Summary

Approved Document C — Site preparation and resistance to contaminants and moisture — is the Building Regulations document that governs weather resistance of the building envelope, including flat roofs. The primary requirement for flat roofs is straightforward: the roof must prevent the penetration of precipitation (rain, snow, ice-melt) into the building structure and interior.

Part C is one of several Parts that apply to flat roof work simultaneously. It is closely associated with Part L (energy efficiency/insulation) and Part A (structural adequacy). When flat roof work is planned, compliance with all three Parts must be confirmed. Part C is typically satisfied by the waterproofing system; Part L by the insulation thickness; Part A by the structural deck design.

The practical details that most often cause Part C failures in inspection — and water ingress in service — are: insufficient upstand height (less than 150mm), inadequately detailed flashings at abutments, poorly sealed penetrations (pipes, rooflights), and inadequate falls leading to chronic ponding. These are the details that both Building Control and the NFRC inspector will focus on.

Key Facts

  • Approved Document C, Section 5 — roof construction; requires roofs to "resist the penetration of precipitation to the structure of the building" and to prevent precipitation reaching the inside of the building
  • Waterproofing membrane — must provide continuous weather resistance; any membrane system complying with NFRC CoP 1, 2, 3, or 4 is accepted as satisfying Part C
  • Minimum upstand height — 150mm above the finished roof surface level (top of membrane) at all vertical abutments: parapet walls, adjacent walls, chimney stacks, roof lights
  • Flashings — required at all junctions between the roof membrane and adjacent masonry; metal flashings (lead, aluminium, or GRP) must overlap the upturned membrane by minimum 75mm and be fixed into the masonry
  • Chase depth — flashing turn-in to masonry should be minimum 25mm; secured with lead wedges or expansion fixings; pointed with non-hardening mastic or proprietary flashing sealant
  • Surface water run-off — Approved Document C Section 5.1.3 states that roofs should be designed to direct rainwater to an appropriate drainage system; this is the Part C basis for the falls requirement (falls create drainage; absence of falls = ponding = moisture risk)
  • Part C and flat roofs adjacent to walls — where a flat roof is at a lower level than adjacent external walls, the junction must be properly flashed to prevent water running down the wall face and into the gap between the roof membrane and the wall
  • Airtightness — Part C does not specifically address airtightness, but moisture-laden air infiltration at junctions is a related risk; flashing details that also provide air sealing are preferred
  • Verification — a Building Control inspector may inspect the upstand height, flashing installation, and drainage falls before the building is signed off; photographs should be taken before the coping stones or finishes cover the details

Quick Reference Table

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Detail Part C Requirement Good Practice
Upstand height 150mm minimum 200mm where ponding risk exists
Flashing overlap on upstand membrane 75mm minimum 100mm
Flashing chase depth in masonry 25mm minimum 30–35mm
Flashing seal (mastic) Non-hardening; weathertight Renew every 10–15 years
Minimum drainage fall 1:80 per AD H reference 1:40 (NFRC recommended)
Roof light upstand 150mm minimum 200mm minimum
Drainage outlet strainer Required Inspect and clean annually

Detailed Guidance

Weather Resistance of Flat Roof Waterproofing Systems

Approved Document C Section 5 permits a range of materials and systems for flat roof construction, provided they prevent moisture penetration. The document cross-references BS 8217 (reinforced bitumen membranes) and other British Standards for specific systems. In practice, the NFRC Codes of Practice are accepted as demonstrating compliance with Part C for:

  • Built-up bituminous felt (NFRC CoP 1)
  • Single-ply membranes including EPDM, TPO/PVC (NFRC CoP 2)
  • Mastic asphalt (NFRC CoP 3)
  • Cold-applied liquid systems including GRP (NFRC CoP 4)

A flat roof constructed and detailed in accordance with the relevant NFRC Code of Practice will satisfy Part C by implication, provided:

  • All laps and seams meet the specified minimum dimensions
  • Upstands meet the minimum 150mm height
  • Flashings are correctly installed
  • Drainage falls are provided

Part C failures in practice are almost always attributable to installation defects (insufficient lap, torn membrane, missing sealant at penetrations) or design defects (insufficient upstand height, no drainage fall), rather than to inherent deficiencies in the waterproofing system itself.

Upstand Height Requirements

The 150mm minimum upstand is the most commonly violated Part C requirement on domestic flat roofs. The consequences of insufficient upstand height are:

  • Water accumulating at the base of the wall during heavy rain can wick up behind the membrane
  • Snow accumulation at the upstand can remain above the membrane level and allow meltwater ingress as it thaws
  • Blowing rain can be driven horizontally under a short upstand

Measuring upstand height correctly: The 150mm must be measured from the finished roof surface level (the top of the installed membrane, including any ballast or walkway pads) to the top of the upstand (typically the underside of the coping stone, or the point where the upstand meets the main wall face). Where tapered insulation raises the finished level toward the outlet, the upstand height must be maintained at 150mm above the highest roof surface level — not the lowest.

Common scenarios where upstands are problematic:

Extension roof against main house: The flat roof abuts the main house wall. The top of the upstand (where the flashing terminates) should be at least 150mm above the finished roof level. If the existing window cill or lintel is too low, the upstand may be compressed below 150mm — this is a design failure that must be resolved at the extension design stage.

Parapets that are too low: If a parapet wall is less than 150mm above the finished roof level, it cannot contain an adequate upstand. Raising the parapet or using a raised coping with a weather-check overhang is the solution.

