A Realistic Guide to Flat Roof Replacement Costs

Updated Flat Roof Prices For 2021

Based on Prices From Dozens of Roofing Specialists

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How Much Does a New Flat Roof Cost?

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 concrete tiles, flat roofs are prone to leaks, often exasperated due to water pooling on the felt surface.

If you want to know how much a flat roof costs, you’ve come to the right place but first you have to choose from one of three 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 this option is to replace the covering with rubber, otherwise known as EPDM. This is a more modern material and should last longer than traditional bitumen felt. See rubber roof prices.

Option 3Fibreglass is the third option and is perfect for smaller roofs like porches and garages. See fibreglass roof prices here.

We Asked 34 Roofers How Much They Charge For Flat Roofs

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Our Flat Roof Price Guide

Below you’ll find our price guide for bitumen felt, this material has been used for decades and 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:

Flat roof bitumen felt system
Felt roof
Flat roof felt

Flat Roof Cost Example 1 – Single Garage

For this project, we asked our chosen flat roofers for a price to replace a roof on a single garage.

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

As a general rule of thumb 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:From:To:
London Area£1100£1300
South, SW and Midlands£950£1100
Outer Region and North£850£1000
All prices exc decking. See notesAll prices exc decking. See notes.
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Cost Example 2 – A 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 us by 34 roofing firms.

Location:From:To:
London Area£2000£2300
South, SW and Midlands£2000£2100
Outer Region and North£1800£1900
All prices exc decking. See notesAll prices exc decking. See notes.
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Flat Roof Price 3 – A 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:From:To:
London Area£650£800
South, SW and Midlands£650£800
Outer Region and North£450£550
All prices exc decking. See notesAll prices exc decking. See notes.
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Decking Costs

Timber decking supports the flat roof felt, and on many roofs, it will need 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 around 19 – 21 square metres. A larger double garage will cover between 35 and 40 square metres.

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

Our guide to OSB decking boards can be found here.

Location:From:To:
Supply only - 2.4m x 1.2m x 18mm boards£30 each£35 each
Supply and fit 2.4m x 1.2m x18mm boards£45 each£55 each

Notes

Decking: As you can see from our 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. 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: Our prices 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.

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 we haven’t considered this in our flat roof price guides.

Insulation: 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). We have not included the cost of flat roof insulation in our guide as the type, quantity and price varies from roof to roof. See our guide to flat roof insulation here.

VAT: We 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. We haven’t included a cost to replace these with Upvc plastic or to paint them.

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Our price guides are based on real quotes given to us by 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:

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

Flat Roof Replacement

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Flat Roof Replacement FAQs

When Were These Flat Roof Replacement Prices Last Updated?

We sourced these prices in late 2020 and published this page in ealry 2021. We don’t expect the price to change much over the next year or so, except for inflation which is currently very low.

Which Companies Supplied You With These Prices?

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

How Long Does Bitumen Flat Roof Felt Last?

Most of the firms we 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. We’ve seen some exceed 30 years.

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

This is sometimes possible, but if the existing flat roof has sagged, has bubbled up or has cracks or other defects, these may transfer through to the extra layer of felt.

In general, it’s best to replace the flat roof covering rather than capping over it. 

In most cases, capping is 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, longlasting 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. In our opinion, 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.

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 our opinion, you’ll get more mileage from your roof if rainwater flows off of it rather than pooling 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?

A single garage can be replaced in a morning but will take three-quarters of a day if the decking needs 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.

Discover More Price Guides:

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Fibreglass Roofs

Fibreglass is a tough and long-lasting material often found on waterproofed items such as kayaks, ships, pond liners. It can also be laid onto flat roofs as it lasts for decades.

Rubber Membranes

Rubber roof membranes come in various thicknesses from 1mm to 2mm, and they all perform very well at keeping rainwater out. Long-lasting and easy to install, it’s easy to see why rubber is a popular flat roof product.

Roofing

Let us show you how much guttering and roofing projects cost in the UK. From repairs to maintenance to complete overhauls and replacements, check out our cost guides.

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