A roof can be pitched or flat. A flat roof may look visually flat but will always have a slight gentle slope to it. This is so rainfall will not pool on the roof, but rain can run off to the lower side and be caught in the gutter rather than eventually seeping through to the house. A flat roof is designed to have a minimum fall of 1:40, which given on-site inaccuracies should result in a minimum fall of at least 1:80 in the finished construction.

Flat roofs are described in the "Scottish Technical Handbook" as  '...the slope of which does not exceed 10º from the horizontal'.

Flat roofs are relatively new to the UK, only since World War 2, as they offered a cheaper alternative to traditional roofs. The downside of flat roofs is that despite being cheaper to replace, their lifespan on average is 35 years (depending on the coverings you opt for).

Flat roofs are typically found on extensions, garages, side returns, garden studios.

Warm roof, cold roof, what is the difference?

When having a roofer replace your flat roof, the term you will hear bounced around a lot is ‘warm roof’ or ‘cold roof’, so what is the difference?

This is about the insulation design of the flat roof.

Warm Roof

A warm roof insulation is attached to the floor decking and rafters. This offers the most thermal protection as the roof and loft area will be a similar temperature to the interior of the building. This will not work if you have limited ceiling height as the insulation will reduce the height of the room. A warm roof allows heat to be conserved and moisture to escape, these are the best roof options for the UK climate.

Cold Roof

Cold roof insulation is placed between the floor joists meaning that this option does not impact on the inside height. This way of insulating is less efficient than a warm roof. A cold roof is easier to install at the build phase, trying to insulate between floor decking retrospectively can be both messy and time consuming.

What are the different types of flat roof coverings?

There are different types of flat roofs to choose from, ranging from cheaper alternatives to more costly but more attractive to look at and all with different lifespans;

Bitumen felt flat roof
Bitumen flat roof felt is affordable and durable. Cheap easy installation yet a great waterproof addition but not as long lasting. These flat roofs last up to 20 years.

‘Torch-on’ felt flat roof
‘Torch-on’ flat roof felt or modified Bitumen. These roofs, tar is mixed with filler components, which can be; limestone, sand or polymers, including styrene-butadiene styrene (SBS). They require little maintenance and cost effective, lasting up to 25 years. 

EPDM flat roof
Ethylene Propylene Diene Monomer (EPDM) is a single-ply rubber roofing system that is tough but lightweight. This roof does not need joins so is easy to install. The synthetic rubber is glued to a clean roof decking. Trims are fitted around the edges of the flat roof to waterproof the roof. These flat roofs last up to 50 years.

GRP flat roof
Glass Reinforced Polyester (GRP) fibreglass roofing system is a liquid roofing resin and reinforcement mat. Suitable for tricky roof shapes as the material is cut to suit the roof. These flat roofs last up to 50 years.

Mastic Asphalt
A popular choice where the roof will be walked on, as the base layer is often paving slabs. Can also be laid on concrete, timber and metal and can be worked around the shape of the roof. As asphalt is so much heavier than felt coverings, the roof needs solid supporting structures. Asphalt is one of the most durable and long lasting coverings, with a lifespan of 50 years.

Sheet Metal
Flat roofs in metals such as lead, zinc or copper have always been a tough and durable option for a flat roof covering. These flat roofs can surpass 50 years.

Living green flat roof
The roofs have a layer of vegetation placed on a rooftop creating a Biodiverse space. Green flat roofs provide wildlife habitats and are attractive to look at and there are many different options of green roofs. They absorb the carbon dioxide in the atmosphere while also having a long lifespan of up to 50 years.

How often do flat roofs need replacing?

On average a well built flat roof can last 35 years, even longer if it is a metal flat roof. Tall trees in very close proximity can damage a flat roof with their branches falling onto the roof. If your home is heavily affected by the weather, this too can have an adverse effect on the lifespan of your flat roof.

Classic tell-tale signs of needing a new flat roof include;

Water on the inside walls - this might be anything from water marks to water staining and if left long term it could be damage caused by water ingress from the roof on the ceilings or walls inside the dwelling from the roof leaking and no longer able to protect the inside from rain.

Sagging roof - if you can see a visible sign that the stability of the roof seems affected, like the roof is sagging, misshapen or sunken, then it may be time to replace the flat roof.

Mould - any signs of mould or moss would mean that there is a damp issue which is possible leaking water from the roof.

How much will a new flat roof cost?

The price of a flat roof will be dependent on the size of the roof and the coverings that you choose.  Another aspect affecting the cost of the flat roof, is what is roof protecting? E.g.; is it a garage flat roof or a house extension. Will people be walking on the top of the flat roof? What access issues will the roofer have to replace the roof and when it comes to replacement of the flat roof, does it include a full timber joist replacement as well?

Taking all of this into consideration, a price including materials for the flat roof could vary from £1000 - £7000. When it comes to the labour of the roofer replacing the roof, plan for the roofer to be with you for 2 - 3 days working on the new flat roof. 

Cost to replace a flat roof	Per	Average total labour cost
Replacing a main flat roof	Per day	£650
Fitting an extension flat roof	Per day	£600
Replacing a flat roof balcony	Per day	£540
Replacing a double garage flat roof	Per day	£400

Does a replacement flat roof require building regulations?

Some repairs to flats roofs will not require an application for approval under the Building Regulations. Planning permission will be needed if :

• there will be structural changes to the roof 
• the new roof covering will be a different covering from the existing and this will affect the roof in the event of a fire
• more than 25% of the roofing area is being replaced, then the thermal insulation must be replaced

Summary of flat roof building regulations;

• the gradient of the fall of the roof must take snowfall into consideration
• Part H of the Building Regulations, drainpipes and guttering must be fitted on at least two sides of the flat roof to ensure the flowing water doesn’t accumulate at the base of buildings.
• flat roofs must be designed and built to be strong, durable and wind resistant; the size of joists and noggins and preliminary timber layer will be determined by the weight of the flat roof
• roofing materials must be the correct size to support an average man’s weight on the roof 
• single ply membranes should extend up adjacent walls at least 150mm from the roof surface for waterproofing 
• thermal layer of insulating material should be inserted either above the roof decking (the ‘warm-roof’ model), which is suitable for homes or permanent residences; or else on the interior of the building just above the ceiling (a ‘cold-roof’), which is cheaper, but less effective
• warm flat roofs must have a VCL (Vapour Control Layer) inserted between the top decking and the first layer of insulation; this can be made of polyethylene membrane, bitumen sheet, polymeric foil or even be a cheap, easy-to-apply self adhesive sheet.
• A Certificate of Compliance on must be received on completion of the new roof, to prove to potential buyers that the flat roof meets all construction and energy efficiency guidelines

Is it better to do a partial or full flat roof replacement?

If it is a case of a sagging flat roof covering but when the felt cover is pulled back the timber structural joints are still in decent condition and not showing any signs of rotting timber, in this case all that will be needed is a replacement cover and possibly an upgrade on the insulation.

If once the timber and joists are exposed to show rotting timber, this will need a full replacement of the flat roof. At this time you can speak with the roofer and be led by their opinion of which covering to choose.

If water is leaking through the roof, and a visual check on the roof shows a tear or a nail that has put a bigger hole in the roof, this area may be able to be patched and repaired to see you through more years without having to worry about a full flat roof replacement.

Estimates for common roofer jobs;

Rates for other trades