What are the benefits of FM Approved assembly numbers for flat roofing projects?


What are the benefits of FM Approved assembly numbers for flat roofing projects?

FM Approved assembly numbers show that a particular flat roof build up, or assembly, has been given FM Approval. This shows that the assembly has been tested against several different criteria and specifiers who ask for them on their projects can do so in the knowledge that they are selecting a robust and proven solution.

Insurance companies are primarily concerned with property loss and business interruption and therefore flat roofs with FM Approved assembly numbers often exceed the national building regulations which are focussed more on life protection.

Insurance companies will advise their clients that roofs with FM Approved assembly numbers are more likely to be awarded insurance cover and have lower premiums than roofs not on the list.

What is an FM Approved assembly number?

An FM Approved assembly number is given to a particular build-up of flat or low slope roofing components, known as an ‘assembly’ that have been rigorously tested. Any deviation from the approved components listed from the deck, through to the waterproofing layer and anything in between, including the method of fixing, will result in a roof that can no longer carry the allocated FM Approved Assembly Number. The performance of the roof can no longer be supported by a FM Approved testing regime and the project loses the reassurance and confidence that comes from using an FM Approved Assembly.

The FM Approved assembly numbers are issued by FM Approvals. They are an American third-party international testing and verification service provider and are a subsidiary of FM Global, one of the leading insurers of commercial and industrial property.

Each individual assembly is assessed by FM Approvals for its resilience against a wide range of potential threats from natural forces such as the weather, self-inflicted issues caused during maintenance or repair, or attack by fire both from internal and external sources. All these threats could compromise the ability of a commercial roof to protect the contents of the building and minimise damage and disruption to business continuity.

What testing criteria does a roof assembly undergo before it can be given an FM Approved assembly number?

The testing criteria is wide ranging, and we explore them in the rough categories we mentioned: weather; self-inflicted and fire.


One of the most effective ways to reduce business loss due to wind storm damage is to ensure the roofing system offers the right level of protection. FM Global looked at a ten-year period of business losses due to wind damage and eighty percent of the losses were due to the failure of the roofing system as it did not offer adequate resistance to wind uplift.

Wind uplift is caused when winds blow across a roof and create an area of low pressure that effectively tries to suck the roof up off the building. FM Approved assemblies are evaluated on special test rigs to ensure the meet the required level of wind uplift resistance. They are also tested for their ability to prevent water leakage into the roof and to ensure that they can withstand the damaging effects of hailstorms. Any metal components in the assembly are checked to ensure they can withstand the corrosive effects of weathering.


Many commercial buildings have air conditioning units and other plant on their roofs. This equipment can require regular maintenance, plus occasional repair or replacements and during these events the roof surface is vulnerable to damage. This can be caused by regular foot traffic in certain areas of the roof or where tools or equipment are accidently dropped causing the waterproofing layer to be punctured. FM Approved assemblies are tested both for their foot traffic and puncture resistant properties.


Depending on the source of the fire, this could be put into the above ‘self-inflicted’ category but equally, the source of the fire could come from elsewhere. Whatever the source, damage by fire has the ability to cause major damage to a commercial roof and the building contents. FM Approved assemblies test the ability of the roof to protect the building both from external, and internal, fire sources.

The external fire assessment uses the American Society for Testing & Materials, or ‘ASTM’ test method called ‘E108’ where it simulates fire from the outside of the building with a 12 mile per hour wind blowing. It is a very comprehensive evaluation, having 6 different ways that the roof can be tested: spread of flame; intermittent flame; burning brand; and flying brand. There are three classifications, Class ‘A’, ‘B’ and ‘C’ with Class ‘A’ being the best and described as being effective against severe fire exposure.

Internal fire testing is carried out under an exclusive test developed by FM Approvals and comes under the FM Approval Standard 4470. It is designed to determine the contribution to the deck of the assembly to the spread when exposed to fire from inside the building. There are three possible classifications for the assembly: Non-combustible (concrete deck); Class 1 – ‘limited combustible’ roof assembly and Class 2 – ‘combustible’ roof assembly.

