Identifying MMC in a completed project

What to look out for

Off-site manufacturers have been relatively successful in designing and building non-traditional buildings that look traditional; so it is easy to be caught out when trying to identify the form of construction. Common mistakes include not identifying:

that the building has a timber or metal frame;
that the whole wall of a house, including the doors and windows were made in a factory;
that the whole roof was made in a factory; and
that behind the render lies an insulated panel, or insulated concrete.

Common problems include:

not being able to see the timber or metal frame;
not being able to see the fixings, or lack of fixings;
not appreciating that replacement of certain elements might prove difficult;
not appreciating that behind the wall lies a metal load bearing frame;
not understanding new technologies, particularly renewalable energy; and
not being able to advise on the efficiency of mechanical and electrical compenents.

There are, however, signs to look out for; some of which are easier to spot than others. As a starting point, consider the use of the building in front of you, as MMC is not used in all sectors. You will find examples in the following 2 categories:

Panel systems (timber, metal frame and concrete): nurseries, houses, food outlets, schools, medical centres, health care facilities and sports halls.
Volumetric systems/modules: accommodation blocks, housing associations and houses.

Then look at the building in more detail. The following headings provide a list of areas to inspect closely. No two manufacturers are the same, so there is no single, common theme that readily identifies the form of construction.

Openings

In a traditional cavity brick construction, you would expect the builder to form window and door openings as the works progressed, and the windows to be fitted at a later date. MMC has developed so that some windows are fitted in the factory, giving a different external reveal depth, although in some instances the depth is similar to brick and cavity construction. In some panel systems the reveal depth is much greater on the inside. Some of the panel systems are delivered and erected on site ready for the contractor to build the outer skin. In timber frame examples, there are likely to be horizontal movement joints located beneath window cills, in the form of mastic joints. If the window is factory fitted, it needs to be able to move in the newly formed outer skin. Horizontal movement of 3mm or more per storey could be experienced within the timber frame, which needs to be taken into account in the design.

The popularity of brick slips has also grown, and some are either factory fitted and pointed on site or installed on site. At the reveal, a return brick is required, which may be referred to as a 'pistol' because of its shape, which butts up to the window frame. There may also be a further cover strip to look out for.

Eaves junction

It may prove difficult to inspect the eaves because of access problems. Peering behind the soffit or fascia, however, may reveal what you've been looking for. The same can be said of other cavity areas, such as meter boxes, if they have not been secured properly.

DPCs and weep holes

The absence of weep holes and DPCs generally indicates the absence of a cavity - a move away from what we have come to recognise as traditional construction. You still need to determine the method by which the building has been constructed, as the inner leaf could be metal or timber. If a half brick outer skin has been constructed there should still be a DPC and weep holes, and the inner leaf could still be metal or timber. Insulated Concrete Formwork (ICF) relies on a solid concrete wall poured into expanded polystyrene blocks. The DPC can be formed in water resistant concrete or via a liquid system. As there is no cavity there will be no cavity trays or weep holes.

Pointing and junction with the ground

Poor pointing, bed thickness, verticality of perpends and colour variations are unfortunately not uncommon, and difficult if not impossible to correct without rebuilding. A site with neat joints and few areas of colour banding is worthy of praise, but may also indicate that brick slips have been used. The horizontal bed joints may be very neat, and it might be possible to see where the perpend has been tooled off.

Where the external wall meets the ground, the external face may continue into the ground (which would need to be probed to see what lies beneath). Starter rails or boarding may also have been used, indicating a non-traditional form of construction.

Floor and wall thickness

Guidelines are laid down in documents about such things as cavity widths and load bearing wall thicknesses, but in some forms of panel systems the overall thickness of the outer leaf may be similar to cavity brick construction. In volumetric systems, however, the modules can be stacked and butted against each other, leading to much greater internal wall and floor thicknesses. It is difficult to detect the form of construction behind plasterboard as mechanical fixings and dot and dab are popular with all forms of construction.

Junctions with adjoining buildings and corners

In a row of terraced housing that steps back between each house, look at the gable. In cavity brick construction you will see brickwork at the party wall. In panel systems, you may see boarding or vertical tiling as there is no way of supporting the outer leaf. Brick slips could of course also be used, but some panel systems still rely on a mitred corner joint (which should be immediately obvious). In modular construction, cover strips or mastic joints may have been used at the junctions of the modules, and vinyl floor coverings, over time, may reveal a joint beneath.

Figure 1: Note the corner of the gable end and front

Roof voids

The roof void of any form of construction often reveals the building's secrets, but getting down into the eaves, often over insulation quilt, is not easy and can be unpleasant without masks and good lighting. It should, however, reveal the inner leaf of construction. The absence of traditional purlins may also indicate roof cassettes.

Health and safety file and other supporting documents

Health and safety file and other supporting documents are likely to provide more information than anything else. They are still sometimes difficult to locate but, together with maintenance regimes, should provide all the information necessary.

Local knowledge

Building surveyors like to get out and about and talk about what's going on within their patch. For residential surveyors, local knowledge is important. Chances are you will be inspecting a house that you watched being built on a local site. If the site is not finished, you will be able look at other buildings at various stages of construction. For commercial surveyors, the opportunity might not arise so readily.

MMC has found a strong client base in the government and local authority sector. Many organisations are happy to answer questions and show you round if you take an interest.