This CPD, sponsored by Georgia-Pacific, looks at how roof cover boards are used in flat roofs, between the outer waterproofing layer and the insulation layer
Roof cover boards are thin substrates, made from various materials, that sit between the insulation and waterproof membrane layers in a flat roof system. They can be used with many types of waterproofing layers, including:
- Mechanically fixed
- Fully or partially adhered systems
- Liquid applied
The illustration below shows how the roof cover board is positioned between the insulation layer and the waterproof membrane. The insulation layer is typically mineral fibre. Rigid insulation boards such as polyisocyanurate or expanded polystyrene can also be used.
The choice of insulation will be determined by several factors including the thermal and acoustic requirements of the roof specification. Cover boards can be used above all insulation types.
The waterproofing shown is a single-ply membrane. These membranes are available from many different manufacturers and the composition of the cover board is important to ensure that it is compatible with the particular membrane chosen.
There are two typical timber-based cover boards. These are treated with a range of different surface materials, depending on the end use of the board in question. Other materials, such as gypsum and cement, are also used in the production of cover boards.
Plywood cover boards are made from three or more thin layers of timber bound under pressure with glue. The grain of the timber is usually rotated at each layer to provide additional strength. There are two types of plywood: structural and general. Structural plywood has been tested to ensure that it is suitable for structural use within the Building Regulations.
Plywood comes in different grades based on the visual appearance of the ply, with A being the best, having no visual imperfections such as knots in the wood, through to D which is generally unsanded and can have large knots and imperfections. A common grade of plywood is CDX, denoting one side as C grade, the other D, with X indicating the glue in the middle.
OSB – or oriented strand board – is an alternative to plywood. Rather than the thin strips of veneer seen in plywood, OSB is made from small strands of wood, criss-crossed and stuck together with resins and waxes. This produces boards that are more consistent than plywood and which have a greater sheer strength than plywood.
OSB is generally regarded as water-resistant; however, it can be treated to make it waterproof. Some manufacturers also produce OSB that has improved reaction to fire properties by using fire retardants during their manufacture.
Both plywood and OSBs will provide the same structural strength; however, being organic based, they are classed as combustible and will struggle to meet specifications where fire protection is paramount.
Asphaltic core boards
Asphaltic core boards are relatively new in roofing applications. They contain a mixture of minerals and asphalt with a fibreglass facer on both sides of the board. They are designed to be used with hot applied asphaltic membranes and torch-on systems and are compatible with all asphaltic roofing systems.
However, they may not be used with some single-ply membranes as they may be affected by contact with the asphalt. Such boards will be used on flat and low-slope commercial roofs. Typically, they cannot be used with single-ply membranes.
This relatively new type of board should not be left exposed to the elements, as moisture ingress can cause compatibility issues that could affect the roof build-up performance.
Perlite boards, in use since the 1950s, are traditionally composed of expanded perlite ore, cellulose fibres, asphalt and starch binders. This type of board is designed for use with hot applied roofing membranes. Some perlite boards are coated with a tacky resin emulsion to prevent absorption of asphalt.
Perlite boards are used in flat residential roofing, with some use in commercial applications. They are not compatible with single-ply membranes.
A distinction must be made between perlite boards for use as cover boards and perlite boards for use as roof insulation. The latter lacks the strength required for a cover board application.
Glass and mineral fibre boards
This type of board is made by compressing fibres of glass or basalt rock bonded with resinous binders. Glass and mineral fibre boards come in different densities. Their porous nature means they are often enhanced with mat facers to ensure compatibility with hot applied modified bitumen membranes.
Applications are flat and low-slope roofing situations, but it should be noted that glass and mineral fibre boards are compressible, and unsuitable for mechanically fastened roofs.
These boards are based on using cement as a binder for wood flakes to produce cement particle boards or cellulose fibres from plants to produce cement fibre boards.
Manufacturers include other additives into the mix to affect board qualities and some also add glass fibre mesh. Cementitious boards are classified as non-combustible, impact-resistant and water-resistant.
They are often used as tile backer boards, but given their properties they can also be used as cover boards, although they are heavy and can be difficult to cut.
Gypsum fibre boards
Gypsum is a naturally occurring mineral and contains 21% chemically combined water and 79% inert calcium sulphate. Gypsum boards with paper facers are commonly used for drylining purposes. However, gypsum fibre cover boards have different facers, such as glass fibre, and the boards are treated to make them more water resistant.
