NHBC Chapter 7.2, Approved Document B · 22 checks · proplot.co.uk
Roof structure and covering checks — truss layout and bracing, membrane laps, batten gauge and fixings, flashings, ventilation and party wall fire stopping.
22 checks · Reviewed by the ProPlot site team · How we write these guides
In short
The roof inspection covers structure and covering. The inspector checks truss type, spacing, bracing and strapping, wall plate fixing, the underlay type and laps, batten size and gauge against the tile specification, fixing and clipping patterns, verge, ridge and abutment flashings, and eaves and ridge ventilation.
The roof is inspected twice in effect — once as a structure, when the trusses are up and braced but not yet felted, and once as a weathering system, when the covering is on. The structural visit is the important one, because bracing and restraint disappear behind insulation and plasterboard, and a truss roof without its full diagonal and longitudinal bracing is a genuine safety issue rather than a paperwork one.
On the covering side the inspector is looking for the details that leak: membrane laps and the eaves support tray, batten gauge and fixing, mechanical fixing of perimeter and ridge tiles under the current wind guidance, lead codes and dressing at abutments, and valley construction. Ventilation gets checked at eaves and ridge — a cold roof that cannot breathe produces condensation complaints in the first winter.
Fire stopping at the party wall head is checked here as well as at superstructure, because that is the last chance to see it. Run the list below before the scaffold comes down; almost everything on it is far cheaper to fix from a working platform.
When it happens
First when trusses are erected, braced and strapped and before felt and battens go on. Second when the covering is complete but the scaffold is still standing.
Who inspects
Warranty provider inspector and building control. The truss manufacturer's bracing drawing is the reference document for the structural check.
If it fails
Missing bracing found later means working in a filled, insulated loft. Covering defects found after the scaffold is struck mean bringing access back to the plot — often the single most expensive re-visit on a housing site.
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 roof visit, each traced to an openly published source. Take a tape to the ones that apply before you book.
| What | Value | Type |
|---|---|---|
| Clear air gap above insulation in a cold flat or shallow roofCold flat roofs and cold roofs pitched below 15 degrees.Measure the gap at mid-span where the insulation is most likely to have bowed upward, not just at the edges. | 50 mm | Minimum |
| Equivalent ventilation areas for a cold roofCold roof construction with the insulation at ceiling or deck level.Compare the ventilator product data sheet against the required equivalent area — a vent's physical size is not its equivalent area. | 10,000 mm²/m at pitched eaves; 25,000 mm²/m at low level on flat or shallow roofs; 5,000 mm²/m at the ridge | Minimum |
| Flue outlet height at a ridgeSolid fuel flue passing through a pitched roof at the ridgeCheck the terminal height from the ridge line once scaffold is still standing — it is expensive to correct afterwards. | 600 mm | Minimum |
| High-level and ceiling air permeability for ventilationHigh-level ventilation, typically 5000mm²/m, is necessary for pitched roofs where the pitch exceeds 35°Check for high-level ventilation where applicable and confirm the ceiling construction aligns with the assumed air permeability for the chosen ventilation strategy. | 5000 mm | Exact |
| Holding-down strap fixing and dimensionsRoof holding-down straps should be 30mm x 2.5mm in cross-section and at least 1 metre longVerify strap dimensions and length, confirm pre-drilling, count fixings, and check the position of the lowest fixing. | 30 mm | Minimum |
| Hybrid roof ventilation and AVCLHybrid roof constructions require a minimum 25mm clear airflow path below the underlay to ensure effective ventilationEnsure a consistent 25mm clear air gap is maintained below the underlay and confirm the presence and integrity of the AVCL or compensatory increased ventilation. | 25 mm | Minimum |
| Minimum eaves ventilation for pitched roofsFor specific pitched roof constructions, a minimum continuous eaves ventilation opening of 7000 mm²/m is a common requirementMeasure the effective clear opening area of eaves ventilation to confirm it meets the minimum specification. | 7000 mm | Minimum |
