Flat Roof Replacement Guide

Roof Care & Maintenance
product guide
Flat Roof Replacement Guide

Practical, reliable and contemporary in appearance, flat roofs can be spotted on a range of industrial, commercial and residential buildings across the UK. With faster, more affordable installation and straightforward maintenance and upkeep, flat roofing is the perfect choice for those who want a sleek, minimalist look minus the hassle.

But what happens when your flat roof is in need of a replacement? On average, flat roofing may need replacing or at least repairing around every 15 – 20 years. Thankfully, the replacement process is pretty straightforward and won’t leave you too out of pocket. In this article, we will talk you through everything you need to know about replacing your flat roof. With materials, budget, timescale and tell-tale signs that a replacement is required, we will discuss all aspects of the replacement process so you can get the job done stress-free.

Flat Roof Materials

A good place to start is to talk you through the various flat roofing materials available and their benefits.

EPDM Rubber

EPDM (ethylene propylene diene monomer) is a highly durable synthetic rubber with its two main ingredients, ethylene and propylene, derived from oil and natural gas. EPDM rubber is a copolymer and has elastomeric properties which means it is both viscous and elastic. This allows the material to return to its original shape as it expands and contracts with the weather.

EPDM is a popular flat roofing choice for a range of different buildings. It offers fantastic environmental benefits with minimal manufacturing process required and 100% recycled rubber materials used. With correct installation, EPDM rubber offers superb insulative properties, retaining heat in the cold winter months and reflecting heat in the summer. This, in turn, generates lower energy costs and greater efficiency for the building.

Highly resistant to weathering, UV and infrared light, EPDM roofing is durable, long-lasting and reliable. You can expect your flat roof to last around 20 years with only minimal maintenance and repairs required. It is highly resistant to fire and can even help to slow down the progression in the event of a fire.


GRP stands for glass reinforced plastic and can also sometimes be referred to as fibreglass roofing. GRP flat roofing is formed when a catalysed resin is applied to decking and a layer of fibreglass matting is then laid on top. A second layer of catalysed resin completes the system with a fire retardant top coat resin applied to create a durable, weatherproof and visually pleasing finish.

GRP roofs are known to last up to 25 years with fantastic durability and long-lasting resistance to weathering. During its lifespan, GRP is easy to maintain, requiring only easy repairs and cleaning when needed. GRP is also known for its flexible properties, easily shaping to any roof design for quick and straightforward installation. When correctly fitted, this flat roof system is completely waterproof so is not prone to leakages or water damage even in heavy rain.

Self-Adhesive Felt

Perfect for DIYers, self-adhesive roofing felt can be easily laid without the need for naked flames. The installation process is faster, safer and easier for homeowners that don’t want to spend the extra cost on an installer. After simply peeling off the release film, the felt will securely self-bond to the roofing structure, quick and simple. It is the ideal roofing material for smaller scale projects such as domestic flat roofs, garages, sheds, outbuildings and porch canopies.

Thanks to its fantastic durability and weather resistance, self-adhesive felt can last up to 15 years with correct fitting and care. It adds a robust protective layer against wet weather conditions and blocks excess moisture to help prevent damp and water damage. The smooth finish provides creates a minimal look that will stylishly blend in with most contemporary properties.

Torch-On Felt

As implied in the name, torch-on felt is applied to the roof using a blowtorch with naked flame. The torch-on felt roof system consists of three layers of modified bitumen melt-welded onto the surface and each other to form a robust, weather-proof seal. The three layers include a vapour control layer, a reinforced felt layer and a cap sheet for larger scale roofing projects. Using the three layers of felt is required by building regulations for habitable home areas.

Incredibly durable and reliable, torch-on felt ensures optimum resistance against harsh weather conditions including UV rays, heavy rain and strong winds. The mineral finish helps to break up sunlight, preventing the bitumen from melting and maintaining cool temperatures within the building all through the summer. The heat applied seal ensures optimum watertightness for the roofing structure, making it a practical flat roofing choice for areas with heavy rainfall.

