Part L1B Existing Dwellings

Building Regulations Approved Document L1B states that when refurbishing a thermal element, such as a flat roof (providing that the area to be renovated is greater than 50% of the surface of the individual element, or 25% of the total building envelope) a check on the insulation within the existing roof must be made to ensure that it achieves or betters, the U value requirements.

The requirements are that :
An existing roof that already has insulation installed must achieve or better the threshold U Value of 0.35W/m²K.

Where an existing roof does not achieve the threshold U Value of 0.35W/m²K, additional insulation should be installed in order to improve the U Value of the roof to a U value of 0.18W/m²K or better.

An existing roof with no insulation must have insulation installed to achieve a U Value requirement of 0.18W/m²Kor better.

For flat roof extensions consult the relevant Part L Approved Document and seek specific guidance from the local Building Control Body (BCB).

Exceptions - Can I avoid the extra expense?
There are some exemptions, including if the roof does not form part of the thermal envelope e.g. unheated buildings, some listed/heritage buildings, where the payback period (cost of insulation versus savings made) exceeds 15 years to achieve, or where the area of roof to be renovated is less than 50% of the surface of the individual element, or less than 25% of the total building envelope.

Ultimately it is down to the local Building Control Body (BCB) to make a judgement on what is required. Please contact your local Building Control office for further details and verification of the above information.

What happens if you do not comply with the regulations?
Whoever carries out the works will be liable to fines from the relevant Building Control Body, although as a homeowner you could also have other potentially expensive problems. If the relevant documentation is not available it can result in problems selling your property, buyers offering substantially lower prices for your property and substantial delays in exchange and completion of contracts.

Extract from Building Regulations L1B
Conservation of fuel & power for existing