England Building Regulations for Flat Roofs

Approved Document L1A looks at the entire building’s performance and CO2 emissions, rather than isolating one particular element. All dwellings (domestic buildings) must be designed and built so their individual Dwelling CO2 Emissions Rate (DER) is no worse than the Target CO2 Emissions Rate (TER). The TER is based upon the performance of a defined Notional Dwelling.

A new requirement to also reach a Target Fabric Energy Efficiency (TFEE) brings the need to specify improved insulation standards and a focus on thermal bridging and air tightness, again based on a Notional Dwelling. Introducing a target for the Dwelling Fabric Energy Efficiency (DFEE) ensures that a ‘fabric first’ approach is taken throughout the building, which ultimately makes it easier to reach the TER.

Both the Dwelling Emission Rate (DER) and Dwelling Fabric Energy Efficiency (DFEE) are calculated using a calculation methodology called SAP (Standard Assessment Procedure) 2012. If the dwelling is built precisely to match the Notional Dwelling criteria, it should meet the Target Emission Rate (TER) and Target Fabric Energy Efficiency (TFEE). Note that other factors need to be taken into consideration including continuous building fabric insulation, air permeability, thermal bridging, information submission (design and build values) and limiting the effects of heat gains in summer.

The actual specification required for a compliant building and necessary building fabric U-value targets may vary depending on the actual overall proposed specification and the outcome of the energy assessment.
The area-weighted average U-value for flat roofs, taken from the Notional Dwelling specification (used to set the TER and TFEE targets), is 0.13 W/m²K.
Existing dwellings differ from new dwellings, in that they do not have to achieve a whole building Target CO2 Emissions Rate (TER, see New Dwellings). However, existing dwellings do have to meet certain target U-values for building elements (walls, floors and roofs).
The target flat roof U-value for existing dwellings or new elements within them are:
 
For replaced or refurbished roofs 0.18 W/m²k
New elements (extension / conversion) 0.13 W/m²k
Approved Document L2A adopts the same approach to Approved Document L1A, taking the entire building’s performance and CO2 emissions into consideration. All new buildings other than dwellings must be designed and built with their Building CO2 Emissions Rate (BER) being no worse than the Target CO2 Emissions Rate (TER). The TER is set from a Notional Building of same size and shape to the actual building. There are three defined Notional Buildings; heated and cooled buildings (that are side lit or unlit), and toplit buildings. The specification for each building type is summarised in Section 5 of ADL2A. CO2 Emissions Rates for buildings other than dwellings are calculated using the Simplified Building Energy Model (SBEM).

Special considerations are made for low energy demand buildings, modular/portable buildings, temporary buildings, industrial sites, workshops, and non-residential agricultural buildings. ADL2A should be consulted for further details. If the building is built to match the Notional Building specification, its BER should meet the TER. Note that other factors need to be taken into consideration including continuous building fabric insulation, air permeability, thermal bridging, performance testing and limiting the effects of heat gains in summer.

Although ADL2A is for new buildings other than dwellings, there are some exceptions if a dwelling is featured within the new building. For more details on these exceptions ADL2A should be referred to.
The area weighted average U-value for flat roofs, used in the Notional Building specifications for setting the TER, is 0.18 W/m²K.
Existing buildings do not require a Target CO2 Emissions Rate, however they do require a target U-value to meet for building elements (walls, floors and roofs).
The target U-value for flat roofs on existing buildings other than dwellings is 0.18 W/m²K.
The figures and information quoted are for guidance only.  Official Building Regulation / Standard documentation should always be consulted.

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