Repairing a Flat Roof Mistakes - Core Sample Patch

Patch in the wrong material

Why would someone cut a hole in your flat roof? And what if they use the wrong material to fix it? This is another in our series of photographs of problems on roofs.

This photograph was taken on one of our recent survey visits to an Academy in the North of England.

The dark patch you can see is a square of bitumen roofing which has been added to cover up a hole which has been made in the roof.

Why would you cut a hole in a flat roof?

Why would someone cut a hole in your flat roof? The most common reason (except perhaps, to put a cable through) is to take a Core Sample during a survey. A core sample allows the surveyor to inspect the layers of the roof.

“Taking a core sample” is the process of cutting out a small section of the roof to expose the different layers back to the deck. This allows us to check what the roof is made of, check for moisture in the construction and inspect the deck and other layers for any faults or alterations. A surveyor will take several core samples across your roofs, in areas which will indicate in his or her experience, the state of the roof as a whole. Find out how core samples were used on a recent project here.

Repairing a Flat Roof the Wrong Way

Of course, if you’re making a hole in the roof, it is absolutely vital that you do it correctly, and repair the hole correctly. In this case, the repair was not done correctly.

Whoever in the past had been on the roof doing a survey, they have sealed all their core samples with a bitumen roofing material. Unfortunately, some of the roofs are PVC single ply roofs, and these should not be sealed with bitumen.

Not only are Bitumen and PVC single plys not compatible products, the waterproof seal between these two dissimilar materials both initially and more importantly over time is far from ideal and will lead to embrittlement of the single ply and failure of the patch.

Risk of Fire

There is a second reason not to use bitumen to repair single ply roofs, and that is the use of flame torches. PVC single ply roofs are not fixed with naked flame torches, rather hand or driven hot air welding tools to seal the over laps.

The use of a naked flame, as well as not being appropriate for the single ply it does increase the risk of fire. One does wonder if the person carrying out the survey was insured to carry out hot works.

Check before you let anyone cut a hole in your flat roof

Before you let anyone cut a hole in your roof, check whether it is under warranty. A professional roofing company will always ask you about warranties before they carry out any survey work.

If the roof has a warranty, it should not have a core sample taken because this may very well invalidate the warranty. Unless there is a leak and a core sample is essential, in which case it is advisable to contact the warranty company for their advice.

How to carry out a Survey and Repair a Flat Roof after Core Samples

Our steps to bear in mind so you have a successful flat roof survey are:

  1. Ensure the person surveying the roof is experienced and qualified to carry out the survey.
  2. Find out if your flat roof has a warranty, and if it has, do not allow core samples to be taken without contacting the warranty company first.
  3. Do not allow a surveyor or roofing contractor to carry out hot works (or any works) on your roof unless they are insured and are signed up to the NFRC’s Safe2Torch pledge. They will have a certificate.

How do we repair our core samples? If it is a single ply roof, most of the time we use a liquid product called AH-25. AH-25 liquid is a heat free and low odour product suitable for a very wide range of existing roof covering. If in doubt we would not open the roof until we were certain about the effectiveness of any patch we apply.

You can rest assured if we send someone to survey your flat roof, all of these things will be covered.

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About the author – Ian Dryden

Ian is an experienced specification manager with ten years working in flat roofing. He works with our technical team of surveyors, technicians and roofing specialists at SIG Design and Technology to deliver better flat roofs on schools across the country.