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Modern Methods of Construction


Modern Methods of Construction (MMC) is the term commonly used to describe the increasingly encountered process of construction involving the use of prefabricated building components (predominantly produced off-site) using fast track assembly and erection procedures. Initially and now widely employed in residential buildings, including blocks of flats (in the case if light timber framed buildings, up to 8 storeys or more in height), MMC is becoming increasingly utilised in the commercial sector, the main perceived benefits of which include:

  • Speed of construction

  • Quality control of factory assembled components

  • Reduction in on-site labour costs

  • Less wastage of materials

The subject of MMC is fully covered in the InFiRes (RISCAuthority) document entitled Modern Methods of Construction and Fire Protection Considerations, published October 2005, posted in ATLAS.

These guidance notes provide a summary of some of the key considerations.


MMC techniques and components can be broadly broken down as follows:

1.     Panellised Units – including ready-made walls, floors and roofs which are assembled on site producing a three-dimensional structure; typically, timber framed buildings. These will consist of:


  • Open panels - comprising a skeleton structure only, or

  • Closed panels – which typically may include internal linings and wall finishes, thermal insulation, doors, windows and services


2.     Panellised Units – Volumetric Construction Units – Modules or Pods assembled in controlled factory conditions to produce three-dimensional building units for delivery to site. These are in fact ready-made rooms for site assembly, complete with all services and fittings. The term “Pods” is normally applied to smaller prefabricated units such as kitchens, bathrooms or, in the context of commercial buildings, plant rooms.


The construction of modules will vary considerably depending on the specific requirements of the project and the manufacturer. Typically, these will be of a timber or light steel frame, supporting composite construction which may include steel sandwich panels with EPS, PUR, PIR or Mineral Wool insulation similar to composite panels, plywood or timber composition board covered externally with a weatherproof membrane and external decorative finish, extensive glazing, plasterboard linings, etc. Floors will be of timber, steel or in some cases concrete depending on the specification. One of the leading manufactures of modular buildings in the UK is Portakabin and examples of this type of construction can be viewed at . A further source of reference is the Modular and Portable Buildings Association

An example of a modular building in the educational sector



3.     Hybrid Techniques – these combine both panellised and volumetric approaches. Typically, volumetric units will be employed for highly serviced and repeatable accommodation such as kitchens and bathrooms, with the remainder constructed of panels.


4.     Structural Insulated Panels Systems (SIPS) – prefabricated insulated panels for walls and roofs of sufficient strength to be self-supporting. SIPS commonly consist of two parallel faces – usually oriented strand board, sandwiching a rigid core of expanded or extruded polystyrene or polyurethane foam of between 100mm and 150mm thickness. Panels in this format are highly combustible. A range of external weatherproofing materials are attached to walls such as brick, concrete block and render, weatherboarding, metal cladding etc. with a standard internal lining of plasterboard for fire resistance. Roofs are finished with slates or tiles, metal cladding or waterproof membranes.


The origins of SIPS in the UK are in the construction of residential buildings, but in recent years SIPS construction has started to gradually extend into other areas, including the commercial, leisure and educational sectors.

SIP Construction

An example SIP product

5.     Others – other forms of MMC will be encountered including Insulated Concrete Forming (ICF), details of which are provided later in this bulletin.



In the FPA (RISCAuthority) Design Guide for the Fire Protection of Buildings, the first three essential principles under the heading ‘reaction in the event of fire’ are stated as:

1. The building shall be constructed in such a manner that if a fire starts, the extent of fire and smoke damage will be minimised and confined as close to the source of the fire outbreak as is practical/feasible.


2. With the exception of joinery products, the building shall be constructed from materials/products that will not make a significant contribution to the early stages of a fire or contribute to the spread of fire.


3. Suitable measures will be taken for the prevention of premature structural collapse and excessive deflection.

It is against these criteria that all buildings should be judged with particular emphasis on MMC.


