NHBC Chapter 5.1–5.2, Approved Documents C & L · 19 checks · proplot.co.uk
Pre-slab and beam-and-block checks — DPM and radon membrane, floor insulation, beam bearings, void ventilation, service penetrations and mesh position before the pour.
19 checks · Reviewed by the ProPlot site team · How we write these guides
In short
The ground floor inspection is carried out before the slab is poured or the screed laid. The inspector checks the sub-base and blinding, the damp-proof membrane and any radon barrier and their laps, floor insulation type and continuity, beam bearings on suspended floors, mesh position and every service penetration.
The ground floor gets less attention than the foundation below it and the walls above it, which is exactly why it produces so many late problems. A punctured damp-proof membrane, a strip of missing perimeter insulation or a radon barrier that was never sealed to the wall DPC will not be visible again after the screed goes down, and the symptoms — cold floors, damp patches, a failed radon test — arrive after the buyer has moved in.
On a beam-and-block floor the inspector checks bearings, grouting, the void depth and the through-wall ventilation before the insulation goes on. On a ground-bearing slab they check the sub-base and blinding, the membrane and its laps and upstands, insulation type and thickness, mesh position within the depth of the concrete, and every service penetration and sleeve. Either way this is a pre-pour visit: it has to happen while the build-up is still open.
Work the list below in the order the floor goes together, and photograph the membrane and the insulation layer before anything covers them. Those two images resolve most future arguments about the ground floor.
When it happens
After the floor build-up is complete — membrane, insulation, mesh and service penetrations — and immediately before screeding or pouring the slab.
Who inspects
Warranty provider inspector and building control. In radon-affected areas building control will look specifically at the barrier and its sealing.
If it fails
Correcting a floor after the pour means breaking out and re-laying, usually with the shell already up. Radon barrier failures can require a retro-fitted sump and fan system plus post-completion testing.
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 ground floor & slab visit, each traced to an openly published source. Take a tape to the ones that apply before you book.
| What | Value | Type |
|---|---|---|
| Ground-bearing floors over deep fill or sulphate-rich groundWhen a concrete floor is constructed over deep fill exceeding 600mm, it should be designed as a suspended floor rather than ground-bearingVerify the depth of fill beneath floor slabs and ensure appropriate sulphate-resistant measures or suspended floor construction are in place. | 600 mm | Exact |
| Ground-supported floor slab thicknessGround-supported concrete floor slab on hardcoreCheck depth pegs or level markers across the pour, not just at the perimeter, and confirm the membrane has not been pushed up into the slab depth. | 100 mm | Minimum |
| Maximum depth of a hardcore bed under a slabGround-supported concrete floor slab on a hardcore bedCheck the reduced-level dig against the finished slab level before hardcore is placed. Deeper fill needs a designed solution rather than more hardcore. | 600 mm | Maximum |
| Minimum thickness for ground-bearing floorsGround-bearing floor slabs, where applicable, must have a minimum thickness of 100mmCheck the thickness of ground-bearing slabs during construction to ensure it meets the 100mm minimum requirement. | 100 mm | Minimum |
| Minimum thickness of a polythene damp-proof membranePolythene sheet used as a damp-proof membrane below or above a ground-bearing slab.Walk the membrane before the pour and look for punctures over hardcore high spots; patch with the same material and tape. | 0.3 mm (1200 gauge) | Minimum |
| Sub-floor ventilation opening areaSuspended ground floor with a ventilated voidCount and measure airbricks on every elevation, then check the ducts through the cavity are clear and not blocked by mortar droppings or insulation. | 1500 mm² per metre run of external wall, or 500 mm² per square metre of floor area — whichever is the greater | Minimum |
| Ventilated void depth under a suspended timber floorSuspended timber ground floor over a ventilated voidMeasure at the deepest point of the oversite before the floor is decked. Check the void is clear of spoil, offcuts and mortar droppings that reduce the depth or block cross-flow. | 75 mm to the underside of the wall plate and 150 mm to the underside of the joists | Minimum |
