Flat roofs are one of the cheapest and easiest roofs to construct. You can use them on outbuildings like sheds and garages or habitable structures like extensions and dormer windows. Also, you’ll benefit from the lower construction costs compared to other roof types. But, a roof of this type must comply with the UK Building Regulations.
The average cost of replacing a flat roof depends on several factors. However, an estimate would be from £40-£110/m2, mainly dependent on the covering material.
This guide will show the prices of different materials, the factors affecting them and the signs that your roof needs replacing. Furthermore, we’ll consider the various regulations restricting construction and how to choose a reputable roofer capable of building a compliant flat roof.
Main Topics
How Much Does A Flat Roof Cost?*
The table shows data for four popular flat roof coverings and compares their typical price ranges for four different-sized flat roofs and their common applications.
| Flat Roof Type | Price/m2 | Roof Area | Typical Range | Average |
|---|---|---|---|---|
| Felt/Bitumen Roof | £40-£60/m2 | Porch 3m2 | £120-£180 | £150 |
| Shed 6m2 | £240-£360 | £300 | ||
| Garage 15m2 | £600-£900 | £750 | ||
| Extension 20m2 | £800-£1,200 | £1,000 | ||
| Fibreglass Roof | £70-£100/m2 | Porch 3m2 | £210-£300 | £255 |
| Shed 6m2 | £420-£600 | £510 | ||
| Garage 15m2 | £1,050-£1,500 | £1,275 | ||
| Extension 20m2 | £1,400-£2,000 | £1,700 | ||
| Lead Bay Roof | £90-£110/m2 | Porch 3m2 | £270-£330 | £300 |
| Shed 6m2 | £540-£660 | £600 | ||
| Garage 15m2 | £1,350-£1,650 | £1,500 | ||
| Extension 20m2 | £1,800-£2,200 | £2,000 | ||
| Rubber Roof | £80-£90/m2 | Porch 3m2 | £240-£270 | £255 |
| Shed 6m2 | £480-£540 | £510 | ||
| Garage 15m2 | £1,200-£1,350 | £1,275 | ||
| Extension 20m2 | £1,600-£1,800 | £1,700 |
*We compiled the data in this guide from primary and secondary sources, with the data being correct at the time of writing (October 2022). The prices are estimated averages and are subject to change depending on several factors we discuss later. For accurate quotes, contact a local professional flat roofing company for fees based on your circumstances after an on-site assessment.
9 Flat Roof Pricing Factors
Pricing a flat roof should consider several factors that happen in the real world. Factors such as area, location and material affect the roof’s price, and you should include these in your research after a professional assessment or survey of the existing site conditions.
1. Type of roof
The most important price factor is the type of roof or roofing material. Typically, top-quality felt roofs cost about £60/m2, rubber roofs cost £90/m2, fibreglass roofs cost £100/m2, and lead roofs cost £110/m2.
2. Roof purpose
Consider the purpose of the roof when you ask for a quotation. Although you don’t want shed and garage roofs to leak, you can use cheaper materials than a dormer or kitchen extension roof, which protect your living space. You’ll find that most garages and other outbuildings have a felt flat roof (£40-£60/m2) while living accommodation uses good quality rubber (£80-£90/m2) or fibreglass (£70-£100/m2). Because of its weight per square metre, people often use lead (£90-£110/m2) over small areas such as dormer windows or porches. However, there are exceptions, and you can satisfactorily use any of these roof coverings for any size roof for whatever purpose you like, as long as you follow the approved building practices for the material.
3. Area
Large roof areas are more expensive to waterproof than small roofs because of using more materials and taking longer to do the work. For example, a 3m2 felt flat roof costs £120-£180, whereas a 20m2 costs £800-£1,200. However, larger roofs cost less per square metre than small roof areas, mainly because of savings in bulk purchases and the standing charges that roofing companies invoice their customers. Typically, you could see a 5% reduction in overall cost for a 100m2 flat roof compared to a 10m2 area.
4. Accessibility
Constructing a flat roof two or three storeys high costs more than a single storey. This is because of the issues with accessing the roof. Although you can climb a ladder to access two or three storeys, you must, by law, use scaffolding as a working platform when operating at this height. Hiring enough scaffolding for a working platform around a typical flat roof costs around £1,000. Alternatively, you can rent DIY scaffold towers that slot together for up to £150/day. The main issue with these is that you must continually move the tower to different locations. In contrast, the professional hire provides access to the entire roof, especially if it’s difficult to reach.
