and each appliance point marked with durable labels. Unlabelled pipes at second fix means guessing, draining down, and a plumber charging for the time.
When: Before any board goes on.
Cap and seal all pipe ends before boarding
Open ends fill with plaster and dust, and a capped system is a system you can leave under pressure to prove it.
When: As first fix is completed, room by room.
Fit backbox protectors to every switch and socket before drylining
Plaster and screw fixings in a backbox is a rewire-the-point job, and it is entirely avoidable with a few pence of protection.
When: Before the boarders arrive.
Photograph every wall and ceiling before boarding
A photo of each elevation with the services visible tells second fix where to drill and proves the notching zones were respected.
When: The day boarding starts.
Mark noggin positions on the floor or the board face
Boiler, TV bracket, grab rails, wall-hung sanitaryware, radiator brackets. If it is not marked, the fitter will find out the hard way.
When: As the noggins go in.
Put floor protection down before boarding starts
Board offcuts, screws and plaster on a new floor deck is a grinding and cleaning job before floor coverings can go down.
When: Before the first board is lifted in.
Leave the plot under pressure test with the reading recorded
A recorded pressure test with a date is the fastest way to prove the leak was caused by someone else's screw.
When: On completion of first fix.
Protect door linings, stairs and window boards
These are already installed at this point and they take the most damage during boarding and plastering.
When: Before boarding, and keep it on.
3 inspection points that apply at the pre-plaster 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.
Insulation cut short at the ceiling perimeter
Boards stop at the joist line and leave a cold strip that shows up on a thermal image at handover. Continuity at junctions is the point of the check.
High — structural, and buried behind plasterboard.
Noggins missing behind heavy fixings
Boiler, TV bracket, grab rails and wall-hung sanitaryware all need timber. Mark them on the layout at first fix, not when the fitter arrives.
High Damage Potential — life safety and a certain RI.
No BREL photographs taken
Once the room is boarded, the evidence cannot be recreated. Photograph each element with the plot and location identifiable in the frame.
High — invalidates the Robust Details registration and forces a sound test.
Pressure test not recorded
The pipework was tested but nobody wrote it down. Log the pressure, duration and date against the plot.
Medium — but re-opening finished walls is the expensive part.
ProPlot's audit tool includes these pre-plaster 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 →
Windows and External Doors
24 checks →
Insulation & Airtightness
23 checks →
First Fix
28 checks →
Fire Stopping
22 checks →
Second Fix
31 checks →
External Works & Pre-Handover
23 checks →
Pre-handover
22 checks →
General Quality
20 checks →
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