Most modern flat roofs have a lifespan of 20 years or more, so that means if you were to live in your annexe for 60 years you would only have to replace it a maximum of two or three times in your lifetime. So unless it’s for style, spending a disproportionate amount of money on a flat roof compared to a pitched roof rarely makes sense for an annexe.

What are the options annexe flat roof materials?

The main options are:

• Felt
• Asphalt
• Fibreglass
• EPDM Rubber
• Single Ply Membranes

Felt roof

This type of flat roof has a bad reputation with some people, most peoples perception is of the old ‘pour and roll’ felts of a decade ago, and these were thin bitumen felts glued together with a hot liquid bitumen between each layer. They had minimum internal reinforcing, with some containing natural hessian fibres which were prone to rotting. This type of felt, sometimes known as felt paper or tar paper, would effectively dry out and crack with age or UV damage from the sun.

Torch on felt roofing

Torch on felt roofing is the modern application method for felt roofing. New breakthroughs continue to be made in bitumen and modified bitumen technology such as Styrene-Butadiene-Styrene modified bitumen (SBS) or Atactic polypropylene modified bitumen (APP), resulting in much better resistance to UV damage and cracking. This type of felt roof comes on rolls and is laid with the aid of a very large blow torch that heats and melts the rear of the felt, melting it onto the roof structure below. Typically it’s applied as a 3 layer system usually consisting of a perforated underlay, a 2mm underlay layer, then thick 4-5mm felt. These layers are all laid in a half lap manner, so all the joints are the furthest away they can be from another joint.

The thickest top layer named the cap sheet is often a coloured mineral finish, and they are available in a variety of colours, as well as a plain felt finished with a silver solar reflective UV finish. All of these felt layers are also reinforced with strong polyester weave sandwiched in the middle of the felt, giving them incredible strength. These newer material are guaranteed for 10+ years, but will last 20, sometimes 30+.

• Cost – Easily the cheapest option
• Suitability – Can be laid to any size of roof, or detail level without problem. Has favourable expansion and contraction properties due to modern bitumen technologies.
• Damage resistance – It is perfectly ok to walk on if you’re careful, but no ladders or step ladders without protection between them and the roof.
• Repairability – Repairable by torching a new patch of felt on the top of a damaged section, but it does look a bit patchy. If you have the plain felt and solar finish it’s the easiest to repair.
• Appearance – A nice mineral finish is pleasant to look at, certainly prettier than a rubber roof. There’s plenty of colours to choose from like green, brown, grey and purple.
• Installation issues – Insurance to work with hot flames.

Asphalt Roofing

Asphalt or mastic asphalt has been used for years, with good results, and the latest asphalt contains modern polymer formulations making it even better.

Asphalt is heated in an asphalt boiler and laid while still hot onto the roof, trowelled into shape, and cools to form a very hard durable roof.

• Cost – Approximately 20-30% more than a Torch on felt roof
• Suitability – Can be laid on most roofs of all sizes, extremely tough, but not heavily sloped roofs or flimsy constructions that may flex. Does not require a hot flame during install.
• Damage resistance – Extremely durable, sets hard like an asphalt pavement, withstands all sorts of abuse
• Repairability – Easy and cheap to repair with a number of solutions such as liquid, torch on felt, or even laying another roof on top
• Appearance – A subtle light grey matt finish with clean lines
• Installation issues – Asphalt is heavy, the roof construction must be strong enough to take the weight without movement or cracking occurring.

EPDM Rubber Roof

The rubber or E.P.D.M. (Ethylene Propylene Diene Monomer) is commonly made by Firestone, and can often be installed in one complete layer without the need for joins (depending on size). It is extremely lightweight per square metre, highly elastic and tough, so it lends itself to many roofing situations. Most new domestic rubber roofs are glued down to a wooden roof substrate.

A well installed EPDM rubber roof, should last up to 50 years.

• Cost – Approximately 20-30% more than a torch on felt roof.
• Suitability – A good choice for green roofs, lightweight, flexible and strong. Does not install well into roofs with complicated details, pipes, curved upstand walls, complicated gulleys or gutters because it becomes full of joints, which can look ugly and leak. Best for nice simple shaped roofs, no naked flames required.
• Damage resistance – A very durable roof, it will resist most forms of abuse.
• Repairability – It can be repaired very easily with rubber patches, but the patches are ugly.
• Appearance – A big sheet of black rubber
• Installation issues – A large rubber roof will shrink over the years, up to 10%. Shrinkage can cause edge details and upstands to pull away from walls if they are not fitted with the correct fitting details. A better quality fleece backed EPDM will resist or stop shrinkage, which can be used with a large roof.

Fibreglass GRP Roof

Fibreglass flat roofs have been used in the UK for over 30 years in small numbers, but over the past 10 years interest in GRP (Glass Reinforced Plastic) fibreglass roofs have really increased. Dedicated roof resins and factory made trims have made this method a good option.

Fibreglass Roof guarantees are for 20-25 years.

• Cost – Depending on specification this can cost slightly more than a EPDM rubber roof, there are many options available which have a baring on the price including the final finish and laminate thickness.
• Suitability – Good for green roofs, lightweight, available in any colour, strong, completely jointless. No hot flame required.
• Damage resistance – Can be slippy when new without the optional anti slip coating. Impervious to all forms of abuse.
• Repairability – Repairs can be from a small patch to a layer bonded directly on the top, and almost invisible.
• Appearance –All colours available for the final finish, can be cleaned with soap and water.
• Installation issues – The weather has to be totally dry to lay a fibreglass roof.

Single Ply Membrane

A single ply membrane is any roof covering that can make a flat roof waterproof in one layer, such as, PVC , TPO, TPE and PIB.

These membranes generally are lightweight, flexible, resistant to UV and micro organisms, handle extreme weather and temperature changes, fire resistant and self extinguishing, show good chemical resistance, and can be very environmentally friendly.

PolyIsoButylene (PIB) was introduced in the 1930’s it is 100% recyclable and the lifespan is typical in excess or 30 years. It’s easy to install and durable, it is however usually the most costly option.

Polyvinyl Chloride (PVC) has been used since the 1960’s. Compromising of two thermoplastic layers reinforced with a scrim it is supplied in rolls, and you heat weld the joints like the other systems mentioned here. The typical guarantee is 25 years with a life expectancy of 30 Years+. However, they may shrink slightly over time.

Thermoplastic Polyolefin Elastomer (TPE) is environmentally friendly, and is 100% recyclable. Introduced in the 1990’s with a lifespan of 30 years, but a typical guarantee is for 20.

Thermoplastic Polyolefin (TPO) is considered to be more environmentally friendly than PVC, but not quite as good as TPE above. Its been around since the 1980’s, and has a typical lifespan of about 20 years, with guarantees ranging from 10 to 20 years. TPO is a relatively new roofing material, and is undergoing changes in the way it is formulated due to initial problems with sun damage, expansion wrinkles and shrinkage.

All of these flat roofs have pros and cons whether it’s price, appearance, durability, flexibility, longevity or suitability. However there is a wide variety of choice for your annexe.