Whilst originating in the residential sector, modern methods of construction as outlined above have in recent years been extended significantly into the commercial sector and there is every expectation that this trend will continue. Typical examples of commercial buildings where MMC may be employed include:

  • Retail outlets

  • Office blocks

  • Hotels – e.g. “Premier Inns”

  • Fast food outlets

  • Hotels and student accommodation

  • Education and healthcare premises

Of these, timber framed buildings are becoming increasingly common and following a number of serious fires in these buildings, major concerns are being expressed by Insurers via the RISCAuthority and Fire & Rescue Services, including a lobby for an urgent review of Building Regulations. It is important to stress that these concerns relate to modern light timber framed construction and do not extend to the more traditional methods of heavy timber construction, typically involving oak and other hardwoods.



Most timber framed buildings in the UK are constructed from prefabricated wall panels, made up of softwood vertical studs and horizontal rails, wood-based panel sheathing and a plasterboard lining. The studs carry vertical loads through the structure which are transferred to the foundations. The sheathing provides resistance to lateral wind loads. The outer cladding provides decoration and weather protection. Thermal insulation is usually incorporated in the space between the studs of external walls, and various protective membrane materials may also be required, depending on the design.

Factory produced wall panels will vary in the degree of prefabrication:

  • Open panels – comprising studs, rails, sheathing and an external breather membrane to which thermal insulation and linings etc are installed on site.

  • Closed panels – as above with the added feature that insulation, protective membranes, linings, external joinery and, in some cases, services already installed.

Additional layers of insulation and board materials may be added on site to provide enhanced sound insulation and additional fire protection where required.

Ground floors can be of concrete or timber, with upper floors of timber joists or prefabricated panels. The joist or panels will normally be installed on top of the wall panels providing a platform on which subsequent storeys are erected – referred to as a “platform frame”. Roof construction will commonly be trussed rafters or prefabricated panels. Externally, a cladding will be applied to the walls which very often is brick linked to the timber frame studs by stainless steel wall ties, although a wide variety of other cladding materials can be employed. Brick slips, commonly 15-30mm thick, may be used (as with other forms of MMC) which can normally be identified by the perfection of the pointing, or by observing the corner detail at the wall terminations.

A timber framed apartment block in Woolwich, SE London

A timber framed building under construction.

Following a number of serious fires in occupied light timber-framed buildings, where it appeared that the fire had entered the premises from outside, the FPA has been conducting detailed research into the possible contributing factors. One of the main findings of this study relates to the use of which are commonly installed in the masonry skin to provide ventilation to the under-floor cavities and the ease with which these can be ignited with nothing more than a cigarette lighter, potentially causing rapid fire spread within the timber-framed wall cavities. Following experimental tests, similar concerns are also expressed with regards to other plastic fittings such as vents and waste pipes.

Typical plastic air brick and cavity sleeve assembly

Arising from this work, the FPA has planned to will be publish a product selection guide embracing all fixtures and fittings that could form a “weak-spot” in the external envelope of timber-framed and other combustible structures, but at the time of review of this Technical Bulletin (February 2023), this has not materialised. It must be recognised that plastic air bricks and other vents are common place in timber-framed buildings, the presence of which may have an adverse effect on the arson risk, or the exposure hazard from the surrounding buildings and storage yards.

Further information on timber framed buildings can be obtained from the Structural Timber Association (STA) at . The Timber Research and Development association (TRADA)  and Frame Homes (a leading UK manufacturer of timber framed buildings)  are further sources of reference.

Of particular note are the following two guidance documents published by the STA, available at


  • Structural Timber Buildings Fire Safety in Use: Guidance Volume 1 – Pattern Book Systems

  • Structural Timber Buildings Fire Safety in Use: Guidance Volume 2 – Cavity Barriers and Fire Stopping   


The purpose of Volume 1: Pattern Book is to provide STA members, clients and specifiers with information on peer reviewed generic wall systems that are backed up by EN test evidence and supplementary research to create resilient fire safe solutions for timber frame wall, floor and roof systems.

