U-Value changes for flat roofing

The construction of flat roofing is governed by a range of building regulations and requirements including fire safety and thermal efficiency. Recently, the minimum U-Value requirements have changed, but what are the changes? And what does this mean for flat roofing projects?

What is a U-Value?

The U-Value refers to thermal resistance, as this is the rate at which heat transfers through a structure, divided by the difference in temperature across the structure. As such, the lower the U-Value, the better insulated the building or structure is.
In the building regulations, the walls, floor and the roof all need to meet minimum U-Value requirements. So what are the U-Value changes for flat roofing? Well, these are outlined in Part L of the Building Regulations.

Changes to Part L documents for building regulations

Changes to Part L of the Building Regulations and subsequent changes to Approved Document L came into practice on the 15th June 2022, across England and Wales. These changes have been implemented to cut carbon as part of the Government’s move towards their target of Zero Carbon Ready Homes by 2025. The aim of these changes in building practices is to cut carbon and heating costs for new homes by 31%, and for other buildings by 27%, when compared to the standards in 2013. This is because heating and powering buildings in the UK accounts for 40% of our total energy usage. As a result, increasing the thermal performance of all properties is essential for reducing the demand for energy, the need for heat and power in homes, and for reducing energy bills too.

As part of this, there have been organisational changes to the documents themselves, with a streamlined approach dividing the thermal performance categories into just two areas: Dwellings and Non-Dwellings.

These changes have also involved a re-examining of the required U-Values, and these have been changed for walls, floors and roofs.

What are the U-Value changes for flat roofing?

In terms of flat roofing specifically, the new U-Values for flat roofs are:

Comparison of the changes to minimum U-Values for flat roofing on dwellings:

Minimum U-Values for flat roofing on buildings other than dwellings:


What does this mean for flat roofing insulation?

With the changes to the minimum requirements for the U-Value, there may be some changes for insulation for flat roofs. The better the rate of thermal performance required, the thicker the insulation needs to be to achieve this. PIR insulation is one good example of this. This type of insulation is widely used for flat roofing across the country, and while the U-Value requirement was only 0.18, 130mm of insulation thickness was suitable. To meet the new standards, this thickness would need to increase to 150mm.

For flat roof build ups, where each component plays a pivotal role, and where sizing and heights need to be carefully considered, this increase in insulation depth can bring a number of potential challenges. For example, a thicker insulation can cause issues with upstand height restrictions, especially for a property where window or door cills cannot be moved, like a refurbishment. In addition, the increase in insulation can also have a negative impact on the amount of weight on the roof. While many insulation options are lightweight, like PIR for example, other insulation options weigh more, like stone wool. As a result, increasing the amount of stone wool used, can add to the roof weight load.

With building practices and regulations changing frequently, you need a team that’s always up to date and can be relied on, like us here at Enviroply Roofing. Why not get in touch today to find out more?