These tests show that a roof assembly undergoes a very comprehensive testing regime before it can be allocated an assembly number. This gives extra reassurance that the roof system will perform as expected when faced with many challenges that a roof could expect to encounter as it protects the building below.

How do roof cover boards and roof boards contribute to the overall robustness of an assembly?

Roof cover boards and roof boards are used either individually, or together, and can enhance the performance of the assembly in all the testing we have just looked at in the previous section of this blog.

Roof cover boards are positioned in between the insulation and the waterproof layer. Roof boards, or roof substrate boards, as they are sometimes known, are placed over the deck of the assembly. The boards can be made from a variety of materials such as plywood, OSB, perlite, asphalt, glass or mineral fibre, and cement or gypsum fibre. The last two are classed as non-combustible, are rigid, with high compressive strength and are most suitable for flat and low slope commercial roofs.

We will now look at how these boards can contribute to the overall robustness of the assembly, starting with the roof cover board.

Roof cover board

Roof cover boards can either be adhered or mechanically fixed to the waterproofing layer and, in both cases, they can offer extra resistance to wind uplift. The mechanical fixings go through to the deck and the extra weight and pull-through resistance of the cover board can reduce the effect of wind uplift on the assembly. Equally, where membranes are adhered to the cover board, the adhesive spreads more evenly on the flat surface of the board and this can help promote better adhesion to the cover board than it would do to the insulation layer. Better adhesion can increase the wind uplift resistance of the waterproof layer.

The hard surface of the cover board below the waterproofing layer can provide extra puncture resistance to dropped tools or flying debris in a storm. It can also offer extra resilience to foot trafficking and helps to resist the damaging effects of hailstorms that can cause small cracks to form in waterproofing membranes. Should water find its way through the membrane, cover boards can provide an extra layer of resistance to the ingress of water further into the roof assembly.

With regards to the fire performance, the non-combustible properties of the roof board can slow the spread of any flames that may form on the outer waterproofing layer and can also act as a barrier to the transfer of heat from a fire. Heat can cause any combustible materials within the roof assembly to ignite, so the barrier effects of the cover board can add an extra layer of fire resistance to the assembly.

Roof board

With roof boards positioned over the deck, they can offer another layer of weight and rigidity to the assembly that can help to enhance wind uplift resistance. They can also provide additional fire resistance in case of fire from the inside of the building. In the same way as the roof cover boards, they can help to protect any combustible materials in the assembly and minimise potential internal fire spread along the underside of the roof or over compartment walls. The smooth flat surface provided by the roof boards over a profiled metal deck can provide more bonding area for compatible membranes or insulation and enhance their bond strength to the roof increasing the resistance of roof to effects of wind uplift.

The inclusion of both roof cover and roof boards into an assembly can enhance many of the properties that FM Approvals test to ensure that their assembly numbers will provide robust roof solutions to specifiers.

How can I learn more about FM assembly numbers?

To learn more about FM Approved assembly numbers, there is an online tool called ‘RoofNav’ that details all the FM Approved assembly numbers. It allows specifiers to search by performance criteria to assist with the selection of specific project requirements.

Specifiers who would like to select robust, resilient, and tested solutions for their commercial flat and low slope roofing projects should be asking for FM Approved assembly numbers on their projects.

Many of the performance characteristics those assembly numbers validate can be enhanced by the addition of roof cover boards and roof boards.

DensDeck® Roof Boards are the leading choice of cover and roof boards in FM Approved assemblies.

A search of ‘RoofNav’ shows the popularity of using DensDeck® Roof Boards:

Different roof assemblies with all waterproof finishes: 1,231,5351
Number of assemblies that contain a roof cover board: 729,683
Number of these assemblies that use DensDeck® Roof Boards: 398,868

As well as showing the prolific use of roof cover boards it demonstrates that 55% and hundreds of thousands of individual assembly numbers with cover boards use DensDeck® Roof Boards.

If you would like to learn more about why DensDeck® Roof Boards are chosen to enhance so many roof assemblies for flat and low slope roofs and are considering the benefits of using tried and tested solutions for your next project please contact one of our team today.

1 Audit carried out on 16th August 2023

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