Building Regulations

Part L1A/B and Part L2A/B in conjunction with Approved Document C

Current Regulations for Flat Roofs as of 6th April 2014 Dwellings Non Dwellings
Maximum (backstop) average U value (new building) 0.20 W/m2K 0.25 W/m2K
Maximum (backstop) average U Value (existing building) 0.18 W/m2K 0.18 W/m2K
Maximum (backstop) U value at any point (new/existing) 0.35 W/m2K 0.35 W/m2K

The U-values come from Approved Documents:

  • The maximum (backstop) average U-values are from Approved Documents L1A (energy efficiency of new dwellings) and L2A (energy efficiency of new non-domestic buildings).
  • The maximum (backstop) average U-values are from Approved Documents L1B (energy efficiency of existing dwellings) and L2B (energy efficiency of existing non-domestic buildings).
  • The current 2014 editions are at
  • These documents give average U-values for walls, roofs and floors that are maximum values for heat losses through these different elements of the fabric.
  • The maximum (backstop) U-values at any point are from Approved Document C, at
  • These values are set for the purpose of avoiding condensation on the coldest parts of the fabric.

However these are maximums and to fully comply in most instances these values will have to be exceeded.

Tapered Layer ‘U’ Value Calculations

Heat loss through a tapered layer is not uniform, varying between the thinnest and thickest parts of the various sections that make up a scheme.

Each tapered scheme we design is individually calculated to allow for factors such as:

  • Rate of fall
  • Length of fall
  • % of Hips and Valleys

These all influence the final ‘U’ value, the building regulations lay down a series of complex logarithms in order to calculate the heat loss for these different shapes. Every tapered scheme is different and therefore a calculation must be carried out for every new scheme as this cannot be accurately estimated.

At CTF we use a specifically designed programme in order to ensure our roof designs are compliant to the specified ‘U’ value.

CTF eliminate the risk of non-compliance for your tapered scheme and ensure you meet your client’s requirements.

Average thickness ‘U’ values are non-compliant.