Written by Daniel Woodley. Fact Checked by Paul Farley. Published to Prices. Updated: 6th March 2023.

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Since the Building Regulations Part L came into force, all new and refurbished flat roofs above heated rooms must meet the minimum requirement for thermal efficiency. For most projects that means extra flat roof insulation.

The Building Regulations requirement is represented by a U-Value, which is a measurement of how much heat is lost through the flat roof insulation and other materials on the roof.

The minimum U-Value in England is 0.18 for domestic extensions and refurbished flat roofs. For new build flat roofs, it’s lower at 0.11.

For non-domestic buildings and any buildings in Scotland or Wales, the minimum U-Value is different.

If this all sounds confusing, keep reading as we’ll explain your options.

On this page I’ll cover:

  • An example showing when you don’t need any flat roof insulation.
  • An image of a typical flat roof that isn’t compliant with the Building Regulations Part L.
  • How to construct an insulated warm flat roof that complies with the Building Regulations, plus a price guide.
  • A look at cold roofs, why they should be avoided and how to make them compliant with the Regs.
  • External resources, calculators for working out the required amount of insulation etc and YouTube videos I like.

Example 1 – No Flat Insulation Required

This photo shows a typical garage roof, you can see timber joists and decking from this angle and when viewed from above the roof, you’ll see the covering.

As this roof is above an unheated building, in this case a garage, there’s no requirement to install insulation.

A garage flat roof

Example 2 – Flat Roof With Obsolete Design

The image below shows how cold flat roofs were constructed decades ago; the design is now woefully obsolete for several reasons:

  • Cold flat roofs require ventilation above the insulation, at least 50mm. This is to stop condensation, but the extra void reduces the amount of insulation that can fit between the joists. It’s unlikely that this design would be thermally efficient enough to meet the regulations, not without extra insulation under the joists.
  • There’s no vapour barrier to stop condensation. One could be installed, but that would mean installing a new ceiling.

With modern thermally efficiency doors, windows and cavities, many homes experience an increase in humidity. A cold roof like this could result in condensation, damp spots and mould due to thermal bridging. I do not recommend roof designs like this.

Obsolete flat roof design

Example 3 – Cold Flat Roof With Insulation Above The Ceiling

As you can see from the image below, this design is still a cold roof, but there’s now 50mm of ventilation, a water vapour barrier and extra insulation between the ceiling and the roof.

Unfortunately, to install the extra insulation and vapour layer, the ceiling would also need to be replaced. This is obviously a costly undertaking for a roof refurb project, and for most flat roofs, there is a far cheaper and easier way to meet the Building Regulations.

There are only three scenarios where I would recommend a design like this:

1) If the roof cannot be raised to allow for the construction of a warm roof, then placing insulation between the roof and ceiling may be the only option. I have seen cases where doors (i.e. to balconies) and window ledges etc. are located directly above the flat roof. Blocking these by raising the roof obviously isn’t an option.

2) If the customer wants or needs to improve the insulation, but doesn’t wish to replace the roof covering, this design is viable as the insulation can be installed from underneath the roof.

3) The flat roof already has adequate insulation between the plaster and the ceiling (i.e good quality and depth of insulated plasterboard).

Insulation above ceiling

Example 4 – Warm Flat Roof Insulation

A warm flat roof is almost always the easiest and cheapest way to ensure the project meets the Building Regulations for insulation.

With a warm roof, there’s no need to create ventilation and all of the work can be completed from above without the need to remove the ceiling.

The image below shows how a typical warm roof is constructed.

Here’s a description of the layers, starting at the ceiling:

  1. The ceiling and plaster.
  2. Timber joists.
  3. Decking, usually 18mm OSB boards.
  4. A vapour layer.
  5. The insulation, usually 100mm – 126mmm foam boards but there are other options.
  6. Decking, can be down to 6mm for rubber roofs but should ideally be 18mm OSB boards for fibreglass roofs.
  7. The roof covering; fibreglass, rubber and bitumen felts are all options.
Insulation above ceiling

The best way to meet the Building Regulations is to install a warm roof and this is our recommendation.

