Written by a roofer. A complete guide.

Written by Daniel Woodley. Fact checked by Hannah Miller. Published to Prices. Updated: 6th March 2023.

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Flat roofs can be found in many locations around the home; above porches, garages, dormers and extensions.

Unlike pitched roofs where water flows quickly off the tiles, flat roofs are prone to leaks, often exasperated due to water pooling on the flat surface.

If you want to know how much a flat roof costs, you’ve come to the right place – I spent over 18 years in the home improvement industry, including 10 years as a roofing contractor.

Here are your options:

Option 1 – Replace the flat roof with traditional bitumen felt; this page explains how much this type of flat roof will cost you.

Option 2 – Instead of bitumen felt, choose rubber, otherwise known as EPDM. This more modern material should last longer than traditional bitumen felt. See rubber roof prices here.

Option 3 – Fibreglass is the third option and I feel it’s perfect for smaller roofs like porches and garages but I’m not a huge fan of it on large areas. See fibreglass roof prices here.

We Asked 34 Flat Roofing Experts How Much They Charge

See how much we think a new bitumen flat roof should cost

My Flat Roof Price Guide

Below you’ll find my price guide for bitumen felt.

I first learnt how to lay bitumen back in 1998, and this material is still popular today.

Here’s a works schedule for the project:

  • Erect access equipment to the desired work height.
  • Remove the existing flat roof covering.
  • Check the existing decking boards and replace any that are damaged, rotten or made from weak chipboard with OSB boards.
  • Lay underlay felt, nailed to the decking boards.
  • Lay mid-layer felt using the torch-on technique.
  • Lay the final cap sheet felt with the torch-on technique.
  • Lower access equipment and remove all the waste.
  • Below are a few photos that show the type of flat roof finish these prices are for:
Bitumen roll of felt
A finished flat roof
Flat roof up a wall

Flat Roof Cost Example 1: Single Garage

To get an idea of how much other roofers are currently charging, I asked a selection of flat roofers for a price to replace a roof over a single garage.

The prices below are just for the felt; a separate price is displayed for replacing the decking, which supports the felt.

Based on my experience and as a general rule of thumb, I’m sure you’ll need to replace the decking if any of the following applies:

  • It’s sagged.
  • It’s rotten.
  • Is wet.
  • Is made from chipboard.
Location:Price inc VAT:
London and South£1100 - £1300
Midlands and North£900 - £1100

Flat Roof Example 2: Double Garage

The prices below are for a double garage; as with the first example, the prices are for the old felt to be removed and a new 3-layer system installed.

The figures displayed are an average of the figures provided to me by 34 roofing firms.

Location:Price inc VAT:
London and South£2000 - £2300
Midlands and North£1750 - £2000

Roof Project 3: Small Porch

The flat roof prices shown below are for a typical porch roof, a few metres long and no more than 2 metres wide.

As with the first two price guides, this is for a 3-layer torch-on felt system.

Location:Price inc VAT:
London and South£650 - £800
Midlands and North£500 - £650

Decking Costs

Timber decking supports the flat roof felt, and on many of the roofs I worked on, it needed replacing.

The new decking should be OSB grade which means it’s solid and won’t fall apart if it gets wet, as chipboard does.

Each OSB decking sheet covers around 2.8 square metres.

A typical small porch roof may measure between 3 and 5 square metres, while a small garage is around 19 – 21 square metres. A larger double garage will cover between 35 and 40 square metres.

(More information and flat roof decking prices can be found here)

Chipboard is significantly weaker than OSB, and no flat roof covering should be placed over it.

Here’s a quote from HPD Consult:

Chipboard is not designed to withstand the elements… it will break down over time due to moisture and temperature fluctuations

The prices below are an average of the figures given to me for the supply and installation of 18mm OSB grade decking boards, priced per board or 2.8 square metres.

Location:Price inc VAT Per Sheet:
Supply Only£33 per sheet
Supply and Install£55 - £65 per sheet

Part L of the Building Regulations Can Increase The Cost (Significantly)

Part L of the Building Regulations states that if you replace more than 50% of the flat roof covering, you must ensure the insulation is up to standard.

This only applies to flat roofs over the home, not unheated garages, sheds or door canopies.

For many of you, that will mean paying for extra insulation and possibly raising the roof slightly to create extra space for the insulation.

Based on my past experience, most flat roofs over the home will require new insulation, and most will need the roof raised slightly to allow for this to be installed.

The average cost for this extra insulation varies from project to project but as a general guide, I think you should budget an extra £50-60 per square metre of roof.


Decking: As you can see from my flat roof price guide above, the cost is much higher should you need to replace the decking underneath the felt. Chipboard should never be used under flat roof felt as when it gets wet, it disintegrates – this is something I’ve seen dozens of times. If the existing roof has chipboard, it should be replaced. If the existing decking has sagged or is showing any signs of rot, it should also be replaced with sturdy OSB decking boards. If your roofer lays the new felt over sagging decking, the roof will hold water, which isn’t ideal.