The STA’s objective in developing Volume 2 is to provide solutions for cavity barrier installations and a clear understanding of who is responsible for design, installation and checking. Consideration has been given to industry good practice and the best-in-class solutions are included within this guidance, with recommendations for increased fire resilience for low to medium-rise structural timber buildings. This attempts to address the major concerns which exist in the fire safety environment.

All light timber framed buildings are to be assessed as 100% EML.


Insulated concrete forming (ICF) otherwise known as permanently insulated formwork (PIF) is an in-situ insulated system of building that is quick to construct and is purported to offer levels of performance over and above those of traditional construction. ICF has an application for walls, floors and, in some circumstances, roofs and has been in use in the UK for the past 15 years. To date, its presence in the UK has been mainly confined to the residential sector including, in recent years, apartment blocks and care homes. ICF has become widely used in the US for commercial and industrial buildings and it is anticipated that this trend will to some extent be mirrored in the UK as the lobby for a sustainable environment becomes greater.

In essence, ICF walls consist of twin-walled expanded polystyrene (EPS) or extruded polystyrene (EXP) panels or blocks which lock together without intermediate bedding materials to providing a formwork system into which concrete is poured, creating a structure ready to accept the floor and roof construction. In some cases, reinforcing rods are introduced for greater strength. Panels or blocks are tied together using a system of plastic or steel ties. The EPS remains in place to provide thermal insulation and provides a uniform surface for the direct application of external and internal claddings and finishes. External cladding will typically include bricks slips attached to panels with the use of adhesives or clip fittings, renders, weatherboarding and proprietary cladding systems, and internal finishes typically plaster or plasterboard.

In most cases, all ICF products are purported to be fire retardant/self-extinguishing, although this may need to be treated with caution. Quad-Lock Building Systems (now part of Aggregate Industries UK) state on their website that a Quad-Lock wall with 150mm concrete and a 12.5mm gypsum wallboard lining has achieved a 4 hour fire resistance rating under US testing programmes. In the absence of certificated fire resistance in respect of the system employed, a cautious approach must be taken.

Typical ICF Wall Construction

ICF flooring (and roofing) systems comprise of EPS floor panels factory made with integral steel beams moulded into the product from end to end. These are set out on site on a proprietary shoring system and steel reinforcement added, including ties to the ICF walls. Concrete is then poured and when set, the shoring system is removed and suspended ceilings and raised flooring systems erected as required, in the normal manner.

Typical ICF Floor Construction

An apartment block of ICF construction in Eastbourne.

Buildings constructed of ICF are to be assessed as 100% EML. For dispensations reference must be made to the Technical Helpline.


Further information regarding ICF can be obtained from the Insulating Concrete Formwork Association and from Quad-Lock Building Systems, a leading manufacturer at (now part of Aggregate Industries UK Ltd, trading as Charcon ICF).



The main fire safety concerns of MMC are centred on their performance under fire conditions and whether the essential principals of the FPA Design Guide as set out above can be achieved.

Many forms of MMC involve lightweight framing with the inherent formation of vertical and lateral flues in the form of voids within the structure, with the potential for rapid and extensive fire development. This can become particularly acute in buildings which incorporate, either entirely or in part, combustible elements of construction. It is therefore essential that the correct attention is given to the provision of effective fire stopping/cavity barriers at the design and construction of the MMC product or component and, very importantly, that this process is applied throughout the erection and fit-out phases on site. During the latter phase, the routing of cable and pipe runs is critical. Formerly routed in traditional buildings in a well-managed fashion to reduce the number of penetrations in blockwork, in timber framed buildings there is a tendency for each service to follow its own shortest route with the result that fire stopping and cavity barriers may become compromised.

Of equal concern, as regards fire stopping, is the need to recognise that this regime should be applied throughout the lifespan of the building, necessitating that all fire stopping measures are correctly managed and are not allowed to be compromised by alterations to the building structure and/or services (experience would suggest that this is easier said than done!).