The downside?

You’ll need to raise the height of the roof and there are things to consider:

    • Are there any roof windows, doors, fire escapes or anything else that would be obstructed by raising the height of the flat roof?
    • Will I need to replace the fascias around the edge of the roof? (Probably as they will need to cover the extra height gained by the roof)
    • If the roof is shared with a neighbour and they don’t want to replace their side, how can one raised roof join with a lower roof? Is it feasible? How much will the extra work cost?

How Much Does Flat Roof Construction Cost?

By using the warm flat roof example above, I can estimate the cost of insulating a flat roof for a refurbishment project.

I sourced the prices below from roofing stores in early 2023 and have displayed them on a per square metre basis:

18mm OSB board decking above joists – £13 per square metre

Water vapour barrier – £0.70p per square metre

Foam insulation board 126mm  – £26 per square metre

The top layer of decking, between 11mm to 18mm – £10 -13 per square metre

Total cost £49 – £52 per square metre + labour

The cost to insulate a flat roof to Buiding Regulations Part L standards is around £50 per square metre plus labour.

I recently published guide prices for flat roof refurbishments that exclude insulation, check this fibreglass flat roof prices, my guide to bitumen flat roof costs or this rubber flat roof price guide.

How To Calculate How Much Flat Roof Insulation You’ll Need

Below you’ll find a list of calculators you can use to calculate how much flat roof insulation you’ll need.

The U-Value target in England for flat roof extensions and refurbishments is 0.18, but the calculation includes all the materials on the roof, not just the insulation.

Most of the calculators will require the following information:

  • The size and spacing of the roof joists.
  • The type and thickness of the ceiling plasterboards.
  • The type of roof covering.

Ecotherm U-Value calculator

Kingspan U-Value calculator (also has an option for cold roofs, but will require insulated plasterboard above the ceiling)

Don’t want to put plastic foam insulation on your roof? After the Grenfell Tower disaster, we don’t blame you. Try this Rockwool U-Value calculator. Rockwool “Hardrock” has low combustibility; Euroclass A1, which is the highest rating.

A Video Worth Watching

The video below is very informative, and the creator explains in detail how the insulated roof is constructed.

My only criticism is that he used 100mm insulation which may achieve a U-Value of 0.18, but for new build flat roofs (rather than refurbishments), the required U-Value in England is 0.11.

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I hope you found this guide to flat roof insulation costs insightful.

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Author: Daniel Woodley

Daniel has over 18 years of experience in the construction, home improvement, and landscape garden industries, including a spell working as a project manager.

In his spare time, Daniel enjoys gardening, hiking and scrambling. He frequently posts videos to his popular YouTube Channel and is well known in the hiking community.

More About Daniel Woodley.

Daniel Woodley

Why Trust This Page? Our Experience

I know there’s a lot of nonsense published online about roofs and the roofing industry, usually by those that have never worked in the industry.

Here’s my experience:

1997-1999: I trained as an installer and fitted fascia and soffit boards and guttering as a roofline installer.

1999-2003: I trained as a flat roofer (built-up systems and fibreglass) and have completed hundreds of projects.

2003-2008: I went self-employed and worked in flat roofing, roofline and general roofing (mostly in domestic settings).

2008-2015: I worked first as a project cost estimator and then as a project manager for a housebuilder, property renovating firm and then a landscaping firm.

2015 onwards: I continued working as a project manager but mostly for larger landscaping projects.

As accuracy is so important, I gathered prices from flat roof installers so I could get a picture of what the going rate is in the UK – I’ve been out of the roofing scene for a while and didn’t want to publish outdated figures.

I also asked Paul Farley to fact-check the important statements I’ve made in this guide.

Explore More of Our Guides Below:


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Rubber Roof Price Guide

Rubber roof coverings are a popular alternative to traditional bitumen felts, and they last for decades with no maintenance required. This is how much a rubber roof costs.

Manual Roof Moss Removal

If you don’t want your roof tiles pressure washed. there is another alternative – manual roof moss removal. This is often cheaper and is very effective.

This guide to flat roof insulation was created by the team here at DIY Gardening

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