Access: The prices displayed are for porches and garages, which are easy to access. If your flat roof is located in a difficult-to-access part of the roof, perhaps 3-storeys high or in another area that can’t be reached easily with a ladder, then other, more costly access equipment might be needed. Scaffold isn’t always expensive but should be budgeted for.

Optional Gravel/Stone Finish: Bitumen torch-on felt looks better (in our opinion) than rubber roofs, but some people dislike the finish. As an optional extra, gravel or stones can be laid on the roof for aesthetic purposes. This will cost more, and I haven’t considered this in my flat roof price guides.

Insulation: As stated, garages and most porches don’t need insulation but a flat roof above any habitable part of the property will need to meet the Building Regulations for roof insulation (Part L). I haven’t included the cost of flat roof insulation in this guide as the type, quantity and price vary from roof to roof.

VAT: I asked 34 flat roofing installers for prices. Some were larger firms that charge VAT while some were self-employed individuals that were trading below the VAT threshold, therefore they didn’t need to charge this tax.

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.

Get Your Custom Flat Roof Price Today

These price guides are based on real quotes from firms and individuals in the UK, but every project is different.

To get an accurate price for your flat roof project, tap the button below and fill in the form:

  • Get a price quickly online.
  • Read vetted ratings and reviews from past customers.
  • Compare the prices you’re given to our figures.

Flat Roof FAQS

When Were These Flat Roof Replacement Prices Last Updated?

I sourced these prices in early 2023 and published this page in March 2023. I don’t expect the price to change much over the next year or so, except for inflation which is currently levelling off.

Which Companies Supplied You With These Prices?

34 roofing firms and individuals from around the UK provided me with prices. I then calculated an average.

How Long Does Bitumen Flat Roof Felt Last?

Most of the firms I contacted offered 10-year guarantees, some 12-year ones, and a single firm even offered a 15-year warranty.

If installed correctly and onto decking that is sound, the flat roof covering should last more than 20 years, and I’ve even seen some exceed 30 years.

The firms I worked with in the past usually offered 10 or 15 year guarantees which is the norm.

When I installed bitumen felts, which was many years ago, we had very few problems and never had to make a claim under warranty.

Can I Save Money by Having a Layer of Felt Laid Over the Existing Roof?

This is sometimes possible, and I have seen it done many times, but if the existing flat roof has sagged, has bubbled up or has cracks or other defects, these often transfer through to the extra layer of felt.

In general, I feel it’s best to replace the flat roof covering rather than capping over it and I see capping as more of a repair rather than a long-term solution.

What is the Best Flat Roof Covering?

Each has its own pros and cons.

Bitumen torch-on felt is easy to repair, affordable, long-lasting and can be laid to any roof of any size. It does, however, require the use of heat and naked flame during the installation process which could pose a safety issue. Also, the finished product may show signs of scuffing and wear-and-tear from foot traffic.

Fibreglass is tough and durable but is prone to cracking when used on very large roofs. We think it’s best suited to smaller roofs such as porches and single garages. Fibreglass is aesthetically pleasing and looks more attractive than rubber or bitumen felt.

Rubber is the newest flat roof material, it lasts the longest of all the options but is the least attractive. Thermal expansion and contraction of the material often creates unsightly ripples and bulges on the surface.

I prefer rubber roof coverings, for several reasons:

  • There’s no risk of fire during the installation process.
  • Aesthetically pleasing.
  • Long lasting.
  • Reasonably priced.

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

Modern flat roof materials such as bitumen and rubber are designed to last years, even with water pooling.

However, in my opinion, and based on my experience, you’ll get more mileage from your roof if rainwater flows off of it rather than puddling on it.

If your existing flat roof is holding water in a few areas that have sagged, replacing the decking may solve the issue.

If the roof is designed so water is flowing in the wrong direction, then one end of the roof will need to be raised. This isn’t as difficult as it may sound, but new decking and some extra timbers will be required. This will obviously add to the overall cost.

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

How Long Does it Take to Replace a Flat Roof?

This depends on how many workers are on the project and their efficiency, but as a guide:

We used to complete a single garage in a morning, but the work would take three-quarters of a day if the decking needed replacing.

A double garage can be completed in a day but will take a day and a half if the decking needs replacing.

Some smaller projects can take an unexpectedly long time, for example, roofs that are difficult to access or are of an irregular shape.

Are There Any Trade Associations Where I Can Find Reputable Installers?

Several roofing associations in the UK hold databases of approved 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 Guide

I know there’s a lot of nonsense published online, 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.

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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Fibreglass Flat Roof

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This guide to installing a new bitumen flat roof was created by the team here at DIY Gardening

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