Written by an expert with 18 years experience

Written by Daniel Woodley. Fact checked by Paul Farley. Published to Prices. Updated 5th March 2023.

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Fibreglass is a tough, durable and long-lasting material that can be found on kayaks, ship hulls, surfboards, pond liners and hundreds of other waterproofed items.

It’s also one of three roof coverings that, based on my many years of experience, I recommend for flat roofs.

The other two are:

Rubber flat roof membranes; rubber is popular flat roof material and lasts for decades.

Traditional bitumen roofing felts are still popular and often work out cheaper than rubber or fibreglass.

Keep reading to see how much I think a replacement fibreglass flat roof will cost you.

On this page I’ll reveal:

  • An updated price guide for 2023/24.
  • How long the works take to complete – based on my experience.
  • A typical project schedule.
  • A look at the materials needed to complete the project.

I Asked 34 Flat Roof Installers How Much They Charge

See how much I think a new fibreglass flat roof should cost

I have over 18 years of experience in the home improvement industry, including over 10 years as a roofing contractor from the late 1990s into the noughties. However, I’ve since changed career and I know that prices have increased significantly recently, so I contacted 34 flat roofing firms and asked them for a price to replace a roof with fibreglass.

This is a Typical Project Schedule For a Fibreglass Flat Roof

This list describes the project, I created it based on my past experience:

  • Erect access equipment to the desired work height.
  • Remove the existing flat roof material.
  • Inspect the existing timber decking boards.
  • If the decking is sound, overlay new OSB decking, so there’s a new, clean surface for the fibreglass to adhere to.
  • If the timber decking is in poor condition, sagged or rotten etc. then it should be replaced with new OSB decking boards.
  • Install edging trims and upstands as required.
  • Install the fibreglass matt and coating layers to the decking.
  • Lower access equipment and remove all waste material.
Fibreglass roof kit

Typical fibreglass roof kit (available on Amazon)

Fibreglass Flat Roof Project 1: Single Garage

I asked the 34 flat roofing firms for a price to replace a roof over a single garage.

The figures below are an average of the price given to me; they include the cost of new 18mm decking and also VAT when it was charged.

Not every project is the same, so check out my guide further down this page.

Location:Price inc VAT:
London and South£1700 - £1900
Midlands and North£1500 - £1700

Flat Roof Project 2: Double Garage

The prices below are for a double garage, and as with the first price example, they include the cost of replacement 18mm OSB decking, which is essential for the fibreglass to adhere to.

This is how much I think a flat roof project like this will cost, based on an average of the figures given to me.

Location:Price inc VAT:
London and South£3500 - £4000
Midlands and North£3000 - £3500

Roof Project 3: Small Porch Canopy

Fibreglass is often found on small porches and canopies but when I worked in the roofing industry I found that smaller projects like this were often quite fiddly, especially if the shape was irregular.

The prices below are an average of the figures given to me, use this as a guide to fibreglass roof prices, but remember that every project is different, and my guide further down the page lists some extra costs to consider.

Location:Price inc VAT:
London and South£800 - £900
Midlands and North£750 - £850

Fibreglass Flat Roof Price Per Square Metre

Fibreglass flat roofs are difficult to price per square metre but based on the figures I was given, and as a general guide for a straightforward project:

£75 – £100 per square metre for the decking, base coat and top coat.

Add £40 – £60 per square metre for insulation, and vapour control layer (if required).

Add the cost of new roofline fascia, soffits and guttering (if required)

Add scaffold (if required).

Extra Costs to Consider

I know from experience that on many projects, there are often “extras” and customisations that need to be considered.

Thicker fibreglass: I only ever laid thicker fibreglass when the roof received foot traffic, I also applied a non-slip finish too, adding to the cost.

Complex detailing/finishing:  Circular balconies, irregular shaped porch hoods and other unusual structures often took me longer to complete and cost the customer more.

Timber Decking: Fibreglass shouldn’t be laid onto old timber so new OSB decking boards must be installed first. This provides a suitable surface for the fibreglass to adhere to and a strong base that should prevent movement and cracks once the fibreglass has dried. The prices on my page include 18mm decking, but an overlay of thinner decking was sometimes possible, it all depended on the condition of the existing roof.

Insulation and Insulated Decking: If your flat roof is above a heated part of the property, your installer will need to make sure there is enough insulation to meet the Building Regulations (Part L). In many cases, the roof will need to be raised so extra insulation can be fitted.

I haven’t included the cost of flat roof insulation because most flat roofs are above porches, garages and sheds; none of these are heated, so they don’t need insulation. However, based on my experience, adding insulation can increase the price by up 40%.

If you’re not sure if your current insulation will meet the Building Regulations, you can contact a flat roofing specialist to discuss your options.

Access Equipment: The prices I’ve displayed are for porches and garages, which are easy to access with basic access equipment. If your flat roof is difficult to reach, perhaps on top of a roof, or three storeys high etc, then other more expensive access equipment such as scaffolding or access towers might be needed.

Optional Gravel/Stone Finish: Beauty is in the eye of the beholder, and I like the neat smooth finish of fibreglass. However, some people choose to lay gravel over the fibreglass for aesthetic reasons. I haven’t included this in the cost guide, and how much extra it costs will depend on how much gravel is needed and the type.

