Modern methods of construction: guidance for building standards verification

1. Introduction

1.1. Purpose of the document

The purpose of this document is to provide guidance to Local Authority verifiers and Certifiers as defined under the Building (Scotland) Act 2003 when assessing building warrant applications and certifying works which use Modern Methods of Construction (MMC) and when assessing the compliance of MMC buildings during construction with the building warrant approved drawings and regulations.

The building standards system is pre-emptive, ensuring as far as possible that the proposed works will comply with the regulations prior to work commencing on-site. Verifiers check that building plans comply with building regulations when an application is made for a building warrant. The system also requires the verifiers to undertake reasonable inquiry to verify that the building work complies with the approved plans, details and with the building regulations. However, verifiers do not inspect all materials and work on every building site as the responsibility for compliance with the building regulations sits with the relevant person (usually the owner or developer). The reasonable inquiry verifier role is to make inspections or other checks during the construction phase on a risk-based basis, to take account of building type and complexity and will, generally have due regard to developer type as well.

Certification of Design or Construction is an option available to building warrant applicants where approved suitably qualified and experienced building professionals and tradesmen can certify certain specified areas of works forming part of a building warrant as complying with the building regulations. Verifiers do not verify such certified areas of design and construction.

Certifiers of construction, where used, will require to certify MMC construction works on site that have been completed in a factory and will need to satisfy themselves as to these factory based works also being compliant and the risks and assurances as highlighted in the guidance is applicable. The current certifier of design schemes under the Building (Scotland) Act 2003 cover structure and energy and the current certifier of construction schemes electrical work, plumbing/drainage and heating installations

It should be noted that for MMC projects - as with all building warrant projects - the proposed design can only be approved if the verifier is satisfied that compliance with the regulations is demonstrated - including Regulation 8 - Durability, workmanship and fitness of materials.

1.2. What is MMC?

Modern Methods of Construction (MMC) encompass a wide range of offsite manufacturing and onsite assembly techniques. Use of MMC can vary from a kitchen or bathroom pod assembled offsite in a factory and transported to site – through to fully factory-built volumetric modules.

Other terms as well as MMC are often used interchangeably – including Modular Construction, Offsite Construction and Offsite Manufacture (OSM).

MMC are typically divided into different categories, as fully defined in the MMC Definition Framework^[1], which relates to all types of pre-manufacturing^[2], site-based materials and process innovation. The most commonly used are:

• 3D structural systems (MMC Category 1 - volumetric), which typically take the form of volumetric modular "boxes" which fully manufactured away from site and assembled on site. This is often called 3D modular construction.
• 2D structural systems (MMC Category 2 – panellised), in which the two-dimensional frame of the building is manufactured away from site and assembled on site. This includes timber and steel frame solutions for instance. This is often called 2D frame construction.
• 3D sub-assemblies (MMC Category 5), in which sub-sections of the building (but not the whole building) are manufactured away from sites. These typically include bathroom and kitchen pods for example. These are often called 3D pods.
• Models that combine different categories of MMC, the most common of which is the combination of 2D frames and 3D pods. This is typically known as hybrid MMC.

This guidance relates to:

• Category 1: Volumetric
• Category 2: Panelised

1.2.1. Volumetric construction

Volumetric construction^[3] is:

'The production of three-dimensional units in controlled factory conditions prior to final installation. Volumetric units can be brought to final site in a variety of forms ranging from a basic structure only to one with all internal and external finishes and services installed, all ready for installation.' and provide the greatest challenge to verify against approved plans and details on site by the usual visual verification inspection methods.

1.2.2. Panelised construction

Panelised construction^[4] is:

'A systemised approach using flat panels used for basic floor, wall and roof structures of varying materials which are produced in a factory environment and assembled at the final workface to produce a final three-dimensional structure'.

Panelised construction describes a two-dimensional unit, typically manufactured offsite, which may or may not have a structural function. Panelised systems can have variouslevels of enhancement in the factory.

Open panels are most commonly used in Scotland – these are a skeletal structure, typically non-insulated with internal finishing and external cladding installed on site. Timber frame open panel kit construction is the most common form of construction for housing in Scotland and the open panel nature readily allows for verification inspections during the construction/assembly phase on site.

Closed panels are more complex in that there is a greater degree of factory-based fabrication, which may include lining materials, insulation, and potentially also services, windows, doors, internal wall finishes and external cladding.

Panels may be made using timber frame, StructuralInsulated Panels (SIPs) or mass timber systems such as Cross Laminated Timber (CLT). On building sites these are often referred to as 'timber kit' regardless of whether they are open or closed.

1.3. Structure of this guidance document

This document provides general points for consideration. It is then divided into a further 5 sections, aligning to the typical construction process and where the verifier role fits in, i.e.

• Building warrant assessment
• Factory assembly
• Transportation to site^[5]
• On-site assembly
• Completion

For each of these stages, the guidance sets out potential risks pertinent to MMC for context, what should be considered as a consequence, and types of evidence or information a verifier could expect to see, what reasonable inquiry should include specific to MMC. References made to standards, accreditation and other types of evidence should be carefully considered alongside guidance in the Technical Handbooks.