Gypsum boards are classified as non-combustible, and in the event of a fire the energy from the heat vaporises the crystallised water incorporated into its structure, calcining the gypsum and creating natural fire resistance. While heavy, gypsum boards are slightly lighter than cementitious boards and tend to be easier to cut.
Depending on its positioning and composition, a roof cover board can bring various benefits to a roof. These can include enhanced integrity and durability, increased resistance to fire, improved weather protection and acoustic enhancement.
Some of these benefits, such as increased durability and weather resistance, will also depend on the specification of the waterproofing system in combination with the cover board.
If fire resistance is a key concern, then choosing a cementitious or gypsum-based board would be the better option. A non-combustible roof cover board can offer resistance from external sources of fire, such as burning debris from a nearby building or from equipment on the roof like a failed solar panel.
The roof cover board offers an additional layer of fire resistance to the roof in the following ways:
Protection from the spread of fire
This is centred around the reaction to fire characteristics of the cover board. It means that should a fire start on the outside of the roof and burn through the outer waterproofing layer, the cover board can help slow down the spread of the fire, helping to minimise damage before the fire is extinguished.
The same ability to slow fire spread would apply if the fire was coming from the inside of the building; however, in this scenario it would have to get past the layers below, such as the insulation, so it would be well developed and the contribution of the board would be less significant to the overall roof protection.
In the event of a fire breaking out, heat from the fire transferring from one side of a material to the other is a problem, as it could be enough to ignite any combustible material on the other side and start a fire elsewhere in the structure.
Roof cover boards can slow the transfer of heat where, for example, a fire has started on the outside of the roof. The heat from the fire rather than direct flames can be enough to ignite any combustible materials below.
It is worth noting that where protection from internal fire spread is required, then a non-combustible board can be secured over the deck; this is simply called a roof board and can be specified in addition to the roof cover boards we are looking at in this CPD.
Several fire tests can be used to determine the suitability of materials based in their performance. Four fire tests combine to allow construction products to be classified under the Euroclass system for reaction to fire.
Euroclass ratings go from A1 a the highest, to F as the lowest. EN ISO 1182 (reaction to fire for building products – non-combustibility) is the most stringent standard. Products rated as A1 have passed this test within the required limits to be classed as products that do not contribute at any stage of the fire, including a fully developed fire.
A2 products do not achieve the criteria on the test to be classed as A1 and have the option of testing to EN ISO 1716 (heat of combustion) and EN 13823 instead. In addition, under the conditions of a fully developed fire, these products will not significantly contribute to the fire load and fire growth.
This leads to all products that achieve an A1 rating being classed as non-combustible. A2 classification is sometimes also referred to as being non-combustible within some regulatory environments, although A2 products should not automatically be regarded in this way, as they technically can still contribute to a fire in some degree.
All other Euroclass-rated products are classed as combustible roof cover boards.
The external fire exposure roof test conducted under BS 476: Part 3 (British Standard specification for fire tests on building materials and structures) looks at how a section of roof behaves when subjected to fire from the outside the roof; this could be caused by a burning brand from a nearby building fire.
The test looks at both resistance to fire – seeing how long it would take for a fire on the roof to burn through to the deck by applying radiant heat to the sample – and the spread of flame over the roof surface.
Before the resistance and spread of flame tests, a representative sample undergoes a preliminary ignition test where the sample is exposed to a standard flame for 60 seconds. If this is failed then there is no need to continue with the other tests; the sample will be assigned the worst result: D for both resistance and spread of flame.
The BS476 Part 3 test also simulates the effect of a wind blowing over the roof, and the sample can be tested either at an angle of 45 degrees for sloping roofs, or horizontally for flat roofs, and it notes whether there were any flaming droplets from the roof sample during the test.
Presentation of fire test results
The best result is FAA or SAA, where the first letter stands for flat or sloping roof, the second letter shows the penetration limits through to the deck in 60 minutes, from A being the best through to D as the worst. The third letter indicates the spread of flame, again from A to D, with A being no spread of flame. If there are flaming droplets, or holes in the sample then an X is added to the end of the test result.
The BS 476 Part 3 test has long been used in the UK and is not one that would be familiar for projects within Europe, where another test – TS 1187 – is used. While BS 476 still appears in the annex of Approved Document B of the Building Regulations, it has now been superseded by TS 1187.