VAT: The 34 roofing firms that gave me prices were both VAT registered and non-VAT registered businesses. I’ve included the cost of VAT in this guide when it was charged. You may find that businesses operating below the VAT registration threshold offer cheaper quotes than the prices I’ve suggested.

Extras: Most flat roofs will have guttering, fascias and sometimes soffits at the edges. I haven’t included a cost to replace these with Upvc plastic or to paint them. You may want to consider this extra work to finish off the flat roof neatly. This page has information about how much a new roofline will cost.

Get a Custom Flat Roof Price Online

I hope you found this guide to fibreglass roof prices insightful.

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

  • Get a custom flat roof price online.
  • Check vetted reviews and ratings.
  • Compare prices from local tradespeople and specialist firms.

Fibreglass Flat Roof FAQS

What is The Best Material For Covering a Flat Roof?

Rubber, also known as EPDM, has become popular over the last decade or so, and it’s easy to see why. This material lasts for decades, I always found it easy and quick to install as it doesn’t require the use of a naked flame. Most of the installers I’ve worked with offer long warranties and I’ve seen the materials patch-repaired, so if it becomes damaged, you don’t have to replace the entire roof.

Fibreglass is a hard, tough material that lasts for decades but I’ve seen cases where it cracked, and upon inspection, I could see that weak, substandard decking led to movement and the cracks. Based on my experience, I feel that the installer should use solid decking and not inferior, thin products. Also, some modern roofs have lots of flex, and I’ve seen some situations where fibreglass just wasn’t a good choice.

Bitumen-based felts have been used on flat roofs for decades and this was one of the first products I learnt to install. They are still a popular product in use today. This product is often cheaper than rubber EPDM but doesn’t last as long and I’ve seen it easily damaged by weathering and sunlight. Also, bitumen must be heated with a naked flame torch during the installation process, which introduces a fire risk.

When Were These Fibreglass Roof Prices Published?

I gathered these prices in 2023 and published this price guide in January 2023. I don’t expect the prices to change much over the next year or so except for inflation which is currently levelling off.

How Long Will a Fibreglass Roof Last?

Most manufacturers offer warranties of between 10 and 15 years, but I’ve seen fibreglass roofs installed 25 years ago that are still watertight now. In my opinion, a fibreglass roof should last for 25+ years.

Of course, the life expectancy of the roof will depend on the workmanship and the condition of the timber decking underneath it.

One problem I’ve seen with fibreglass roofs is the potential for cracking, however from what I’ve seen this usually only occurs on larger roofs and when the decking and timbers underneath move, as fibreglass is very rigid.

Do I Really Need New Decking?

Yes. Fibreglass should be laid over clean decking that’s strong enough to support the roof without too much flex. I’ve always used 18mm OSB decking boards for this type of roof.

There are only two occasions when I feel you can leave the original decking boards in place:

  1. If they are in excellent, “as new” condition.
  2. They are in good condition, and new tinner decking can be secured over the top of them.

More information about 18mm replacement and 11mm overlay decking boards can be found here.

My Existing Flat Roof is Pooling Water, Is This a Problem?

Modern flat roof materials are designed to last years, even with water pooling on them.

However, in my opinion, and based on past experiences, your roof will last longer if most of the rainwater flows off of it rather than pooling on it. A few millimetres is fine, but any more than this could be a problem if you get a leak.

If your existing flat roof is holding water in a few areas that have sagged between the joists, then new decking should resolve the problem.

I’ve seen some roofs so badly built that rainwater flows in the wrong direction and pools, sometimes several centimetres high. The solution is for the installer to fit firrings under the new decking to raise one end of the roof.

If your existing flat roof is pooling water, contact a roofing specialist, make them aware of the issue and request a fibreglass flat roof price that includes the cost for new firrings.

How Long Does it Take to Install a Fibreglass Flat Roof?

Based on my experience, it should take a team of two about a day to replace the decking and install the edgings and fibreglass finish to a single garage. A double garage will take between one and two days while a small porch will take ~half a day.

Are There Any Associations Where I Can Find Vetted Roofers?

Several roofing associations in the UK hold databases of approved flat roofers.

The National Federation of Roofing Contractors and the Confederation of Roofing Contractors are good places to start.

You can also use our network of tradespeople, and of course, there’s also Checkatrade, Which?, Trustmark and other general associations.

Why You Can Trust This Price Guide

The author of this guide, Daniel, has nearly two decades of experience in the construction industry, including ten years in the roofing industry.

The information he has provided regarding specifications, timescales, options and extras is based on his experience; however, the prices are calculated from figures provided to us by 34 roofing firms.

We asked Paul Farley to fact-check this guide and he told us:

Pricing fibreglass for garages, porches and outbuildings is fairly straightforward but if the roof is over a heated part of the home, the finished roof will need to meet Part L of the Building Regulations. This relates to insulation and how much this costs will depend on the existing insulation and how the roof is built. Therefore, it’s very difficult to provide even a ballpark price for more complex flat roof projects such as this.

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

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