Flat Roof Insulation – A Look at Options

Updated Flat Roof Insulation Info For 2021

Prices For Flat Roof Insulation

Links to Calculators and Resources

Welcome to Quotation Check

Prices and Options For Flat Roof Insulation

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 we’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 we 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.

garage roof without insulation

A straight-forward and typical garage roof with no insulation.

Example 2 – A Cold Roof With An 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. We do not recommend roof designs like this.

Obsolete cold flat roof design

An obsolete cold flat roof design.

Example 3 – A Cold Roof With Extra 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 we 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. We 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).

Cold roof

Not the easiest or cheapest option but is suitable for some scenarios. Image from Quinn

Example 4 – A Warm Flat Roof

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.


Warm flat roof construction

The easiest and most popular way of insulating a new flat roof or refurbishment to Part L standards. Image originally from Kingspan.

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 it Cost to Insulate a Flat Roof?

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

The prices below were sourced from roofing stores in early 2021 and are displayed 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.

We recently published guide prices for the flat roof refurbishments that exclude insulation, check our fibreglass flat roof prices, our guide to bitumen flat roof costs or our 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.

We Like This Video:

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

Our 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.


The regulations and recommendations for flat roof insulation and design differ in Scotland and Wales. The information in this guide is for flat roofs in England only.

Compare Flat Roof Refurbishment Prices

We hope you found our guide to flat roof insulation costs insightful.

To get a custom price for your replacement flat roof project, tap the button below and fill in the contact form:

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By Quotation Check

Bitumen Flat Roofs

An alternative to fibreglass is traditional bitumen-based torch-on felts. They don’t last as long but are often cheaper and have been used successfully for decades.

Rubber Roofs

Rubber EPDM roofing materials are long-lasting, quick to install, and there’s no need for the installer to use a naked flame, so there’s no fire risk. They’re also repairable, a good point to bear in mind when choosing a roof covering.

Fibreglass Flat Roofs

From ship hulls and kayaks to pond liners and car parts, fibreglass can be found on thousands of waterproofed products. It also perfect for many flat roof refurbishments.

Their prices for roofline installations were correct; we were given quotes from several firms, all were similar to [Quotaton Check’s]

Quotation Check’s guides to flat roof prices are very accurate, they include all the extras many other price guides conveniently exclude.

Why only rely on three or four quotes for comparison? This website called Quotation Check gathered prices from hundreds of firms for various home improvement tasks and published their findings online.

Roofing firms often charge more than they should because they think consumers have no idea about how much the parts and labour should cost. Quotation Check’s guides are easy to read and they explain in detail about the materials needed and how long the work will take.

Go see the prices on the Quotation Checker website; they’re suggesting a price for a new roof that’s much lower than that.