What is a warm decked flat roof?
This is a construction build-up where insulation is fitted above the structural deck of a building. Warm deck constructions offer a number of key advantages. As the structure of the roof is kept insulated and warm, meaning there is no cold areas on which condensation can form. In most cases this eliminates the need for the construction build-up to be ventilated. With careful preparation, it can also allow insulation to be fitted above an existing roof surface, allowing its thermal performance to be easily upgraded.
How can foot traffic impact flat roof insulation?
When you walk on any surface, your feet apply a compressive load on the insulation underneath. Even flat roofs which are not designed for pedestrian access can be exposed to these loads on a fairly frequent basis either during the initial installation or during maintenance. Regular maintenance is particularly important on flat roofs as their extremely low pitch angle means that debris on the surface or in drains can easily build up. Typically, these surfaces should be assessed and cleaned at least twice a year – before and after the winter period which is generally the most challenging time for roofs.
When exposed to this occasional dynamic loading, some insulation materials can lose their compressive strength. As a result, the insulation can begin to collapse, thermal performance can start to reduce and divots can develop leading to trapped water. Trapped water and water ponding is a serious threat to flat roofs. It can allow moss and algae to grow and, in winter, can cause the membrane to expand and contract as any water freezes. These issues can reduce the lifespan of the roof and may require costly repair work to fix.
How is the compressive strength of insulation measured?
Insulation materials are assessed using the standard BS EN 826:2013 - Thermal insulating products for building applications - Determination of compression behaviour.
In this test, a square section of insulation is placed on a compression testing machine. A load is applied and steadily increased until the material either gives way (yields) or the stress from the loading causes the material to deform by 10% (typically referred to as 10% compression). The pressure measurement taken at this point provides the maximum compressive strength which should not be exceeded.
Within this test, pressure is measured in kilopascals (kPa). The measurement is affected by the weight of the object and the size of the contact point. For example, a person standing on one foot would have a higher kPa measurement than if they spread their weight across two feet. On average, a typical person will have a ground pressure of around 110 kPa when walking in flat shoes. Materials with higher pressure measurements (in kPa) have greater compressive strength. This can make them suitable for applications where they may be exposed to greater loads.