Roofs renewed

Kingspan Insulation’s Adrian Pargeter explains why tapered insulation systems should be considered when refurbishing flat roofs in order to avoid water ponding

Water ponding is an all too common sight on flat roofs across the country. When carrying out refurbishment work on these areas it is important to tackle the underlying issue rather than simply patching over the problem. The latest tapered insulation systems can provide a lasting solution, enhancing drainage and thermal performance without adding significant weight to the deck. Fall angle Despite their name, even flat roofs require a gradual slope for drainage. Without this, rain and snow will simply sit on the surface forming ponds. In addition to being unsightly, this can cause significant long-term issues such as alkaline formation and mould growth. The weight of the water may also lead the surface to deflect over time. This will result in further ponding and may mean the entire deck has to be stripped and replaced at significant cost to the owner. The obvious question when creating a fall on an existing surface is what gradient is required to ensure the water will actually drain away. The British Standards recommend that installers design surfaces to a fall of 1:40 (that is a one unit fall for every 40 units of horizontal roof length). This should ensure that all areas of the actual construction achieve a minimum fall of 1:80.

Thermal performance

When undertaking any major re-roofing work, the Approved Documents for the Building Regulations in England and Wales and Section 6 (Energy) of the Building Standards in Scotland require project teams to look at whether the thermal performance of the surface can be improved. Typically, contractors should look to achieve a U-value of at least 0.18 W/m²·K for refurbished flat roofs in England and Wales or 0.15 W/m²·K for flat roofs in Scotland.

Typically, this is either achieved with a warm roof construction, where the insulation layer is installed above the deck, or a cold roof construction, where insulation is installed between and below the roof rafters with a minimum air gap of 50 mm between the insulation and the deck. Recently, some installers have adopted a hybrid approach – fitting a thin layer of insulation above the deck with the rest below. In theory, this allows a thin layer of insulation to be fitted above the deck and eliminates the need for a ventilation gap below the deck. In practice, however, it can lead to some significant issues. The above-deck insulation layer often offers insufficient performance to keep the deck warm, allowing moisture to form below the deck and with no ventilation gap, this is not removed before it forms condensation. As such, this construction should only be attempted with a full condensation risk analysis and, even then, installers should be aware that workmanship will need to be almost perfect to avoid the potential pitfalls. Tapered insulation systems can often provide a more practical solution.


Tapered systems typically comprise tapered boards, hip and valley boards and flat packer boards. They can be installed above a variety of decks either with standard waterproofing or as part of a green roof system. The systems can significantly reduce installation times. In most cases they can be fitted directly over the existing surface and their dry installation process means no time is lost waiting for the surface to set. They can also weigh as little as 1.5 per cent that of a screed to fall system – removing structural loading concerns. The latest tapered insulation systems combine polyisocyanurate (PIR) sloped, hip and valley boards with phenolic insulation packer boards which have thermal conductivities as low as 0.018 W/m·K. This enhanced level of thermal performance can allow desired U-values to be met with thickness savings of 20 – 40 mm, depending on the individual scheme and build-up, when compared with a full PIR system. Beyond these benefits, some manufactures also provide tapered design services to support installations. These provide contractors with a detailed layout ensuring drainage and insulation performance are met with a minimal construction depth – streamlining the retrofit process.

A fresh start

Adding a fall to an existing flat roof was once a complex, costly and time-consuming process. However, by taking advantage of the latest tapered roof insulation systems it is now possible to transform the thermal performance and drainage of these areas with a quick and simple installation which doesn’t add significantly to the structural loading of the surface.

Adrian Pargeter is head of technical and product development at Kingspan Insulation