| Minimum masonry thickness around a flueMasonry chimneys built with a suitable liner.Measure the wall thickness at the chimney breast before plastering, and check the liners are the right way up with sockets uppermost. | 100 mm around the flue; 200 mm between flues serving different dwellings | Minimum |
| Minimum strap dimensions for holding downA strap used for holding down purposes, particularly in roof structures, must have a minimum cross-section of 30mm x 2.5mm and be at least 1000mm long to effectively resist uplift forces.Visually confirm the dimensions and length of any installed holding-down straps. | 30 mm | Minimum |
| Minimum tile or slate width at a vergeAll verges on plain tiled, interlocking tiled and slated roofs.Sight along the verge from the ground: narrow cut pieces are obvious against the course above. | 150 mm | Minimum |
| Roof underlay horizontal lapPitched roof underlay beneath tiles or slatesCheck laps before battening starts. Confirm the underlay drapes into the gutter over an eaves carrier rather than stopping short behind the fascia. | 100 mm at pitches of 15° and above; 150 mm below 15°, and at least 150 mm where the underlay is unsupported | Minimum |
| Separation between a flue and combustible materialMasonry chimneys and fireplace recesses serving combustion appliances.Check joist trimming, floor decking and skirting runs around the chimney breast before the trimming is boarded over. | 200 mm from the flue face, or 40 mm from the outer face of the masonry | Minimum |
| Specification and fixing of lateral restraint strapsLateral restraint straps must conform to design specifications for length, size (typically 30mm x 5mm), and corrosion protectionCheck strap dimensions, corrosion protection, the correct length spanning at least three trusses, and proper downturn anchorage into the masonry. | 30 mm | Minimum |
| Thickness of Code 4 milled lead sheetMilled lead sheet to BS EN 12588 used for flashings and weatherings.Check the code marking on the roll or the delivery note before the lead is dressed in; once fitted the code cannot be read. | Code 4 = 1.80 mm | Exact |
| Typical timber roof member dimensionsStandard timber roof member sizes, such as minimum 100x50mm for struts, 32mm thick for valley rafters, and rafter cut plus 25mm for ridges and hips, are generally acceptableMeasure key timber members to confirm they meet minimum specified sizes, or match engineer's drawings. | 50 mm | Minimum |
| Upstand, Warm Roof, WFRL, Waterproofing, Zero Fall Roof definitionsAn upstand provides perimeter waterproofing continuity between a flat roof or deck and vertical wall construction, typically 150mm high, but potentially 75mm at accessible thresholds | 150 mm | Exact |
| Ventilated batten space beneath a roof underlayRoofs designed with a ventilated space beneath the underlay, including counter-battened details.Check counter-battens are continuous and that the underlay drape between rafters has not closed the gap. | 25 mm | Minimum |
| Ventilated Cold Roof Construction DiagramIn a ventilated cold roof construction, a clear void, typically a minimum of 50mm, is maintained between the insulation layer and the roof deckDuring construction, check that the specified ventilation void depth is maintained consistently across the roof area. | 50 mm | Minimum |
| Ventilation openings to a cold pitched or flat roof voidCold roof construction with insulation at ceiling levelLook into the void from the loft hatch before insulation is topped up. Check eaves trays are in place and insulation has not been pushed tight into the eaves. | 25 mm continuous at the eaves; 5 mm at the ridge for pitches of 15° or more; 25 mm at both sides of a cold flat roof | Minimum |
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Verify truss positions match layout drawing with correct centres and bracing specification (BS 5268-3)
BS 5268-3Check bracing (longitudinal, diagonal, chevron) installed per truss engineer design (BS 5268-3)
BS 5268-3Verify truss clips or framing anchors installed at correct positions to all bearing points (AD A)
AD AVerify water tank platform is correctly positioned and supported per truss engineer detail (NHBC 7.2)
NHBC 7.2Confirm any dormer / roof window construction matches structural design and is weathertight (NHBC 7.2)
