If you are a looking to replace your roof and you are aiming for a flat roof, you are certainly worried about how much it is going to cost you. Fortunately, we share your concerns and this is why we have created this detailed cost guide.
Flat roofs date back to 2000 BC, during the Minoan period. In those days they were constructed of timber or thatching to block out bad weather, but now things are a little more advanced than that! Before you decide on installing a flat roof, it is worth considering the cost vs. maintenance factor – whilst flat roofs generally cost less to install, they often cost more in maintenance costs over the years.
On this page
- How to use this guide
- What are the benefits of a flat roof?
- What are the disadvantages of a flat roof?
- What types of flat roofs are there?
- How much does a flat roof cost?
- Flat roof cost
- Flat roof cost per square metre
- Flat Roof Prices – Examples
- Flat roof insulation
- GRP flat roofing
- Flat roof repairs & maintenance
How to use this guide
Even though this article is well researched and tries to provide as much as possible accurate estimations, flat roof prices will always vary depending on the local roofer’s rates. Please try to read carefully this article and use it mostly as a guide.
What are the benefits of a flat roof?
Cost: Constructing a flat roof usually takes less time and uses less materials than a traditional roof, which means it costs less to install.
Warmth: Flat roofs can keep your home considerably warmer than their competitors, especially during the cold winter months. This is because the flat surface area attracts sunshine which then penetrates into the home below.
Versatility: Versatile internal and external space. It can be used as a patio, garden, outdoor swimming pool or just a communal outside area.
Accessibility: Flat roof are more accessible, making it easier for new installations and repairs.
What are the disadvantages of a flat roof?
Draining: The biggest disadvantage to installing a flat roof is the danger of water and other debris gathering, which gradually can make the roofing material to break down or to crack.
Maintenance: This is a very important factor when considering whether to install a flat roof, as they require a lot of it.
Lifespan: The average lifespan of a flat roof is 20 years, and that’s only if it is properly maintained.
Materials: There is a lack of roofing materials available to use and the vast of majority of flat roofs use rubber, EPDM, TPO, or bitumen, which are relatively cheap and easy to install but have a short lifespan of 10 -15 years.
Styling: This is another limitation mostly because of the lack of materials. However, rubber singles can now be used in conjunction with rolled up roofing and several other materials such as polycarbonate roofing and PVC roofing, that can improve the styling of the roof at a higher cost.
What types of flat roofs are there?
Any material used to construct a flat roof must be watertight, because even though flat roofs are built on a slight incline, they are still very susceptible to leaks. That said, there are five main types of flat roof: roll asphalt, single-ply, multiple-ply, modified ply, modified bitumen and flat-seamed metal.
- Roll asphalt flat roof
Roll asphalt is made up of layers of asphalt-coated fiberglass, tar, felt and water resistant materials, all of which are kept in place with nails or asphalt cement. The top layer is covered with gravel, which protects the roof from ultraviolet rays. This type of flat roof has been used for over 100 years and is generally the cheapest; however it doesn’t usually last much longer than 10 years.
- Single-ply flat roof
Single-ply flat roofs are made from one layer of materials that are a bit like rubber. These materials cost less, but are studier and more energy efficient then roll asphalt roofs. Usually single-ply roofs are made from EDPM, uPVC, ethylene and TPO. These are all mould resistant and low maintenance. Single-ply roofs usually last for up to 12 years.
- Multi-ply flat roof
This type of flat roof is made up of lots of layers of fiberglass, which is packed between layers of asphalt. The big benefit of multi-ply flat roofs is that they are reinforced, so they last longer and are less likely to get damaged. They can last as long as 30 years.
- Modified Bitumen flat roof
Some multi-ply flat roofs use modified bitumen membrane, which is made from a mixture of polymers and asphalt which makes it flexible and water resistant. The top later is covered with gravel. These types of flat roofs could last up to 20 years.
- Flat-seamed metal flat roof
This is the most expensive type of flat roof, largely because it is made of metal such as copper and stainless steel. Metal flat roofs usually last between 20 and 50 years.
