Flat roof replacement costs explained: budget your project with confidence

While flat roof replacement costs can vary widely depending on a number of factors, such as the materials you are using, the size of the roof you are covering, who is carrying out the work and where in the country you live, it is still possible to get a good idea of what this job is likely to end up costing you. 

Flat roofs can require replacing for a number of reasons, although it must be said that modern construction methods and materials have made it much less likely for them to suffer from leaks or to be as poorly insulated as they once were. 

Here, we take a look at the kinds of figures you can expect should your old flat roof need replacing so you go into the project knowing exactly what lies in store. 

Flat roof replacement costs vs repair costs

Many homeowner with a flat roof in a poor state face the dilemma of whether to attempt to repair what they already have or to opt for a full replacement. 

"If your home does have a poorly fitted flat roof, its replacement will probably be much more cost effective in the long run than repairing it," says author of the Housebuilder’s Bible and experienced builder Mark Brinkley.

With this in mind, in this guide we'll take a look at the cost of buying new materials and the work involved in replacing these types of roofs. 

Mark Brinkley

Mark is the author of the ever-popular Housebuilder’s Bible and an experienced builder. He has written for publications such as Homebuilding & Renovating for over three decades. An experienced self builder, his latest self build, a contemporary eco home built to Passivhaus principles, was created on a tight urban brownfield plot.

How much does a new flat roof cost?

The first question those looking to replace their flat roof will be likely to ask is how much does a new roof cost? 

When it comes to construction costs, Tim Phillips, experienced senior quantity surveyor and estimator, advises being prepared for figures of upwards of £1,500.

"The construction cost of flat roofs is typically between £1,500 and £5,500," says Tim. "The price will vary depending on the type of flat roof you choose and it will also vary based on its size."

While roofer's rates will vary a little, it is possible to get a rough idea of labour costs for installing a new flat roof. 

"Depending on the size and complexity of the job, you can expect to pay between £25 and £35 per hour for a roofer," advises Tim. "If the job is large, you may be quoted a day rate, which will typically range between £200 and £280, depending on your location and the level of expertise required for the project."

Tim Phillips

Tim is a quantity surveyor and runs Quantiv.uk, with almost 30 years of experience across the commercial and residential sector. He also regularly writes articles for Homebuilding & Renovating magazine and website. 

How much do materials for flat roofs cost?

Of course it isn't just construction costs that you need to budget for when it comes to your new flat roof — the materials you choose will also play a huge part in what you finally end up paying, as well as how long the roof lasts and the kind of look and performance you can expect. 

"In flat roof construction, four materials are most often used," says Tim Phillips. "Felt, single-ply membrane i.e. EPDM, fibreglass/GRP and also lead."

"Generally speaking, the issues to watch out for are the cost and the guarantee offered," advises Mark Brinkley. "As a rule, don’t go for anything less than 20 years in terms of a guarantee. Costs vary from around £35/m2 up to £80/m2."

Although felt still tends to be the cheapest type of flat roofing material, it is a product that has seen considerable development in terms of performance over the years and as such will not always be the least expensive option. 

"It remains potentially the cheapest covering but there are several different grades you can specify," explains Mark. "The best felt systems are now comparable with any of the alternatives performance-wise but cost just as much."

Flat roof material costs compared

Which factors could push costs up?

While it is all well and good offering average and starting from prices for flat roof replacement, there will always be circumstances where certain issues may arise to push costs up. Being aware of what these could be should prepare you for the unexpected. 

"Opening up an existing roof may uncover rotten timbers or perished felt (which was used predominantly in the pre-1990s, before the introduction of breathable membranes)," warns Tim Phillips. "And timber rafters and wall plates may need renewing, too. The main cost factor will depend on the existing type of roof and the material that has been used to cover it."

Other hidden costs to be aware of include:

• Roof removal: Don't forget to factor in the costs of skips and waste disposal for the old roof. 
• Asbestos removal: Some old flat roofs have been found to contain asbestos. "Before any work can begin, a survey must be conducted to determine the location and condition of the asbestos roofing materials," explains Tim Phillips. "If asbestos roof tiles are identified, a licensed contractor must be hired to carry out the removal process. The removal of asbestos roof tiles is a highly regulated process that must be carried out by licensed professionals who have received specialised training and certification. Allow a minimum of £1,200 for the works."
• Complex roof structures: In some cases a house will feature both a flat roof as well as a pitched roof, and in these instances costs are likely to be higher. "Where a flat EPDM roof adjoins a slate/concrete tile pitched roof, this will require weathering and also the possibility of expansion joints," points out Tim. "The materials will move differently over time so make sure this detail is allowed for in the quotation."
• Location: The area you live in will most certainly have an effect on the quotes you receive — London and the surrounding areas still tend to be amongst the most expensive. 
• Scaffolding: If your project requires scaffolding, ensure this has been included in the quotes you get back.
• The addition of glazed features: If you plan on fitting a roof lantern, rooflights or areas of glazing, you are going to be looking at higher costs than if not. Not only will you be paying for the glazing/windows themselves, but labour costs are likely to be higher too. "Dormers, rooflights, roof lanterns, sun pipes, etc. may require re-weathering with lead, cladding or EPDM," says Tim.

Whether you urgently need to repair a roof or simply want to factor a flat roof extension into your future renovation plans, making sure you are up to speed with the likely costs will ensure you can get a proper handle on the budget before work starts.