When flat roofs fail

Flat roofs fail for all sorts of reasons. Many asphalt and bituminous-based roofs can last for up to 30 years, but will eventually fail. UV radiation causes effects such as crazing, blisters and splits and thermal or building movement can lead to cracking.

Some roofs fail earlier than expected. Poor design, such as incorrect or non-existent wind uplift calculations can lead to the roofing surface lifting off the substrate. Poor detailing or workmanship around pipes, rooflights or upstands can also lead to water finding its way in.

Many flat roofs suffer from a lack of maintenance. An annual inspection and the removal of a few leaves from the drainage outlets might be all that is required. Meanwhile, damage is often caused by other maintenance work, such as dragging heavy plant across the surface or attaching cables using nails.

A damp patch on the ceiling is a sure sign that you’ve got problems with the roof. But what should you be looking out for if you want to spot a problem before things get that bad? Below Andrew Leggett ACIOB, a specialist refurbishment surveyor working for flat roofing manufacturer Sarnafil, shares some of his survey shots illustrating typical signs that may indicate your roof is about to fail.

What action to take on a failed roof depends on a number of factors: whether the client requires a long- or shorter-term solution, the size of the roof, and the state of the existing roof covering and substrate. It should be noted that the latest version of Part L of the Building Regulations, introduced in 2006, means that if more than 25% of the roof is to be refurbished then it must be thermally upgraded to meet the new standards.

Often the best solution is to overlay the old roof covering with a new one. This causes minimum disruption to the building’s occupants, and negates the need to send the old stripped material to landfill.

With traditional flat roofing membranes, however, overlaying would present a problem if moisture was trapped in the old roofing system. However, Sarnafil membranes are permeable, which means moisture can escape.

A layer of insulation can be glued or mechanically fixed to the existing roof with the membrane glued on top.

If the existing roof is wet, or if the underlying substrate is in poor condition, then the best solution could be to completely strip and replace the roof. This is often the case if a roof has been left unmaintained for a number of years.

Occasionally, a partial strip could be a good solution. This would work if the leaking has occurred in a specific contained area or if the type of roofing used originally can be removed without damage to the thermal insulation or structure. This solution is also governed by the size of the roof.

When you have determined whether to replace or repair a flat roof, it is important to make sure that the new roof covering is properly designed and installed to avoid further problems. Sarnafil can take on the responsibility for the whole process from the initial roof survey and report, through design and specification including wind uplift calculations, through to installation, inspection and guarantee.

To take on the full responsibility, Sarnafil uses the top tier of its registered roofing contractors to carry out refurbishment work. These firms require specific management and practical experience. By using these specialists, Sarnafil can issue a final inspection certificate which protects the client against defects within the installed roofing system. An extended product guarantee also protects against the risks of incorrect design or defective materials.

If you own or maintain a building with a flat roof, now could be a good moment to get up there. Some simple roof husbandry now could save you a lot of money later.

Cracks: Structural or thermal movement leading to excessive differential movement between the substrate and the roof covering can cause this. As the roof covering gets older it is less able to cope with this type of movement and it is often an indicator of the end of the serviceable life of the roof covering.

Asphalt creep: The surface cracking here is caused by the movement of the asphalt due to flow and/or creep. One theory is that mastic asphalt behaves in a viscous-elastic manner and creeps slowly to assume the profile of the supporting substrate under normal in-service temperatures. If high temperatures are experienced then the flow properties will become more fluid, such that the asphalt could sag into undulations in the substrate.

Mechanical Damage: This photo shows damage caused by something being dropped onto the asphalt roof surface. Asphalt, although soft when warm, is brittle when cold. If a roof is regularly trafficked, it’s best to use protective walkways.

Slumping: Problems often occur at details such as this abutment. Here, some slumping and cracking of the asphalt has occurred.

Poorly detailed penetrations: Where pipes or services penetrate the roof, it is important to get the detail right. Here the waterproofing to the roof penetrations has been poorly formed, allowing the opportunity for water ingress.

Nailed Wires: Wires have been attached to the roof with clips nailed through the waterproofing. Any punctures to the waterproofing system will allow water to penetrate.

Ponding: This bituminous cover does not have adequate falls as recommended in BS6229:2002 British Standard Code of Practice for Flat Roofs with Continuously Supported Coverings: Section 7.1. The minimum fall recommended for this roof is 1:80. When falls are below this figure the roof undulation and any deck deflection increases the risk of ponding.

Blocked rainwater outlets: It is very common for rainwater outlets to become blocked with general roof debris. Good regular roof husbandry is recommended, which includes the cleaning out of all roof outlets, and replacing leaf guards as appropriate.

Open laps: Open laps were observed on this bituminous felt roof in the main field sheet and in the flashings. This is usually caused by shrinkage of the roof membrane or excessive thermal and structural movement of the substrate. Older roof membranes have less inherent flexibility and they are more susceptible to the effects of movement. A more fundamental cause of open laps is bad workmanship.