Other considerations, as fully detailed in the InFiRes (RISCAuthority) document Modern

Methods of Construction and Fire Protection Considerations, include:

·       Resilience, particularly as regards passive fire protection

·       Reparability/Replacement

·       Long term durability

Fire risk is not the only concern and a case can be made that buildings of MMC may not always be as durable as buildings of traditional construction, particularly when considering the perils of flood and storm damage.

Dealing with construction sites, a number of fires have been experienced in the UK, as a result of which the latest edition of The Joint Code of Practice on the Protection from Fire of Construction Sites and Buildings Undergoing Renovation incorporates Annex A, specific to timber framed buildings.

Serious concerns regarding MMC have been expressed by the National Firechiefs Council; these have been published (November 2022) in a positioning statement which can be viewed here.


LPS 1501

Arising from the concerns of Insurers, Fire and Rescue Services and other stakeholders, the LPCB (now part of BRE Global) has launched LPS 1501 – Fire Test and Performance Requirements for Innovative Methods of Building Construction. The standard incorporates a large-scale fire test to investigate system performance in relation to structural behaviour and fire spread between units, including the performance of fire stopping and cavity barriers. It is intended to apply to panelised, volumetric and hybrid systems across all purpose/occupancy groups. Regrettably, to date, there have been no product approvals issued under this scheme.



An essential element of all surveys is one of obtaining and reporting on the details of the building construction. This importantly includes MMC, although it is recognised that occasions will arise where this may not be an easy task. Fortunately, the majority of buildings of any size or significance employing MMC are likely to be under 20 years old and information may be contained within the construction files retained on site. Alternatively, where dealing with a property owner’s case, access to construction information should be obtainable from the managing agents, architects and other parties.

Any suggestion that such information is freely available from local authority Building Control is not valid as such information is not in the public domain.

Dealing with timber framed buildings, some key pointers from UKTFA include the following:

  • In a timber framed building, windows are affixed to the frame and not to the brickwork outer leaf. This will often result in a deeper reveal as compared with traditional construction. 

    Also, there will normally be some indication of allowance for differential movement around windows in brick or concrete externally clad timber framed buildings.

  • Where brick or concrete block clad, look for “perpends” (small vertical gaps between bricks, usually with a ventilated plastic spacer above ground level at about 1.5m spacing – see figure below) which are provided for ventilation in the cavity between the cladding and the timber frame (this would also apply to brickwork applied to SIP construction).


  • Inspection of the loft space may reveal the top of timber wall panels. Also, in terraced or semi-detached property, presence of a plasterboard lining to the adjoining wall will almost certainly indicate timer frame construction. The presence of oriented strand board (OSB) or plywood sheathing in the gable wall will also indicate the same.



  • Also, in the roof space, look for the timber wall plates which support the trussed rafters. In timber framed construction these will normally be planed, but in traditional masonry construction are invariably left sawn.


  • By removing a cover plate to a light switch or socket on an external wall (following isolation of the electrical supply) it may be possible to see the wall’s construction. With timber frame, you are likely to see a polythene vapour control layer, insulation partly or totally filling the void, backed by a wood-based board sheathing such as plywood or strand board. In many cases this is a simple and reliable method of determining timber framed construction, but must only be undertaken by a competent person employed by the policyholder or tenant, as witnessed by the Consultant, and under no circumstances undertaken alone.

Identifying ICF is extremely difficult and the only reliable method is to take core samples internally and externally. Access to the roof space or visual inspection of voids, cupboards and other enclosures may reveal unfinished plasterboard, exposing the EPS panels.


There can be no doubting the fact that MMC is here to stay and will play an increasing role going forward, not just in connection with the construction of residential buildings but also in the commercial sector. Hopefully by reference to these guidance notes and the InFiReS (RISCAuthority) document mentioned, Consultants will have a greater awareness of the types of MMC employed and the potential fire safety issues encountered.

Queries or concerns relating to a specific case are to be referred to the Technical Helpline. We would also ask that Consultants keep the Head of Risk Engineering and Surveys abreast of any technical developments that are identified in the field which may benefit the entire surveys team.



Technical Bulletin (57) Cross-laminated Timber Construction.


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