How much does a flat roof cost?
The price of flat roof replacement in the UK depends on factors like:
Flat roof specifications: Each job is different and presents different difficulties.
How accessible is the roof: The height of the roof and whether scaffolding is required or not.
The location: If the cost of living is significantly higher, then you should expect the charges to be higher too.
Tradesman Mark Up: Each business needs to make a profit. Also, if a tradesman has a very good reputation and is very busy, the prices are usually higher as they can choose the jobs at a certain extend.
The size of the project: For small projects the cost is usually higher per square meter, due to time saving and man power considerations.
Flat roof cost
According to our research the cost of building a flat roof in the UK ranges from £1,700 to £4,500, depending on the number of rooms and the type of property you own. Flat roof extensions cost from £700 (lowest possible) and can reach up to £3,000. For bay window roofs expect to pay from £350 – £900 and for dormer window roofs between £700 – £1,300. More details can be seen in the table below:
Cost of a new flat roof
|1 Bed House||2 Bed House||3 Bed House||4 Bed House|
|Main Roof (Cost of flat roof)|
|£1,700 – £3,000||£2,000 – £4,000||£2,500 – £4,500||£3,300 – £4,500|
|£700 – £1,700||£1,000 – £2,500||£1,300 – £2,700||£1,500 – £3,000|
|£350 – £650||£400 – £700||£450 – £750||£500 – £900|
|£700 – £1,100||£750 – £1,200||£800 – £1,200||£800 – £1,300|
|£300 – £500||£400 – £600||£450 – £600||£500 – £600|
Flat roof cost per square metre
Flat roof prices start from £40 per m2 for felt roofs. Rubber roofs costs start from £80 per m2. For fireglass expect to pay from £90 per m2 and for fire glass bay roof from £250 per square metre. Lead bay roofs are even more expensive starting from £300 per m2.
|Flat Roof Prices Per Square Metre|
|Job Type||Cost Per Square Meter||Specifications|
|Felt Roof||£40 – £50 + £40 if the boards are damaged||This is a modern felt roof or built up felt roof. It is also known as Torch-on flat roof.|
|Rubber Roof||£80 + additional cost if the boards are damaged||A Modern Flat Roof similar to a firestone that is also known as E.P.D.M.|
|Fire Glass||£90 (trims and lead flashing included)||A modern fibreglass GRP flat roof system with specialised roofing grade resins.Price does not include over boarding with OSB which is absolutely necessary to adhere a new fibreglass roof to it.|
|Fire Glass Bay Roof||£250 – £490 + 50 (stripping and insulating)||Even though the size is really small, the cost increases due to trim cost and time spent fitting.For approximately additional £50 the window can also be also stripped and insulated|
|Lead Bay Roof||£300 + lead costs||Prices can vary significantly due to different project specifications and the many different options of lead code and cost of raw material.|
Flat Roof Prices – Examples
Flat roof insulation
Heat rises and will escape quickly through an un-insulated roof. Installing insulation in your flat roof will help to prevent this from happening; keeping your home warm and saving you money on your fuel bills. Flat roofs are made up of a number of layers which are usually: the roof deck, a vapour control layer, a thermal layer, a waterproofing layer and a surface layer.
The layers that make up a flat roof
The roof deck can be made of metal, timber, concrete, composite or plywood and will support your flat roof. The vapour control layer can be made of polymeric foil, bitumen sheet, polyethylene membrane, or a self adhesive sheet. The thermal layer of the flat roof is used for insulation and is made of materials that trap air bubbles and air pockets in order to insulate the flat roof. Common materials used for insulation are mineral wool, fibreglass or polystyrene.
The waterproofing layer of the flat roof is commonly made up of single-ply membranes such as thermoplastic or elastomeric; uPVC or resin-surfaces are also an option. A surface layer of stone chippings, hard resin or mineral finish is often applied to protect the waterproofing layer. It can be difficult to know which is right for your roof but your roofing contractor will give you advice on what is needed.