As the fitting process does incorporate the use of naked flames, it may be dangerous to attempt DIY installation without the correct training. Most feel that leaving the job to a professional is the safest option as it does depend on the level of experience you have with this type of flat roofing material.

Signs That Your Flat Roof Needs Replacing

There are a few factors that may prompt you to consider replacing your flat roof system.


One of the tell-tale signs that your flat roof is in need of replacing, is consistent or large areas of leakages. In most cases, very minor leakages here and there can often be fixed with easy repairs. However, if you are having significant issues with water leaking through your flat roof, then it may be down to the entire roofing system just not working properly.

Your roof could have been incorrectly fitted when initially installed, large impact may have inflicted damage or the roof could have just reached the end of its lifespan.

Flooding, Pooling & Sagging

Once again, with a lot of cases of minor water pooling, you may be able to fix the issue with repairs to the existing roofing system. However, if the water pooling/flooding/sagging has been a long-term problem, this may have led to further damages such as rotting or leakages. At this point, it may be time to consider replacing the entire roof and starting again.

Punctures, Splitting, Cracking, Blistering & Stretching

Unfortunately, when heavily exposed to UV rays and harsh weathering over a long period of time, you may notice that your flat roof has begun to split, crack, blister or even stretch without returning to its original shape. This can be a common issue with materials such as EPDM as they reaches the end of their lifespan.

Damages such as these can compromise the structural integrity of your roof so it may be the safest option to go for a complete replacement.

Ready For an Upgrade?

You may find yourself wanting to replace your flat roof due to the simple fact that there are now far better options available than your current system. With significant advancements within technology and material quality in the last 10-20 years, your existing flat roof might feel quite outdated. Before waiting for your flat roof to succumb to damage, you can upgrade now and enjoy a better quality roof without having to deal with costly repairs in the future.

How Much Will Replacement Cost?

As with any DIY project, considering the cost of materials, tools and any additional extras is important to ensure you can correctly budget for the job. The cost may also have an impact on the type of flat roof material that you end up picking, so carefully considering each calculation is one of the very first steps you should take.

The cost of materials will of course vary from brand to brand. The quality of the product will have a direct impact on the cost so you may find a material that you think is extremely affordable, however you may face additional costs down the line in repairs if it has not been manufactured to a high standard.

Cost will also depend on the scale of the project you are taking on. Obviously, a porch will be considerably less expensive than a larger scale property flat roof replacement, based on the amount of material you need per M2.

Below is a table compiled by priceyourjob including average costs per M2 of each roofing material across the UK.

Flat Roof Material

Average Cost Per M2

Felt Roof

£40 - £60

GRP (fibreglass) Roof

£70 - £90

EPDM Rubber Roof

£80 - £90


To calculate the costs, you first need to get up on the roof and measure the space. Once you have the measurements you can relate the cost of the materials per metre2 and get a clearer estimate.

Example – EPDM

Using a tool such as our handy EPDM roofing calculator, you can easily enter the length x width in metres to help estimate complete project costs. Let’s talk through an example.

After carefully measuring up our roof space, we can see that the length is 4m and the width is 3m. This equates to a roof space size of 12m2.

The next step is to choose the appropriate trims for the job. In this example, we are using 3 black plastic wall trims and 1 black plastic gutter trim. These options have been added to the calculator.

The roofing calculator has measured that the cost for a 12m2 ClassicBond Kit for this project will come to around £290.51 (excluding vat). This incorporates all the volumes and materials required to cover 12m2 of flat roof area, including extra membrane required for each edge as well as adhesives, sealants, trims and corners for installation.

At this stage, I am also able to add any further upgrades to help with my flat roof project including a DIY installation kit.

Example – GRP

This time, we will be using the GRP roofing calculator to estimate the price for a 4m x 3m flat roof.

We have a roof area of 12m2 and have added 3 wall edges and one gutter drip edge.

The next stage is entering the installation surface (OSB3 Standard Square Edge) and the installation temperature (5 - 8°C).

