Household retrofit guidance for private properties - Flat and pitched roofs

Flat roof insulation

The easiest way to insulate a flat roof is from above. This can either be done by adding insulation on top of the existing roof (if the roof is in good condition) or by completely renewing the roof by combining this insulation with a new covering.

It’s best to insulate a flat roof when the roof needs replacing, as this allows you to upgrade it to meet current building regulations. 

Insulating under the roof is also possible, but it carries a higher risk of condensation issues, so it should only be done by a qualified professional such as a retrofit designer or coordinator.

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Regulatory considerations

• Insulating a flat roof is regarded as permitted development and typically allowed without planning permission. This is as long as the new roof does not extend more than 150mm above the existing roof.
• Planning permission is required for flats, maisonettes, and listed buildings.
• Listed buildings require listed building consent.
• Major changes to walls, roofs, or floors usually need building regulations approval.

The relevant building regulations which are required include:  

• Part L (Energy Efficiency)
• Part P (Electrical Safety)

The Energy Saving Trust has a very useful roof and loft insulation guide which covers flat roof insulation in detail.

Cost: £500

Disruption: Low

Pitched roof insulation

Most homes do not need insulation in pitched roofs because the insulation layer is usually placed in the loft floor. 

However, if you are planning to convert your loft into a living space, the pitched roof will need to be insulated. You will need a professional to ensure the insulation is properly installed and that there is enough ventilation.

Risks

The biggest risk with insulating pitched roofs is condensation and moisture. Proper ventilation and ways for moisture to escape are essential to prevent further issues.

Other things to consider are that any repairs to the roof structure should be completed before adding insulation – this is to ensure that the roof stays in good condition.

Regulatory considerations

• Alterations to the roof (such as re-roofing or adding skylights) is considered a permitted development and is usually allowed without planning permission.
• Flats, maisonettes, and listed buildings will require planning permission.
• Listed buildings will also need listed building consent.
• Major changes to walls, roofs, or floors will need building regulations approval.
• One of the required building regulations is Part L (Energy Efficiency).

The Energy Saving Trust has a useful roof and loft insulation guide with a comprehensive section on pitched roof insulations. 

If you are planning a pitch roof insulation for the first time, find out about the building regulations required from the Planning Portal.