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Flat roof construction

Flat roofs construction – Types and issues

We often see flat roofs at our UK Kingston surveyors office so we’ve written up a piece all about flat roofs and their construction. Flat roofs are often found on extensions and outbuildings however they can sometimes be found on flats and commercial buildings. In general, they tend to be less effective than pitch roofs as they are less effective in shedding water which can lead to more defects.

Types of flat roofs

Cold roof

A cold roof is a flat roof that has insulation fitted within the roof structure and joists. This was used until around 40 years ago. It tends to be less effective as condensation can be an issue. Therefore, it needs ventilation in void above insulation. This type of roof experiences thermal movement due to solar radiation.

flat roof

Warm roof sandwich

This is when the insulation is located above the roof. the benefits of this is that there is less condensation, however, the waterproof layer is vulnerable to more weather and thus more likely to break.

Related: 4 reasons why property owners hire surveyors

Warm roof inverted

This kind of roof is when the insulation is placed on top of the waterproof layer. this needs frost proof insulation.

flat roof

General problems with flat roofs

  • Poor falls: The fall for flat roofs should be at least 1:40 to ensure rainwater drains off the flat roof to the drain but this isn’t always the case.
  • Ponding: This is caused by unevenness in the covering and is when water pools at certain points in the roof, can lead to break up of the roof covering, and different temperatures in the roof.
  • Outfall problems: drains must be of the correct size and in the correct place.
  • Damage by foot traffic and humans.
  • Organic growth can lead to inefficient water runoff, lead roofing can corrode from the acidic excretions of certain growths.
  • Thermal and moisture movement- covering materials tend to expand and contract with heat mainly caused by the sun. Expansion and contraction will weaken the material and eventually lead to cracking.
  • Differential movement- when there is different movement between different elements or parts of the roof itself. This can lead to cracking as well as inefficient detailing.
  • Defects at upstands and skirtings- the most common of these splitting along the 90-degree turn in the covering,
  • An insufficient height of the upstand: the height at the upstand should be 150mm. A lead flashing should be applied to prevent differential movement, these should also be placed around outlets. This will seal the roof in the most vulnerable areas.
  • Decking problems-chipboard should be moisture resistant grade, however, this can still absorb moisture in certain situations. Plywood must be of external quality or marine grade. Softwood timber board may warp and shrink causing the DPM to be affected.

Related: Wood rot: the difference between dry rot and wet rot

Wind-related damage

  • Suction- this is a type of wind pressure which can especially affect the edge of the covering which may lift if it is not fitted properly.
  • Scouring action on protective dressings- if a gravel covering is used this can be lifted and then damage exposed roof material. This will wear away the roof over a fairly long period of time

Failure of roof coverings

Many different materials are used to create the surface of a flat roof. these are normally vapour impermeable to create a waterproof seal, however, this will depend on the type of roof.

Built up mineral felt

This consists of two or three layers of felt bonded with bitumen, or polyester felt membranes which consist of two layers glass and other fibre membranes of three layers. They were very Popular in the 60s and 70s. Life expectancy of these roofs can reach 15-25 years although they can last as short as 10they may last longer with modern techniques such as better-quality installation and materials

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Common defects of built-up mineral felt

  • Blistering-this is when there is expansion of trapped air underneath the covering which will expand and contract as it is heated by the sun. after time this may cause a split in the felt.
  • Loss of surface protection- surface protection prevents the bitumen from oxidising from ultra violet light from the sun. protective ballast can be used although this can be removed if the fall is too great. Solar reflective paint can be used but must be reapplied every 2-5 years
  • Problems with jointing of the top layers- must be staggered and overlapped for the top layer which is then bonded with hot bitumen.

Single ply membranes

Manufactured from plastics and rubber-based materials which are then fixed together with glue or by welding. These Are normally mechanically fixed to a warm deck roof and have a Lifespan of 50-60 years.

Mastic asphalt

This consists of Fine and course aggregates which are mixed with bitumen over a separating layer to account for movement.

Common defects with mastic asphalt roofs

  • If bitumen is too thick on the top layer the covering is susceptible to surface crazing in cold weather.
  • Adverse effects of the substrate: this type of covering is very susceptible to movement, so any movement of the substrate can lead to cracking.
  • Surface crazing- this occurs due to solar radiation, however, is only a problem where there is ponding as well.
  • Blistering- same as felt

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Lead Roofing

Majority of lead roofing is milled lead sheet and has a lifespan of 100 years plus. Thinner sheets should be used for areas of low exposure and thicker for greater exposure. Lead sheets Are greatly affected by thermal movement and so should be applied in sufficiently small sheets with overlaps to accommodate for movement.

Common defects in lead roofing

  • Corrosion- underside of the lead is vulnerable to condensation and is unprotected and so may corrode, especially if there is no ventilation or impermeable layer/vapour check underneath.
  • Excessive bay size- individual sizes of lead should be limited in size as larger pieces may ripple and crack due to thermal movement.
  • Incorrect fixings- should use copper or stainless steel as some materials can act as an anode and corrode at a greater rate than the lead around them.
  • Incorrect underlay- if a bitumen or resin underlay are used this can bind to the lead as it is heated from solar radiation. the lead then cannot move and will crack.
  • junction problems rolls- junction along the ridge of each bay, must only be fixed down if completely necessary

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