NHBC 7.2Check no truss members have been cut, notched or drilled without engineer approval (BS 5268-3)
BS 5268-3Confirm felt / breathable membrane is correctly lapped (150mm horizontal, 100mm vertical) (BS 5534)
BS 5534Inspect tile batten gauge matches tile manufacturer specification with correct fixings (BS 5534)
BS 5534Check ridge, hip and valley details per manufacturer guidance with correct mortar / dry-fix system (BS 5534)
BS 5534Check verge detail is correct — mortar bedded or dry verge system properly clipped (BS 5534)
BS 5534Check valley tiles / trays are correctly cut and dressed with adequate side lap (BS 5534)
BS 5534Confirm perimeter and every course of tiles are mechanically fixed as required by the wind uplift calculation (BS 5534)
BS 5534Verify lead flashings and soakers are correctly dressed, code weight matches specification (LSA)
LSAConfirm eaves detail — tilting fillet, felt support tray and bird comb correctly installed (NHBC 7.2)
NHBC 7.2Check all penetrations through roof covering (flues, vents, aerials) are correctly sealed (NHBC 7.2)
NHBC 7.2Check any solar panel / renewable energy fixings have adequate weatherproofing detail (MCS 012)
MCS 012Confirm vapour control layer (VCL) is installed where specified (warm roof construction) (BS 5250)
BS 5250Confirm soffit and fascia ventilation meets requirements (10mm continuous or equivalent area) (BS 5250)
BS 5250Check roof space insulation plan against design — no gaps, compression or missing quilt (AD L1A)
AD L1AConfirm party wall fire stopping extends to underside of roof covering with no gaps (AD B)
AD BVerify roof space pipework (SVPs, overflows) is correctly supported and insulated (NHBC 8.1)
NHBC 8.1Verify roof ladder / walkway boards are fitted where specified for maintenance access (NHBC 7.2)
NHBC 7.2The roof stops the water. Almost all early roof callbacks are damage and debris, not tiling workmanship. No inspector will fail the visit on these — they are the difference between a plot that passes and a plot that hands over clean.
Keep gutters and outlets clear of mortar, felt trimmings and nails
A blocked outlet overflows into the fascia and the customer sees a water stain on a brand new soffit.
When: At the end of tiling, and again before handover.
Protect the roof edge, verge and rainwater goods from scaffold strike
Scored uPVC and cracked verge tiles are the most common scaffold-down snag on the whole plot.
When: Before the scaffold is struck.
Sheet down and secure loose materials before every weather window
Loose tiles and battens in a gale are a safety event, not a snag.
When: Ahead of any forecast wind.
Keep loft insulation and services out of the way until the roof is sealed
Wet loft insulation has to be replaced and it is expensive to get back out through a finished hatch.
When: Delay insulating until weathertight.
Photograph the underlay laps, ventilation path and any valley or abutment detail
These are all concealed by the tiles and they are the details raised most often when a leak is investigated.
When: As each area is covered.
Check the loft is clean and complete before boarding the hatch
Every trip back into a finished loft risks ceiling damage.
When: Final walk before the hatch is fitted.
96 inspection points that apply at the roof 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.
Bracing "to be completed later"
Diagonal bracing is fitted, longitudinal is not, and the loft is boarded before anyone returns. Complete the full bracing pack before the felt goes on.
High — a bracing failure is a structural collapse risk.
Insufficient mechanical fixing
BS 5534 requires fixings based on a site-specific wind calculation. "Two nails a course as usual" is not evidence — keep the calculation with the plot file.
High — a cut truss needs an engineered repair detail.
Insulation stuffed into the eaves
Quilt pushed tight to the eaves blocks the ventilation path. Fit eaves ventilators or rafter trays and keep the airway clear.
Medium — but leads to condensation claims in the first winter.
Fire stopping stopped short at the ridge
The last 300mm at the party wall head is the bit that gets missed, and it is the bit the inspector photographs.
Medium — fixable but usually means lifting tiles.
ProPlot's audit tool includes these roof 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 →
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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