0 of 19 ticked — saved on this device
Confirm hardcore is a suitable inert material, laid in layers and properly compacted (AD C)
AD CCheck sand blinding is complete and free of sharp material that could puncture the membrane (NHBC 5.2)
NHBC 5.2Verify finished floor levels and thicknesses match the drawings before laying the build-up (NHBC 5.2)
NHBC 5.2Confirm DPM is the specified gauge, continuous and free from punctures or tears (AD C)
AD CCheck DPM laps are at least 150mm, taped, and dressed up to lap the wall DPC (AD C)
AD CVerify radon or gas membrane, where required, is sealed at all penetrations and perimeters (BR 211)
BR 211Confirm any radon sump and pipework is installed and capped ready for future connection (BR 211)
BR 211Check beam type, spacing and bearing length match the manufacturer's layout drawing (NHBC 5.2)
NHBC 5.2Confirm infill blocks are the correct strength and the floor is grouted and brushed in (NHBC 5.2)
NHBC 5.2Verify the ventilated void depth beneath the floor meets the minimum required (AD C)
AD CCheck through-wall ventilators or periscopes are installed and clear of debris (AD C)
AD CConfirm floor insulation type and thickness match the SAP specification (AD L1A)
AD L1ACheck insulation is tight-butted with no gaps and perimeter upstand insulation fitted (AD L1A)
AD L1AVerify mesh or fibre reinforcement is correctly positioned within the slab depth (NHBC 5.2)
NHBC 5.2Check underfloor heating pipework is clipped, pressure tested and pressurised during the pour (BS EN 1264)
BS EN 1264Confirm movement or edge strips are fitted where the screed abuts walls and columns (NHBC 5.2)
NHBC 5.2Verify all service entries through the floor are sleeved, positioned per drawing and sealed (AD C)
AD CCheck drainage stubs and SVP positions match the layout before the floor is closed (AD H)
AD HConfirm photographs of the membrane and insulation are on the plot file before the pour (BREL)
BRELThe floor is the datum for the whole plot. Damage and contamination here shows up in the tiling, the kitchen and the door thresholds. No inspector will fail the visit on these — they are the difference between a plot that passes and a plot that hands over clean.
Sweep, cap and protect service penetrations through the slab
Concrete or screed in a soil branch or a conduit is a chase-and-break job later.
When: Before and immediately after the pour.
Protect insulation and the DPM from foot and barrow traffic
A punctured DPM is a damp snag that surfaces months after handover, and torn insulation is a cold spot on the thermal image.
When: Board out walkways before the pour.
Record the finished floor level against the datum in each room
Level errors here become out-of-tolerance stairs, doors and worktops, and they are cheap to catch now.
When: Within 24 hours of the pour.
Cure the slab properly and keep it off the drying critical path
Rushed drying means curled edges, cracked screed and a floor covering claim.
When: From pour until the moisture reading passes.
Cover the finished slab before following trades start
Mortar droppings, paint and plaster splashes on a power float finish all have to be ground off before floor coverings.
When: As soon as it has cured.
Photograph insulation, DPM laps and perimeter upstands
This is Part L evidence and it is also your defence if a damp query comes back.
When: Before the pour, room by room.
1 inspection points that apply at the ground floor & slab 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.
Membrane punctured by traffic
Barrows and boots after the membrane is laid do the damage. Lay it as late as possible, board any route across it, and inspect immediately before the pour.
High — a breached membrane is a health and safety issue, not a snag.
Radon barrier not sealed to the DPC
The floor membrane and the wall DPC have to form one continuous barrier. The junction at the perimeter is where it is nearly always missed.
Medium — but it shows on the SAP as-built and in thermal imaging.
Missing perimeter upstand insulation
A cold bridge right around the room, and a SAP input that no longer matches the build. Fit the upstand before the insulation field is laid.
High — leads to damp claims and is buried under the slab.
Void ventilators blocked
Periscopes filled with mortar or buried by landscaping stop a suspended floor breathing. Check them again once external levels are finished.
Medium — recoverable if caught before the screed.
ProPlot's audit tool includes these ground floor & slab 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 →
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 →
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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