What is an inverted roof?

…And how is it constructed?

An inverted roof is quite simply where the waterproofing layer is installed over the structural deck rather than over the insulation.

Rigid extruded polystyrene insulation has minimal water absorption due to its closed cell structure. This makes it the only material suitable and approved for inverted roof applications.

Inverted roof applications are becoming increasingly popular for new construction projects due to a number of benefits that we think are important to highlight:

• Extruded polystyrene, such as Kingspan Styrozone, has notable compressive capabilities that make it an ideal solution where there may be considerable foot traffic.
• Installation of the insulation is not weather dependent.
• Due to the open-jointed roof finish, the insulation can be easily lifted to allow inspection of the waterproofing system.
• The waterproofing system can be expected to have a life in excess of that obtained in an exposed situation due to protection against mechanical damage and UV degradation.

Now that we understand what an inverted roof is and some of the benefits, we’ve put together a listed build-up to help make designing your roof that bit simpler.

Internal to external roof build-up

• Concrete structural deck
• Waterproofing layer eg singly ply membrane
• Geotextile layer (non-woven polyester fleece separation layer) – this layer prevents a reaction between the insulation and the waterproofing layer
• Kingspan Styrozone insulation
• Water infiltration layer, such as Kingspan Aquazone – this layer reduces the amount of water flowing between and beneath the insulation boards, therefore minimising heat loss
• Open-jointed ballast system (gravel / paving slabs on supports) or roof garden

You’ll note that we recommend an open-jointed ballast system in conjunction with an inverted roof construction. In this next section we will explore the ‘why’ behind this.

A process called ‘freeze thaw’ occurs when a porous material such as concrete or screed is exposed to wet weather conditions. When the temperature drops to low temperatures, the water that has been absorbed into the screed or concrete will freeze. As the water expands in volume, changing from liquid water to solid ice, it contracts, breaking the once continuous substrate apart.

You might be asking yourself how is this is in any way relevant to an inverted roof construction and the content of this post. Well, this natural cycle is the very reason we do not recommend installing a continuous concrete or screed layer over Kingspan Styrozone for inverted roofs. Instead, we recommend avoiding the headache of a potentially cracked substrate and sticking to an approved open-jointed finish that is guaranteed to work all year round.

You can find out more about Kingspan Styrozone inverted roof insulation and the correct method of installation within the product literature.

You’ll also find inverted roofs on our U-value Calculator.

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