The European system has been running alongside the British Standard system for a number of years, although that changed on 30 August 2019 when the European classification system became the main reference for external fire performance of roofs in the UK.
Euroclass B can be achieved for some types of OSB, with the addition of fire retardants in the manufacturing process, but the board will still contribute to the fire when compared with the non-combustible options.
Only cementitious and gypsum fibre boards achieve A1 Euroclass ratings.
Differences between UK and European standards
The main difference between the UK’s BS 476 test and the European TS 1187 test is that fire penetration is the most important factor in the latter. This is due to the main focus being on protecting the lives of the people under the roof and not the materials on the top.
As long as the sample passes the preliminary test, only the result of the penetration test is needed.
The table below shows the classification of the TS 1187 test and how this compares to the results of the BS 476 Part 3 test. The Building Regulation limitations column in the table refers to the separation distances required between buildings in the Approved Document B.
These are the minimum distances from the roof, or part of the roof, to the so-called “relevant boundary”, defined as “the boundary or notional boundary that one side of the building faces and/or coincides with, and that is parallel or at an angle of a maximum of 80 degrees to that side of the building”.
Integrity and durability
Using a cover board over the insulation and under the external waterproofing membrane helps strengthen the roof and helps spread static loads and resist damage from foot traffic.
Many buildings have equipment on the roof, from air-conditioning through to PV panels.
Cover boards can help to improve the load distribution and protect the insulation installed below it.
The use of cover boards can allow a wider choice of insulation where heavy static loads will be placed on the roof. For example, where heavy static loads will be placed on a roof, expanded polystyrene insulation may be preferred due to its high compressive strength and ability to suffer less compression under load.
However, use of cover boards could allow mineral fibre slabs with their lower compressive strength to be used and allow the roof to benefit from the mineral fibre’s superior acoustic and fire properties.
Frequent foot traffic in concentrated areas can lead to insulation being damaged in the long term and compromise the effectiveness of any fixings used to secure the insulation to the deck. And where the waterproofing membrane is fully or partially adhered to the insulation, foot traffic can cause delamination of the membrane from the insulation.
Use of cover boards can provide some protection for both the insulation fixings and the waterproof membrane increasing the integrity of the roof.
Things to look for
Not all cover boards are compatible with all waterproofing layers, so it is always worth ensuring that the board specified is compatible with the waterproofing layer intended to be used.
A good bond between the cover board and the membrane will increase resistance to foot traffic as the membrane will be less likely to delaminate when exposed to such mechanical loads.
When deciding which type of cover board to choose to enhance durability of the roof, its compressive strength should be considered. This will give a good indication as to its load-bearing properties and its ability to protect the underlying insulation layer.
There are waterproofing systems, such as “hot mop”, that would not be compatible with some insulation types, such as foil-faced rigid insulation boards – here the use of the cover board acts as a separation layer, increasing the choice of insulation and ensuring that the integrity of the waterproof system is maintained.
Cover boards can provide benefits to help the roof do its key task of protecting the building from wet and windy weather; help protect the roof membrane against the effects of accidental impacts from tools dropped during installation of the roof itself or when installing or maintaining additional plant on the roof such as air-conditioning units or photovoltaic panels; and offer resistance against more random events, such as falling objects during storms.
All of these impacts have the potential to damage and puncture the waterproofing layer of the roof. The cover board will increase the impact resistance to minimise damage and limit the possibility that a puncture that will lead to damaging water leaks within the roof.
Where buildings have lot of mechanical plant on the roof it becomes a difficult and time-consuming task to trace and eliminate the source of leaks, so installing cover boards helps can help minimise these costly events.
When considering which type of cover board to choose to enhance the weather protection of the roof, it is always worth ensuring that the board you have specified is compatible with the waterproofing membrane. This will increase the impact and puncture resistance of the roof surface.
Acoustic benefits of using cover boards
Roof cover boards also provide a way to reduce the sound coming through the roof assembly.
If higher-density cover boards are chosen, the extra mass can help to reduce sound transmission. This is especially useful where lightweight flat roofs or metal roofs are being considered, since they require careful design to ensure that the desired acoustic performance is achieved.
A high level of sound attenuation can be achieved with the use of a single layer of cover boards. However, this can be further enhanced with a second layer of cover boards and/or a layer of roof boards over the deck. The combination needed will depend on the acoustic design requirements of the building.
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