Cost of Replacing or Repairing a Flat Roof

All you need to know about replacing/repairing a flat roof including costs of materials, costs of labour and time frames.

replacing a flat roof

What the job entails

Flat roofs send some people into a panic, some will not buy a house with a flat roof or will spend a fortune having it removed and a traditional pitched roof fitted - but are they really that bad? The short answer is, no! In fact a professionally sealed flat roof should last 20 years or more without leaking, so the worst case scenario for most of us would be having to replace it 3-4 times at most, with the total cost still being much less than building a pitched roof (on average you could completely replace a flat roof around 15 times to spend as much as one single pitched roof build).

But no matter how professionally a flat roof is fitted, there will come a time when it begins to fail, leaking at first, but eventually bowing in and in danger of collapse. When you see signs of leaking, you should consider repairing or replacing the roof, putting things off will not fix the problem, but will in fact make it worse and more expensive! As to whether you should repair or replace the roof, that depends on the roofing material and the amount of damage. Funnily enough flat roofs are generally built with a slight incline to allow rain water to drain off into gutters, so basically most flat roofs aren’t actually flat!

Flat roofs are made up of roofing timbers or joists which are are laid across walls at a specified distance apart, then they are covered with some sort of boards or sheeting that usually need to be waterproofed. There are many different types of flat roof materials, but felt, EPDM or rubber and fibreglass are the most popular types. Typically a repair job will involve essentially re-waterproofing a section of the roof, whereas a replacement roof means stripping away the roof covering and replacing it with a new one, which my also involve new joists depending on the damage.

When replacing a flat roof, it makes sense to fit new fascia boards at the same time, as these will have to removed anyway so you are already paying the labour for half the job anyway! In addition, a flat roof that has been leaking for some time, may also need the roof joists themselves treated or even replaced if they are damp and rotting. It obviously makes no sense to fit a new roof on top of rotting joists! So a joiner or carpenter may be required, along with a decorator to finish the interior.

Some flat roof repairs can be done on a DIY basis, there are a number of waterproofing and sealant materials which can make effective though temporary repairs, giving you time to gather quotes and save up for longer term professional repairs to be carried out. You can even buy paint on "waterproofer" which is emergency roof repair in a tin! But if damage to the roof is localised to just a small area, it may be possible to effect a permanent longer term repair, rather than replacing the entire roof. However, if you cannot fix the roof material yourself on a DIY basis, it will probably not be cost effective getting a contractor to carry out repairs, as for a little extra, you could have a complete new roof.

In many cases making repairs to a flat roof is a complete waste of money, for example if the roof timbers underneath are damaged. In addition on certain types of roofing material it is difficult to patch small damaged areas without damaging the rest of the roof, thereby making things even worse, falling through the roof when attempting repairs is not unheard of with DIY either!

Cost of Replacing or Repairing a Flat Roof

The average cost of replacing a flat roof is usually in the range of £700-£1300 depending upon what materials the roofer uses, whether or not scaffolding will be needed, and the size of the roof.

Most roofers operate in pairs and will usually charge around £200-£300 per day. Usually the job will be completed within a single day, and the roofer should always come back the next day to ensure there are no leaks.

Here are a few average costs for replacing a flat roof:

Job Size Scaffolding? Avg. Cost Duration
Single Garage No £700 1 day
Double Garage No £900 1 day
2-storey Extension Yes £1200 1.5 days


Cost Breakdown

Individual costs for replacing or repairing a flat roof on a 2-storey extension - Total Cost: £1200

50%

Materials
£600

25%

Tradesmen
£300

25%

Scaffolding
£300

FAQ's

The cost of a new flat roof will depend on a number of factors including the specification of roofing materials, height and access, location, size of business carrying out the work, and the actual size of the flat roof area. But the costs for a replacement felt roof using the common bitumen based felt that is applied using a gas-powered blow torch to a flat roof over a double garage, would cost around £1000 on average.

This figure would include the removal of the existing roof felt, inspection of existing timber, supply and install of underlay, supply and install of cap sheet, supply and install of all edgings, and the disposal of all waste material from the site. The cost would increase for a flat roof in a habitable part of the building as more insulation would be required, the presence of asbestos in the old roof would also considerably increase costs, as would access or height problems requiring scaffolding.
No, actually cowboy builders do this all the time, leaving the original felt in place and just put the new roof material on top. This can lead to a whole host of problems which will no doubt begin to appear just shortly after the builder has disappeared into the sunset with your money! Removing the old felt is an essential part of the job as the timbers underneath need to be inspected or replaced. This will last for a year or less until the movement of the roof, UV rays from sunlight, and damaged existing roof cause it to split and leak.
It all depends on size, access, height and type of materials used. But on average a single garage should take one day with two men working, and with a double garage allow for a day and a half. Note that even on small jobs taking less than one day, a competent and professional roofer would always come back the next day to ensure that the flat roof was watertight overnight. Obviously unusually shaped flat roofs or roofs where there is a lot of cutting around roof windows or pipes required will take longer, as will flat roofs on second or third storeys which will require scaffolding or access towers.
All flat roofs should be inspected twice a year in Spring and Winter, ideally just after some rain or snow so you can check water on the roof is draining away correctly and not pooling. Note that before you attempt any flat roof inspection you must take suitable safety precautions, unless 100% unsure of the condition of the roof do not step onto the roof, or if you must then use a sheet plywood to distribute your weight evenly across a large surface area.