Wood fibre roof insulation
Wood fibre insulation is an organic board made of wood, cane or vegetable fibres mixed with various binders and fillers. It can be asphalt impregnated or coated to improve its water resistance. It has a thermal resistance value per inch of 2.78, making it fairly mediocre as insulation for a flat roof but good value for money.
Perlite roof insulation
Perlite insulation is composed of perlite combined with organic fibres and binders. Air cells are trapped within the perlite, which provides the insulation for your flat roof. The insulation board is treated with an asphalt coating to limit asphalt absorption into the insulation. Like wood fibre, it has a thermal resistance value of 2.78.
Expanded and extruded polystyrene roof insulation
Polystyrene insulation for your flat roof comes in two forms – expanded and extruded. Expanded polystyrene is made up of polystyrene mixed with a foaming agent. When exposed to heat the material expands and traps warmth. It has a thermal resistance value of 3.80. Extruded polystyrene is made of polystyrene that is heated, exposed to atmospheric conditions and then expanded. Its thermal resistance value is 5, making it a good choice for your flat roof insulation.
Polyisocyanurate roof insulation
Polyisocyanurate insulation is sandwiched between organic and non-organic felt within your flat roof. A blowing agent such as pentane is used to expand the foam material which gives it its high thermal resistance of 6.0.
Cellular glass roof insulation
Cellular glass insulation for your flat roof is composed of crushed glass that is placed in a mould and heated to a high temperature. It is available in flat or tapered blocks and provides a thermal resistance of 4.69.
Living, green roofs
“Cool” flat roof : Although you may not have considered it before, a flat roof covered in grass and plants is a quirky and very efficient way of insulating and protecting your home. Green flat roofs have increased insulation because of their thickness and also keep your home cool in summer. Although they reduce your energy costs as well as being environmental, they can be expensive to install as the roof may require fortifying to deal with the extra weight. However, they soak up water better and reduce run off by 50%, reducing the chances of leaking from your flat roof. Green roofs are said to be able to last twice as long as a normal roof because of this.
You may want to consider a “cool” flat roof if you live in a hot area. The dark, heat absorbing materials such as tar and asphalt that are usually used in the construction of a flat roof can soak up the heat from the sun and transfer it down to your home. A cool roof is made by applying a reflective coating and can reduce the summer cooling costs of your home by 70%. You will also get a tax break if your cool roof meets Energy Star standards.
Flat roofs are sensitive to human traffic and are liable to split or crack if they are trodden on, leading to leaks. When having a flat roof installed it is highly recommended that you hire a certified professional because of this which also ensures your roof is safe and efficient.
GRP flat roofing
GRP is a popular flat roofing material and it is commonly used in order to ensure flat roofs remain leak-free and up to scratch.
What is the difference between GRP and fiberglass?
GRP (Glass Reinforced Plastic) is the technical name for what is more often known as fiberglass. It was originally developed back in 1938 for use in insulation, however it is now used for a variety of purposes in many different industries. Also sometimes referred to as FRP (Fibre Reinforced Polymer) GRP is extremely versatile as well as being hard-wearing and waterproof. These qualities make GRP perfect for use as a roofing material. If you are concerned about GRP’s appearance then rest assured: it can be made in a range of different colour options. It can also have special finishes applied, such as a non-slip surface. This all means that GRP is now a feasible alternative to more conventional roofing materials.
How long will a GRP flat roof last for?
A lot of tests and research have been carried out regarding the life expectancy of a GRP roof and studies have estimated that they can last somewhere in excess of 50 years. This is proven by the fact that many glass fibre laminates, which were manufactured back in the 1940’s, are still used today. Research has shown GRP doesn’t really deteriorate, in fact in the testing periods that have been trialled, it has remained in the same condition throughout. Some experts would even argue that GRP products applied by a specialist company could potentially last a lifetime.
What colours does GRP come in?