Equipment and materials must also be delivered, requiring vehicle turning space and access for mechanical lifting equipment. Where the roof is will determine whether you need to remove fences and gates, enlarge driveways, provide better side access to the property, and reinstate the original access on completion. All these factors increase the costs
5. Removing an existing roof
If you’re building a new extension, porch or whatever, you won’t have the expense of removing the original roof. But, if you’re replacing it, the first thing is to remove the old one.
The amount of time needed depends on the original’s age, material, and the tools needed for the removal. For example, removing an old felt roof usually requires a small blunt blade such as a spade. In comparison, a rubber roof requires a sharp-bladed utility knife to cut the old rubber into manageable-sized pieces. Finally, the lead roof needs a grinder with a diamond-tipped blade. Therefore, using various removal tools for the different materials affects the job’s overall cost.
6. Waste disposal
Replacing an old flat roof produces a lot of waste which needs proper disposal. Therefore, you need a waste skip hired from a registered waste disposal company. The skip’s size depends on the roof area and the amount of old covering material. However, most individual flat roofs would fit into a mini skip costing £90-£150 or the next size up, which costs £100-£250.
Many old flat roofs used corrugated asbestos cement sheets, which are carcinogenic if the sheets break and the fibres become airborne and inhaled. By law, you must use a registered asbestos handling and disposal company. The UK government has an extensive website detailing the available information about asbestos. It recommends contacting your local council for advice on the many removal and disposal regulations, and information on local asbestos handling specialists.
Usually, you need a permit to park a waste skip on a public road or other communal areas. Generally, however, the skip hire company will organise this for you and charge you the cost of the permit. Every council has a different permit charge. Therefore, for more information on fees, contact your local council.
7. Contractor markup
All companies have overhead costs they must pay from their business. Typically, contractors factor these overheads into their prices. But, you’ll find that larger companies have higher overheads, making them more expensive to hire. However, selecting the cheapest quotation is not always a good idea, as standards are often significantly compromised if a contractor charges unusually low prices. Therefore, selecting a company offering a mid-range price is better.
8. Location
Where you live always affects the price you pay:
Rural areas
Often, rural areas have a low population density, so there will be fewer roofing contractors nearby. Usually, this results in higher prices.
Travelling
Suppose a contractor travels a significant distance to reach your property. In that case, they often charge a fee based on travelling time and additional fuel. Therefore, a long drive could cost you more. However, most contractors have a specific radius, inside which they don’t charge a travelling fee. They set a sliding scale outside this radius, depending on the distance from their base.
Cost of living
Living in a city or affluent area is usually expensive, with higher living costs increasing wages locally. As such, you’ll find that prices could be higher than expected. In the extreme, London and Southeast England have the highest cost of living in the UK and, subsequently, the highest wages. Often, salaries can reach up to 15% more than elsewhere in the country.
9. Additional costs
Installing a flat roof doesn’t only need the covering. Several other materials are required:
- Wood fibre insulation board – £5-£15/board
- Foam insulation board – £5-£35/m2
- Chippings – £5-£10/bag
- Deck boards – £60-£140/board
There are more, but the materials used depend on your flat roof’s type, size and quality.
4 Signs Your Flat Roof Needs Replacement
You might decide to repair a flat roof. But, this isn’t good practice as once the waterproof covering begins to deteriorate enough to cause leaks, the remainder of the roof isn’t far behind. Therefore, replacing a flat roof as soon as problems start is always a good idea. Let’s look at some obvious signs that the roof is deteriorating.
1. Leaks
Leaks are one of the commonest signs that your roof is on its way out. Roofs keep water out of your home and protect from wind, rain and cold. Therefore, the roof has a problem if you see signs of water inside the house. But, it’s not always obvious where the leak is. There could be a crack, split, hole or blister on the roof, or the adhesive might deteriorate, which is common with felt or rubber roofs. Sometimes, you can see the problem and temporarily fix it with a sealant. But, if you have fibreglass or lead roofing, the leak might be from an invisible hairline crack.
2. Puddles
The slight slope of a flat roof ensures that rain sheds from the surface. However, the wooden boards or supporting joists become damp and sag over time, forming indentations across the surface where water accumulates and pools. Often, these puddles speed up surface deterioration, and the underlying structural movement causes surface cracks. Therefore, the puddles indicate that you either have a leak or will soon have one, and the wooden structure has started to collapse. Then, it’s time for a new covering and probably new structural supports, or you will have a severe leak or roof collapse.
