Does installing a flat roof on our extension increase the risk of leaks?

Property Clinic: Is there anything we can do to prevent leaks?

I’m putting a flat roof on our extension, but I’ve heard that there is a greater danger of water leaking in from a flat roofs. Is that true? Is there anything we can do to prevent leaks?

Flats roofs are often problematic. In my opinion, flat roofs should be avoided in a country such as ours where there is an abundance of windblown rainfall. Our ancestors, with their pitched thatch and slated roofs, even galvanised sheeting, did not entertain flat roofs and neither should we; however, the architects would probably pay no heed to that precedent.

In my experience, the issue is poor workmanship in most cases, with construction failing to meet the manufacturer’s recommendations. Even where the contractor is recommended by the product supplier, that is no guarantee they will do a good job.

A further difficulty is when it comes to diagnosing problems with a flat roof, the water can enter at point A, travel between membranes or along a roof deck to a weak point and emerge within a building at point B, very often several meters away from the offending puncture.


The most common issues with an Irish flat roof include:

Felt damage

Water leaks due to damage to or deterioration of the felt over time – most felt, or coverings have a limited lifespan – usually 15 to 25 years depending on the product. Pooling of water on the roof surface can happen when the drainage fall towards the gutter or outlet that was designed into the roof is lost due to natural settlement of the building. Very little can be done to repair this and it may require an improved fall being engineered using tapered insulation boards during recovery. Prolonged ponding of water on any roof will lead to early deterioration of the covering.


The upstand is like a kerb around the perimeter of the flat roof, a vertical element that protects the building fabric from rainwater. This is the weakest point in any flat roof and issues arise where the upstand felt sags or loosens its bond with the upstand. This can occur at a parapet wall, roof light, plant equipment upstand or a patio door threshold. Flashing can work loose or crack over time, rendering it ineffective.

Lack of maintenance

Leaves, for example, can be allowed to build up, choking rainwater outlets, or debris left on the roof can be walked upon, causing damage to the roof covering. Avoid foot traffic on the roof if possible.

Unfortunately, in many domestic extensions a pitched roof often will not work for many reasons, including there being inadequate height to fit a pitched roof under a first-floor window or the blocking of light to a neighbour, for example.

If you must go with a flat roof, you need to discuss with your designer and builder the weak points in your proposed roof and make plans to mitigate risks. In any case, avoid any unnecessary penetrations, maintain an adequate fall towards the gutter, and select a good, well-established felt product.

Finally, once constructed, ensure your flat roof is inspected at least twice a year and regularly maintained. Larger commercial flat roofs should be checked quarterly.

Pat McGovern is a chartered building surveyor and a member of the Society of Chartered Surveyors Ireland

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