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All you need to know about: Inverted roof ballast

by Jennie Ward

In the second instalment of our new column from the experts at Polyfoam XPS, Rob Firman explains what contractors need to know about the ballast used to finish inverted roof constructions.

Rob Firman, Polyfoam XPS.

Designers and installers of flat roofing solutions are often told what they ‘should’ know about different systems or components. In this series of articles, I will look at different parts of flat roofing design and construction and explain what contractors really need to know. This month’s focus is the ballast used to finish inverted roof constructions…

Why does an inverted roof need ballast?
The thermal insulation and water flow reducing layer (WFRL; see last month’s Total Contractor, page 60) that make up an inverted roof system are loose laid. To guard against wind uplift and secure them without using adhesive or mechanical fixings requires ballast of a certain weight.
Two other important functions of ballast are: its weight helps to resist any potential flotation of the insulation boards during particularly heavy rainfall, and it protects the insulation and/or WFRL from UV degradation.

What types of ballast are there for inverted roofs?
The two most commonly specified ballast roof coverings are gravel and paving slabs. A potential alternative to these is screed, while a green roof covering can also be used as ballast if it is of sufficient weight.
Third party certification for inverted roof systems details suitable gravel specifications, including what height of building it can be used on. The size of the aggregate has to resist ‘wind scour’ – movement of the ballast by wind – and maintain consistent coverage.

For taller buildings, it may be necessary to load the perimeter with paving to help resist wind scour. For other conditions, project engineers should be able to offer advice based on wind uplift calculations.
Particular advantages of paving include faster installation, simple access for any inspection of the roof or services running below the slabs, and the ease with which a single damaged slab can be replaced. Visually, it is easier to assess complete coverage of the roof. Checking that gravel is installed to the appropriate depth over the whole roof area requires more investigation.

Why are gravel and paving slabs so popular as ballast?
The choice of roof covering is, more often than not, made easier by the external fire performance of certain inorganic roof coverings listed in a document called Commission Decision 2000/553/EC. The following roof covering specifications are deemed to satisfy performance requirements without the need for testing.

• Loose laid gravel at least 50mm thick, or with a mass greater than 80 kg/m² (subject to maximum and minimum aggregate sizes).

• Cast stone or mineral slabs at least 40mm thick.

• A sand cement screed at least 30mm thick.

Despite being listed in the Commission Decision, screed is rarely used as an inverted roof covering and ballast. The performance of WFRLs depends on the air pockets that occur with gravel and paving slab coverings, but are unlikely to occur with screed coverings, and which allow moisture vapour to pass through the WFRL and disperse.

Some roofs, of course, require other finishes, such as timber decking for aesthetics. In these situations, a combination of finishes can cause a specification headache if relevant fire testing is unavailable. Designers or specifiers may be conscious of failing to meet fire safety regulations by combining multiple finishes.
In situations where a roof cannot be seen and will only ever be accessed for maintenance, it’s little surprise that a single finish of gravel or paving is the preferred choice.

Green roofs as inverted roof ballast
Arguably, a green roof covering is the most notable alternative to gravel or paving that is likely to be employed. Like any ballast, a green roof build-up above the WFRL is considered to contribute no thermal performance, so does not impact on a U-value calculation.

The Green Roof Organisation (GRO) Code of Practice should be the first port of call for anybody seeking more detailed advice on green roofs, as there are no formal standards covering their design, installation and maintenance.

For more information on different types of ballast and their function on inverted roofs, visit polyfoamxps.co.uk.

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