3. Algal stains
Over time algae settles on the roof and colonises the surface. Unfortunately, they don’t only cause unsightly discolouration; they also start to react chemically with the exterior covering. If you don’t do something soon, you might develop leaks.
4. Moss
With algae comes moss. Moss is a primitive plant that latches onto a damp roof, usually on a north-facing or tree-shaded surface. Their roots burrow into tiny cracks and spaces, eventually making them broader and deeper. Also, moss maintains high moisture levels on the roof surface and produces acidic decomposition products, often affecting waterproofing. Generally, moss is easy to remove but beware of damaging the roof’s surface when you do.
Planning Permission & Building Regulations
Flat roofs are subject to specific regulations.
Planning permission
If you are repairing or recovering an existing roof, you don’t need to apply for planning permission, as it comes under “Permitted Development“. However, if you’re changing an existing roof with structural or size alterations, you must apply for planning permission before beginning work.
A new roof doesn’t usually require separate planning permission, as the submission for the complete structure includes the roof. Look on Planning Portal for details of planning permission for a roof
Building regulations
Several UK Building Regulations govern the construction and replacement of a flat roof. However, the rules recognise a significant difference between a roof on a habitable and a non-habitable structure, with the habitable building having stricter controls. Typically, this is the difference between an extension or dormer roof, and a garage or shed.
The regulations for habitable buildings include the following:
- Insulation and thermal elements – Approved Document L
- Structure – Approved Document A
- Fire Safety – Approved Document B
- Resistance to moisture – Approved Document C
- Ventilation – Approved Document F
- Rainwater removal and drainage – Approved Document H
- Positioning of rooflights and windows – Approved Document K
- Removing excess heat – Approved Document O
- Quality of materials and workmanship – Approved Document 7
Read the information supplied by the government before starting on new or existing flat roof work. Probably, some won’t apply to your project. But, if you’re confused or want more information, contact the building control office in your local council.
What To Ask Your Roofing Contractor
Contractors must build flat roofs to comply with UK Building Regulations, requiring qualifications and experience. Therefore, asking your contractor a few questions before accepting their quotation makes sense.
- What qualifications do they have? Typically, a Level 3 NVQ Diploma in Roofing Occupations is a good example.
- Do they have references? Reputable contractors often keep a portfolio of photographs and contact details from previous customers. Follow up on the references and see the work for yourself.
- Does the contractor rely on “Word-of-Mouth” or newspaper advertising? Although both methods are acceptable, word-of-mouth will get you someone with a solid customer base.
- What experience do they have? Depending on your budget, hiring someone with at least two years of experience is advisable.
- Do they belong to the National Federation of Roofing Contractors (NFRC)? And are they certified under the organisation’s Competent Roofer Scheme?
- Alternatively, the UK government and local councils use a general Competent Person Register.
The NFRC and UK government competent person schemes provide a list of people whose work standard is good enough to self-certify as complying with the building regulations without inspection by the local authority.
FAQs
How often does a flat roof need to be replaced?
Replace flat roofs when they become defective, and start to leak. Generally, this varies with the material, construction procedures, and work quality. Typically, a felt-covered flat roof lasts up to 15 years. In contrast, a fibreglass roof lasts about 20 years. Next, more durable rubber often has a life of 40 years. Finally, a flat lead roof lasts 60 years or longer.
Does building insurance cover flat roofs?
If the flat roof damage happened because of an accident, you should be covered by your building insurance policy. However, damage due to normal expected wear and tear, or poor maintenance probably won’t be acceptable. But, to be sure, read your building insurance policy to determine the coverage.
What qualifies as a flat roof?
Many people don’t realise that a flat roof isn’t genuinely flat or, should we say, horizontal. The term “flat roof” covers all roofs with a low angled pitch, covered in a continuous membrane, rather than individual tiles, shingles or slates. The accepted slope gradient should be around 1:80. But, most roofers work on the easy measurement of 1cm fall over 1m or 1:100. This gradient allows the rainwater to run towards the guttering on one or two edges.
To End
Many people use flat roofs on extensions, garages and sheds to provide weatherproofing without the overbearing height of a pitched roof. They are straightforward to construct while complying with UK Building Regulations if they cover a habitable area. Therefore, it’s essential to use a qualified and experienced roofer, certified under a competent person scheme.
Complete the form on this page, and you’ll receive up to four quotes for a flat roof from a local contractor.