MMC System Review

Guidelines on the scope and procedures for the review of MMC systems

MMC System Review

The NHBC registered builder is responsible for ensuring that the homes they build meet the NHBC Standards regardless of the form of construction. Where MMC systems are proposed, NHBC carries out a review of the system to determine if it can be accepted for use in homes provided with Buildmark warranty.

NHBC undertakes a broad review, taking account of the following headings. Each system should also demonstrate how compliance with Building Regulations is achieved.

1. Scope and design responsibility

Describe the scope of the system along with its intended use and the limits of application, including:

  • Applicable geographic location
  • The building regulations that apply
  • Ground conditions, wind loading and storey height limitations

Confirm which parties have responsibility for design and coordination of the system.

2. Strength and stability

Demonstrate structural performance of the system against appropriate relevant and recognised standards. This will usually comprise a design philosophy statement that describes how loads are sustained and transmitted to the ground. The statement should include:

  • List of design standards and other sources of reference
  • Materials, design parameters, deflection limits, movement & tolerances
  • Loadings and description of load path and overall stability and the provision of any movement joints
  • Drawing and calculations of primary structural systems
  • Details of the design provisions made to prevent disproportionate collapse
  • Assessment of the fire resistance of the structural elements.
  • Dynamic performance criteria (where required)

Where the MMC system is manufactured off site, an assessment of the performance of the structure under the temporary loads during transportation and erection is expected.

3. Behaviour in relation to fire

Demonstrate the performance of the system in relation to fire. Particular consideration to be given to:

  • Internal fire spread (linings). Evidence of how the internal linings provide adequate resistance to the spread of flame over their surfaces and if ignited that they have a reasonable rate of heat release or rate of fire growth
  • Internal fire spread (structure). Evidence of how the stability of the building is maintained for a designated period in the event of fire and how the spread of fire is inhibited within a building and between adjacent buildings
  • Specification and design details of fire stops and cavity barriers that demonstrates they have adequate resistance to fire and smoke
  • Details of the materials used and their behaviour in fire together with the fire test data of those materials (tested in the configuration in which they are proposed to be used).

4. Resistance to moisture

Demonstrate how the system prevents moisture ingress to the home from the ground and from precipitation and how the risk of interstitial condensation is mitigated.

  • Demonstrate resistance to moisture from the ground including the use of suitable damp proof membranes, damp proof courses and other separation of the structure and internal environment from ground moisture.
  • Demonstrate how the building envelope resists the passage of water into the home. This should include management of moisture ingress and vapour movement including the use of flashings, vapour control layers and breather membranes. Drained and vented cavities are required in all external timber framed walls.

5. Resistance to the passage of sound

Demonstrate how the MMC system provides adequate sound resistance:

  • Between adjoining homes (through party walls and floors)
  • Within the home (internal walls and floors)
  • From outside of the home (through external walls)
  • Around soil pipes.

6. Energy efficiency

Demonstrate how the MMC systems reduce energy use in the building:

  • Through heat losses and gains through walls, floors, roofs, windows etc
  • From heating and ventilation services.

7. Durability of materials

Demonstrate that the materials used are suitable for their intended purpose and that the structure of the home has a design life of at least 60 years. The measures taken to protect the materials should be specified. Methods of demonstrating acceptable performance include:

  • Performance in accordance with the NHBC Standards
  • Compliance with relevant British Standards or equivalent European Technical Specification
  • Satisfactory assessment by an appropriate independent technical approvals authority
  • Use of materials and products in accordance with well-established satisfactory custom and practice accepted by NHBC.

8. Interfaces, compatibility of materials and interaction between components

All interfaces should be considered within the system and how the system fits with the rest of the building to form the home. Standard details should be provided by the system supplier to demonstrate how the system can be used in different configurations, including:

  • Details of how joints and junctions are formed, including consideration of vertical steps and horizontal staggers in the building plan or elevation
  • Interfaces between elements manufactured off-site and on-site construction, including services, foundations and methods of protection of the home from ground gasses
  • Type of cladding or façade systems, how they are supported and details of their assembly and/or finishing on site
  • Interfaces with window and door openings (head, cill, jamb details) and other openings in the building envelope
  • Detailing to account for thermal movement and the differential movement between different materials and structural elements.

9. Quality assurance and production controls

Demonstrate how the factory production control systems can ensure the prefabricated elements are manufactured to the design described in the system manual and that measures are in place to enable the system to be constructed to the design. This should include:

  • Audit of the factory production controls and quality management systems carried out by a third party, acceptable to NHBC
  • Method statement for packaging, transport and on-site assembly
  • List of on-site checks to manage QA at delivery and during erection of units.
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