The tool has calculated that the cost for a 12m2 ProGRP Kit for this project will be around £382.82 (excluding vat). This incorporates all the volumes and materials required to cover 12m2 of flat roof area over OSB3 Standard Square Edge at a 5-8°C temperature.

This includes an extra 10% resin, topcoat, catalyst bandage and matting as well as all the trims required to install the GRP roof kit.

How Long Will Replacement Take?

Depending on the type of flat roof you have chosen, there will be a variation in the amount of time you should expect to spend replacing the system. Remember, you need to factor in time for both removal and installation. The table below, estimated by myjobquote, provides average estimates for replacement times based on the scale of the roof space.

Roof Size/Type

Average Duration

Flat roof (main property)

2-3 days

Extension roof

2-3 days


1-2 days

Double Garage

2-3 days


1-2 days


Timeframe will also depend on the type of material you are using and the experience you have with using it. For example, EPDM can take up to 8-12 hours and then up to another full day to re-install. Whereas a simpler option such as self-adhesive felt may take less due to a simpler installation process.

It is recommended that you give yourself at least a weekend to complete and go back for extra checking. This will ensure that job is completed safely and not left unfinished.

What Does the Replacement Process Involve?

The replacement method is different for each roofing material. We have included a brief overview of each removal and installation process to give you an idea of what replacement involves. Please note that this is only an overview and the step-by-step process will involve further steps/work.


To first remove the EPDM from your roof, you will need to cut the membrane into sections and pull up each piece of the rubber. It is advised that this is worked through in sections, so can be time consuming if you have a large roof space.

An EPDM roofing system is best installed onto a timber deck, either exterior grade ply or OSB3. Before installation you will be required to clean and prepare the roof surface as EPDM does not adhere to wet surfaces.

In most forms of EPDM, the material will be rolled out and laid into position across the full surface of the flat roof, then smoothed out using a broom or similar to ensure it is completely flat. You will also likely need to trim off excess pieces of the EPDM rubber material to ensure a sleek fit for your roof. A water-based adhesive is applied to the decking with half of the membrane pulled back. Edges and details will then be secured using a stronger contact adhesive to ensure the EPDM membrane is fully fixed in corners and more complex sections of the roof space.


A simple removal method for GRP fibreglass roofing involves cutting up the material using a tool such as a circular saw. Again, you will need to work across the roof in sections so this could take some time.

A GRP roof must be applied in dry and mild conditions and should not be installed in conditions under 5 degrees Celsius as this will not allow the resin and topcoat to cure correctly. If during the process it does begin to rain, you should always halt the project and cover the roof until the rainy weather has ceased.

OSB3 decking boards will need to be laid followed by a layer of catalysed resin. A layer of fibreglass matting is then laid on top with a layer of catalysed resin to finish the system. After curing, a fire retardant top coat resin is applied to form the final finish.

Self-Adhesive Felt

The felt removal process is pretty straightforward. You will need to prise up the sides of the felt, then tear away and dispose of it, section by section. You should always wear protective gear such as goggles and gloves whilst doing this.

Self-adhesive roofing felt is praised as one of the easiest flat roof materials to install. After installing the first layer of underlay felt to the wooden decking, the second layer of underlay felt will be rolled out but should not coincide with the first. After installing gutter drips, the top layer of felt is applied using the same method as the second. The technique involves simply peeling off the release film and then self-bonding straight to the surface.

Torch-On Felt

To remove torch-on felt you will need to slice the surface using a utility knife and hook blade. Then use a roofer tear-off shovel or pitch fork to level under and roll back into sections.

Installation involves three layers of modified bitumen melt-welded onto the roof surface and each other using a blowtorch. The felt is rolled out whilst heating the underside of the bitumen, allowing the felt to adhere with a strong bond.


If you are thinking of starting your own DIY flat roof project but are wondering where to start, why not check out our high-quality, affordable range of flat roof materials.

For any further questions or queries, don’t hesitate to get in touch with our award winning customer service team who will be more than happy to help. Give them a call on 01295 565 565, email [email protected], or just leave a message in our handy live chat.

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