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Flat Roof Costs Explained

A flat roof can be a great and inexpensive way to complete a new single-storey extension, ideal if you are working to a tight budget or want to keep funds to spend on other elements of the build. Perhaps you want to replace an existing flat roof on a part of your home or outbuilding or you want to have a covered area at the back or the side of your house.

The flat roof cost per m2 on an extension or new build is £1,500; this figure includes the materials, preparatory work and all the labour charges plus any finishing and VAT.  £1,500 is a countrywide average so you may find in some locations that your new roof comes in under this figure.  In London and the south-east, the cost is always higher and the flat roof cost per m2 will be nearer £2,000 on average.

Last Updated: October 2022

How to work out the flat roof cost per m2?

To work out the area on your building just measure the length of the roof by the width and multiply the two figures together to produce the total square meters.

Is the flat roof cost per m2 the same for a new roof as the replacement of an existing roof?

The flat roof cost per m2 will be similar for a roof on a new build as it is to replace an existing worn out flat roof with perhaps a slight increase if you have to dispose of an old roof as part of the project for which there will be an additional charge. Let’s look at the flat roof cost per m2 in more details.

Expect to pay for:-

  • Materials
  • Labour to construct the frame on a new build and then fit the roofing material
  • Disposal costs of an existing flat roof
  • Reinstatement of the frame on an existing roof if it is not in good repair

What are the advantages of a flat roof?

For most householders, the most obvious upside to a flat roof is the cost per m2.  There is much less risk in installing flat roofs so the labour is cheaper; they are quicker and easier to construct so the labour costs are reduced twofold.  There are other advantages to a flat roof some of which will also impact your pocket and these are:-

  • Flat roofs can create single storey covered areas where a pitched roof would not be possible as it would interfere with the accommodation on the first floor
  • Flat roofs are easy to clean, maintain and repair with many simple tasks within the reach of the householder –literally - as they provide easy access often with just a ladder.  This can save a lot of money on regular maintenance and repair costs as well as general cleaning and weatherproofing.  Flat roofs are less complicated than the structure and materials of a pitched roof so not only is the initial flat roof cost per m2 lower but repairs are usually much cheaper in comparison too

What type of materials can you use for a flat roof?

The choice of roofing material can influence the flat roof cost per m2 and there is a huge choice of flat roofing materials available.  Options include liquid rubber, asphalt and torch-on felt.  There are next-generation products that not only look attractive but last far longer than the old-style flat roof materials.

How often should you replace a flat roof?

This depends on the integrity of the original structure, the quality and type of materials used and whether the roof has been well maintained and kept in good repair throughout its life.  A common criticism of flat roofs is that although they are much cheaper to install, they need replacement far more often than a pitched roof which theoretically should last for decades.

A good quality flat roof will easily last about ten years.  As well as the lower initial flat roof cost per m2, you can make your money go much further if you re-seal the roof and look after it properly.

When to coat a flat roof?

The main reason to recoat a flat roof is for water protection; if water builds up and ponding occurs then the integrity of the roof is compromised and water can leak into the interior below causing damage.

It is a reasonable expense to reseal the roof every five years to prevent this from happening.  A flat roof left unsealed can have its lifespan reduced by up to 50%.  This is not a cost normally associated with a pitched roof which will naturally have a much better drainage angle but even with a five year reseal, the initial flat roof cost per m2 and the maintenance will still be less overall than a pitched roof alternative.

What are roof coatings?

Roof coatings are essentially a coating designed to renew the surface of the roof.  Generally, these coatings are made from materials that are durable against heat and other elements and resistant to impact.  A flat roof coated at the appropriate intervals can last for decades and make your money go much further reducing the flat cost per m2 even further.

Does the local climate make a difference to the integrity and longevity of a flat roof?

Without question, a location that is exposed to severe weather is going to take a much greater toll on all aspects of a building and not just the roof.  Harsher climates or locations which are not sheltered from the prevailing weather will call for more frequent roof sealing.

Bad weather – and this is not just wind and rain but also long periods of hot sunshine – can break down the sealant on a flat roof much faster than temperate climates or locations which don’t have intense sun or heat and high rainfall.  Repetitive cycles of rain and sunshine can cause the roofing sealant to expand and contract repeatedly which will require more frequent recoating in the same way that wooden windows and doors will probably need more regular maintenance and repainting.  But even the expense of recoating the roof every five years plus the initial flat roof cost per m2 will be cheaper than installing a pitched roof at the outset.

How to tell when a flat roof needs coating?

Householders can be very proactive with this rather than relying on roofing companies that may be keen to increase their business.  The great advantage of a flat roof is that you can see for yourself what state the roof is in.