Roof light kerbs: Many proprietary roof light units are supplied with 100mm kerb height. This is less than the 150mm minimum upstand required by Part C. A purpose-built upstand of min 150mm should be constructed on the roof structural deck before the roof light is installed; the roof light then sits on top of the upstand. Structural manufacturers who supply roof lights with kerbs below 150mm are not providing a Part C-compliant solution — confirm the total upstand height including the kerb and the distance to the finished membrane level.

Flashing Installation

Flashings cover the junction between the upturned roof membrane and the adjacent masonry, providing a weather-tight seal and preventing water from penetrating behind the membrane.

Materials for flashings:

  • Lead (Code 4, 1.8–2.0mm): the traditional material; most durable; malleable; can be dressed to irregular brick surfaces; complies with BS EN 12588; requires adequate support
  • Aluminium: widely used proprietary self-adhesive flashings (e.g. Flashband, Roflex); easier to install but shorter lifespan than lead; suitable for domestic installations
  • GRP: used to match GRP roof systems; non-corrosive; typically moulded to a specific profile
  • Proprietary systems: various EPDM, rubber-aluminium, and butyl composite flashings available

Lead flashing installation (Code 4):

  1. Rake out the mortar joint to a depth of 25–35mm and a width sufficient to accept the lead
  2. Cut the lead to length; form the upstand portion (typically 75–100mm height) by bending against a straight edge
  3. Insert the lead turn-in into the chase; secure with lead wedges at maximum 600mm centres
  4. Point the chase with non-hardening mastic (Mastic A, or equivalent); do not use cement mortar (rigid pointing cracks with thermal movement and allows water behind)
  5. The downward face of the flashing should overlap the upturned membrane by minimum 75mm; press the flashing against the membrane and seal the lower edge with a lap sealant appropriate to the membrane type

Step flashings: At pitched roof abutments or where the wall is not flat, individual step flashings are used over soakers (lead trays over each course of brick). Soakers are cut 175mm × (gauge + lap)mm and turned 25mm over each brick; step flashings cover the soakers and lap minimum 65mm into the brick course. This is a heritage technique but is the correct detail at complex junctions.

Surface Water and Drainage — Part C Perspective

Approved Document C does not specify the minimum fall for a flat roof (that is in AD H and NFRC guidance), but it does require that the roof directs water away from the building. Roof drainage is therefore both a Part C concern (weather resistance) and an AD H concern (drainage design).

From a Part C perspective, the key risks are:

  • Ponding at upstands: if the roof surface falls toward a wall rather than away from it, water accumulates at the base of the upstand; this is the most likely point of water ingress and must be avoided by ensuring falls direct water toward outlets, not toward walls
  • Blocked outlets leading to overflow: overflow provision (secondary outlets or weir notches) prevents water accumulating to levels above the upstand height; see flat roof drainage design for calculation guidance

Gutter abutments: Where a flat roof discharges to a gutter, the gutter fascia board must be robust, the gutter must be secured to prevent movement, and the gap between the gutter back and the building wall must be sealed. A gap at this junction is a common source of water ingress to the soffit and rafter ends.

Part C and Penetrations

Every penetration through a flat roof membrane is a potential Part C failure point. Common penetrations include:

  • Drainage outlets (discussed in flat roof drainage design)
  • Pipe penetrations (gas flues, soil pipes, drainage pipes): must use purpose-made pipe boots or collars; sealant alone at pipe-to-membrane junctions fails within a few years
  • Electrical cables and conduit: should be avoided; where essential, use purpose-made waterproofed cable glands
  • Roof light openings: the roof light frame must be on an adequate kerb (150mm upstand) with the membrane sealed to the kerb face

At any penetration, the principle is: do not rely on sealant alone. Mechanical fixing (collar, boot, flange) plus sealant is the standard approach. Sealant degrades with UV and thermal cycling; a mechanical fixing provides the long-term seal even after sealant failure.

Frequently Asked Questions

My customer's flat roof drains toward the house wall — is this a Part C problem?

Yes. Falls toward walls rather than away from them concentrate water at the most vulnerable junction (the upstand/wall flashing). This is a design defect that should be rectified — tapered insulation to redirect falls, or repositioning the drainage outlet so the roof drains away from the wall. Inspect the upstand and flashing for signs of existing water ingress if this condition has been in place for some time.

Do I need to notify Building Control for every flat roof replacement?

Flat roof replacement that involves improving the thermal performance (adding or increasing insulation) is notifiable under Part L. A full strip and rebuild is notifiable under Part A and C as well as L. Like-for-like felt replacement without improving insulation may not be notifiable in all circumstances — but where doubt exists, notifying Building Control is always the safe approach. The fee is modest and the completion certificate provides legal protection.

What is the correct mastic for sealing lead flashings?

Use a non-hardening (flexible) mastic approved for exterior use and compatible with lead and masonry. Mastic A (BS 4254) or a proprietary equivalent is standard. Do not use silicone (poor adhesion to lead), polyurethane foam (not weather-resistant), or cement mortar (rigid, will crack). The mastic must remain flexible throughout its service life to accommodate thermal movement of the flashing.

Can a flat roof drain directly onto a lower pitched roof without a gutter?

This is acceptable in principle if the lower roof can handle the combined runoff from both roof areas. The discharge point onto the lower roof must be controlled — a lead or aluminium flashing directs water onto the lower roof surface cleanly, without undercutting the flat roof membrane. A gutter between the two is always preferable and avoids issues with the lower roof being directly under the splash and flow from the upper level.

Regulations & Standards