Thanks to advances in recent technology, GRP can be manufactured in a range of different colours, with something to suit every need.
What are the benefits of GRP for flat roofing?
Whilst GRP is a useful material for all roofs, it is particularly beneficial to flat roofs because:
Easy installation: GRP products are cold-laid (this means that they do not require any type of heat treatment and can just be laid as they are). This eliminates a certain level of risk to both the GRP roofing installers as well as anybody else on the site.
Low insurance: because GRP roofing systems are cold-applied they do not attract the high levels of insurance premiums that other roofing materials which require hot installation do.
Durable & waterproof: GRP flat roofs are not only very durable but they are also 100% waterproof, this gives them the edge over other materials that may have been traditionally used for flat roofing.
Low maintenance: Due to the nature of GRP, the entire GRP area of the roof is one huge GRP membrane; this eliminates the need for seam joints or welds, along with the consequential maintenance.
UV resistant: GRP roofing is entirely UV resistant.
Adaptable: GRP forms a very adaptable roofing system, which easily allows for new features such as balconies or a walkway on the roof area. You could also easily add extras such as roof lighting.
Leak-free: The overall and largest benefit of having a GRP roof installed, however, is that as long as it is correctly fitted it should remain leak-free for life. A particular advantage to those with flat roofs, who may encounter leaks more frequently.
Can GRP be fire retardant?
If fire safety is a must in your household then you can use GRP which is formulated with special fire retardant resins and topcoats.
What does the end result look like?
The end result can look however you choose, as the surface and colour of a GRP laminate can be altered to suit your requirements. You could opt for a smooth or textured finish, as well as choosing from a wide array of different colours.
Is GRP ok in serious weather conditions?
GRP is perfect for those who frequently encounter adverse weather conditions and temperature extremes alike. GRP is often used on boats, which shows that it performs well when put in cold climates and/or rain.
Flat roof repairs & maintenance
If your flat roof is need of repair, you will first need to locate the problem. Usually the first signs of a problem in a flat roof can be found in the room directly below; either damp patches or water dripping from the ceiling will be visible. Before climbing up to have a look though, think about the strength of your flat roof; it is old and has been damp for a while it could be rotten in areas and very dangerous. You should always hire a professional roofer to inspect and repair your flat roof safely.
A roofer will inspect the condition of the surface of your flat roof, looking for tears, puddles of water (refered to as ponding), holes or bare patches. The leak could also be coming from blocked drainage around the edges of the roof. If your flat roof is in a very poor state, your roofer may recommend replacing the roof entirely.
Hire a professional to repair your flat roof, which can be dangerous and cause structural problems if not carried out correctly. Before any repairs are carried out, a professional roofer will make sure the area is clean, removing any chippings and using a fungicide to get rid of any mould growth. It’s best to carry out repairs after a long dry spell of weather, meaning there is less moisture in the roof. Below is a guide to how a professional will repair a few basics in your flat roof:
Blisters in the felt
A roofer will clean the surface over and around the blister, making sure there are no chippings there and ensuring all mould and dirt is removed. They will use a knife in order to a make a cross cut through the top layer, being sure not to press down too hard. Peeling back the segments created will allow the roofer to dry the area inside, perhaps using a heat gun. Once that is down, they will apply some bitumen adhesive to the surface and then put the segments back and press down to ensure it stays. The roofer is then ready to apply new pieces of felt, making sure there’s sufficient overlap to cover the repaired blister.
Splits and tears
If the roofer can lift the split or tear without damaging the felt, then they might do that, cleaning the space underneath it. They will then fill the space under the lifted split or tear with adhesive and press it down. Any larger spaces can be filled with bitumen compound.
You will find the sealing flashing (usually a zinc or substitute strip) where the flat roof touches the wall. The best way to repair this is to use one of the self-adhesive repair tapes. A roofer can do this by first of all cleaning the surface, cutting the tape to length and then applying it to the area, making sure that the overlaps are well sealed at either end.
Flat Roof Cost