Undertake regular inspections when you clear guttering and rainwater goods.  Look carefully at the surface of the roof and check for any signs of leakage or ponding.  Check when it is raining is the best option – also keep an eye on the interior of the building, the ceiling inside the rooms, the tops of the walls and the tops of the windows.  Rainwater that is ponding will cause staining and damp patches.  Even if you don’t think the roof is in bad repair, it should still be routinely resealed every five years to safeguard it and protect your investment.

Does resealing a flat roof always work?

If a flat roof has major issues or structural damage then resealing it won’t improve either its water-repellent abilities or its longevity; there may be no option other than a full replacement.  Don’t throw good money away by resealing a roof that needs replacement – get the roof assessed by a professional.  If you are buying a house that has a flat roof on part of the building then pay close attention to the age and condition; if it looks like it needs replacement then you can negotiate with the vendor on the sale price.

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What are the different types of roof coating?

Each coating has strengths and weaknesses, advantages and disadvantages.  Always take professional advice if you are unsure about the right type of coating to use even if you intend to do the job yourself.  Cheaper coatings may seem more attractive initially but might not offer such good longevity.

  • Elastomeric – a popular coating in sunny spots as it reflects the UV rays, it is also cost-effective and helps insulate the building thus saving energy costs
  • Silicone – a very versatile roof coating and incredibly durable with a very reasonable price point.  Made from silicone polymers, it is UV damage resistant.  It is bright white in colour hence its ability to reject and repel heat
  • Acrylic – acrylic is unique because it allows colour customisation.  It is a water-based coating made with acrylic resin and it handles the constant expansion and contraction associated with temperatures changes with consummate ease
  • Urethane – urethane roofing is becoming more popular as it has high tensile strength so it is good for roofs that are regularly walked on, a factor if you are thinking about a first-floor balcony above a flat roof extension.  Urethane is a more expensive option and may require professional installation

Are there any other benefits to recoating a flat roof?

A coated roof will contain the temperature better in the accommodation beneath it.

How to save even more money on a flat roof?

The flat roof cost per m2 is already a big attraction to the householder looking to extend their property but what can you do to drive the cost down even further?  Here are some suggestions:-

  • Write a maintenance plan for your current flat roof and keep it in the best repair possible.  Regular checks will spot small problems before they go beyond the point of no return which could mean a new roof.  Most simple repairs can be easily undertaken by the householder
  • Act quickly on any necessary repairs and damage
  • Remove trees or overhanging branches to prevent the surface of the roof from becoming scratched or damaged
  • Recoating the roof will maintain better heat in the rooms below and help with energy bills
  • Take professional advice quickly if you have a serious repair or suspect that the roof may need replacement

Frequently Asked Questions

When does a flat roof become a sloping or pitched roof?

The definition of a flat roof is a roof with an angle of less than ten degrees – any slope greater than this is classified as a pitched roof.

Do I need planning permission for a flat roof?

Planning permission will usually cover all aspects of a new build and not just the roof.  There can be an issue if you live in a conservation area or have a listed house and want to build an extension or separate building with a flat roof that is not in keeping with the style of the house and/or the surrounding properties.  Equally, you may not be allowed to replace a pitched roof with a flat roof on an existing building if the property is listed.

If you are building a single-storey extension that does not extend to more than three metres beyond the rear wall of the house then you may not need planning permission for this build at all.

What are the disadvantages of flat roofs?

The main drawback for many people is their appearance – they lack curb appeal and sometimes they just don’t look as stylish as a pitched roof alternative.  Flat roofs also lack the extra living or storage space within the building which can provide additional room height or a loft area.

Flat roofs do not drain as well as pitched roofs for obvious reasons but this can be taken into account when the roof is installed and managed with appropriate guttering and rainwater goods.

Is there such a thing as an eco-friendly flat roof?

There is a material called EPDM or Ethylene Propylene Diene Monomer which is a synthetic rubber membrane which as well as being environmentally friendly is also a great economical choice.

A lot of the EPDM membranes used in the UK have come from recycled materials such as car tyres – this is the first big tick for green credentials as otherwise these products are disposed of via incineration, releasing harmful gasses into the environment.  When the roof has come to the end of its life, it can be recycled back into the system too.

EPDM is one of the most effective and enduring flat roofing materials with low maintenance and easy repair so your eco-friendly roof can save you money too.  The membrane is non-toxic and therefore excellent for harvesting unpolluted rainwater for use inside and outside the house.  None of the adhesives used to bond the rubber to the boards creates toxic emissions.

EPDM is also much safer than traditional flat roofing systems as it is free of any naked flame or hot tar so does not present any fire safety issues.

Flat roofs used to suffer from a bad reputation but a new generation of materials can now produce roofs that not only last a long time but look good.  The flat roof cost per m2 means that flat roofs are one of the most economic choices on a new building or extension and have the added advantages of speed of installation and easy maintenance for the householder.

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