Flat Roofs – Typical Defects & How to Deal With Them

Flat roofs are roofs that have a pitch of typically 1-12.5 degrees, explains Toby Bowser, Partner at Chartered Building Surveyors Hamilton Darcey. Any more than this and the roof is considered a pitched roof. Flat roofs became more prominent after World War Two, due to them being more economical and quicker to construct than pitched roofs. The trade off is that flat roofs generally have a lower life expectancy and require regular maintenance to ensure longevity.

Flat roofs are one of the most common building components to fail. Here we will discuss typical defects and how (and how not) to repair them.

Typical Defects

Most defects with flat roofs manifest themselves in issues with moisture, either through water ingress or condensation. Below are the most common defects we come across:

• Blistering – Blistering occurs when moisture is trapped under the surface of the roof covering, either through water ingress or from when the roofing membrane was laid. As the roof membrane heats up (usually due to solar gain) the moisture turns into vapor and expands. This results in debonding of the membrane and visible domes form on the surface. The surface of these domes crack, due to expansion. These cracks, in turn allow more water to track in.
• Mechanical Failure – Mechanical failure is an all-encompassing term for punctures or general wear and tear. Given flat roofs are often used to house plant (such as air conditioning, air handling units and lift motor rooms) they are often trafficked for maintenance, causing wear and tear. Punctures can also occur from tools and materials left on the roof by workmen.
• Blocked Rainwater Goods – If gutters and downpipes are not subject to regular maintenance, they can become choked and blocked. This reduces the capacity of the roof drainage, which means that water does not drain from the roof effectively, leaving it with standing water (ponding). Eventually this water has to go somewhere and as the roof fills up with rainwater it will track up and over any membranes and enters the building below.
• Detailing – The junctions where the flat roof adjoins other elements of the building, such as parapets, drip details, rooftop plant rooms, rainwater goods, etc. are often weak points of the flat roof. These areas need to be properly detailed. Typically, we would see lead or the flat roofing membrane used as an upstand. Problems usually occur when upstands or drip details are not detailed correctly or are not correctly sized.

At Hamilton Darcey, all too often we come across poor quality repairs to flat roofs. These typically utilise low cost liquid products, or self-adhesive flashing tape. These should only be seen as a quick fix, to be used for emergency repairs only. Whilst they may stop the immediate water ingress, you should not assume the problem has gone away.

Additionally, these short-term emergency types of repair are likely to invalidate any warranties or guarantees on the roofing membrane. These guarantees and warranties come at a premium and so once voided this investment is wasted, due to lack of foresight from the persons undertaking the repairs.

Once an emergency repair has been undertaken you should begin to plan for a long-term solution to the water ingress, to prevent further issues in the future. Repairs should be undertaken by a reputable roofing contractor. If the roof is covered by a guarantee or warranty, the original installing contractor or the roofing membrane manufacturer should be engaged with, as more often than not, they should undertake the repairs at their cost.

It is also important to take core samples at locations throughout the roof. This will help to identify the extent of the ingress and water damage, so you are not replacing more of the roof than you need to.

At Hamilton Darcey, we regularly provide options for long term repairs, based on budget and access. We recommend coinciding any major roof renewal works with the next cycle of major works to the block to benefit from the cost saving by utilising the scaffold and access.

The unpredictable British weather can make managing property tricky. However, remember even though there are time pressures to immediately halt a water ingress issue, you need to consider the long-term repair, as the quick fix is just papering over the cracks.