Part L Building Regs Explained – Flat Roof Repair & Construction

What is Part L?

Part L sets the energy performance targets that all buildings in England and Wales have to meet. As control of the Building Regulations is devolved, the requirements are different in the two countries. Both countries also provide their own technical guidance handbooks – called Approved Document L – which explain how to keep your construction compliant with the regulations.

What thermal performance (U-value) will roofs need to achieve under the new Part L?

The minimum U-value is 0.16 W/m2K in England. It is highly recommended, however, that installers look to go above and beyond these minimum requirements. Roofs can act as a key source of heat loss on a property, and by insulating them correctly from the outset, it is possible to achieve significant energy and cost savings year-on-year. This is even more important with performance targets for existing homes also set to tighten over the coming years. By getting the envelope right from the start, you can allow customers to avoid costly insulation refurbishment work further down the line and ensure you are properly prepared for the future changes.

How does this effect you?

Basically, if you’re refurbishing more than 50% of the roof, then Part L comes into effect; the whole of that roof must then be brought up to the thermal efficiency demanded by the regulations. In practical terms that means a strip and re-roof, or even just installing a new layer of waterproofing, for example when stripping and re-felting a flat roof, will usually require upgrading the roof’s thermal performance.

It’s also important to be aware that the homeowner CANNOT opt out from having their insulation upgraded to Part L. An exception to this is when the payback period is more than 15 years or the work is not technically feasible. However, even then the roof must be upgraded to the best standard that is within this criteria.

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Frequently Asked Questions – Warm Flat Roof Construction

When to specify each type of roof?

So given the thermal benefits of a warm roof structure, why would you want to install a cold roof?

For the construction of new roofs in the UK, warm roofs are almost always the answer. The exception is where there are restrictions on the depth of the roof build up. This is of particular importance when considering balconies and terraces with openings to take into account. Warm roofs commonly require insulation of around 150mm which significantly adds to the overall height of the roof. In contrast, insulation adds not additional height to the structure of a cold roof when installed between rafters.

Thermal Efficiency

Warm roofs offer better thermal efficiency than their cold counterparts and are particularly well suited to the UK climate. The reason for the increased efficiency is the reduction of thermal bridging.

Warm roofs eliminate the cold bridging issues associated with cold roofs to deliver thermal efficiencies.

Thermal bridging occurs in cold roof structures because the rafter is not insulated. Because wood conducts heat, cold areas will be present on the underside of the rafters which can encourage the formation of condensation if dew point is reached. This condition can be diagnosed with the aid of thermal imaging cameras which will clearly show cooler areas consistent with the location of rafters.

What is the cost of a warm roof?

Typically a warm flat roof will cost double that of a standard cold roof. Depending where you are in the UK, the specification of the project and material prices at the time, typically warm flat roofs cost between £200-280 m2.