18/06/2026
The importance of core samples. Or, more specifically, knowing when not to take one.
There is a lot of talk in surveying circles about core samples. And rightly so.
In most flat roof investigations, core samples are incredibly valuable. They can confirm the roof build-up, identify trapped moisture, expose hidden defects, and give the client evidence that a surface-level inspection cannot.
In many cases, I would say they are essential. But not always. And this is where experience and independence matter.
I recently inspected a two-level flat roof that, on paper, had all the ingredients for a more invasive investigation: single-ply membrane, reported woodwool deck construction, ponding water, previous concerns, questions over the build-up, and a scope that allowed for core samples.
It would have been easy to arrive on site, cut holes in the roof and say, “That’s what we were asked to do.” But that is not surveying. That is following a process without thinking.
Once on the roof, the picture was different. The single-ply membrane was generally well installed. The laps were checked and no significant delamination was found. There were no reported active leaks. The moisture mapping did not identify elevated readings to the accessible mapped areas.
The main issues were not hidden within the roof build-up. They were on the surface: blocked outlets, vegetation, debris, a gutter that could not discharge properly, a slipped downpipe partially restricting an outlet, and localised mastic and masonry defects.
In those circumstances, cutting into the roof would not have added value. It would have added risk. Every core sample is an opening through the waterproofing. Yes, it can be repaired, but it is still a deliberate breach in a roof that, based on the evidence, was performing reasonably well.
That is the bit that sometimes gets missed. Core samples are not a badge of thoroughness. They are a tool. A very useful tool, but still a tool. The skill is knowing when to use it, where to use it, and when the evidence says it is not justified.
Had this been a “free survey” from someone with a system to sell or a job to win, I suspect the answer may have been different. The ponding might have become a reason to overlay. The woodwool deck might have become a reason to strip. The blocked outlets might have been treated as a symptom of failure rather than a maintenance issue.
But the independent answer was much simpler: retain the roof, clean it, clear the outlets and gutters, carry out localised repairs, and maintain it properly.
Not every roof with ponding water needs replacing. Not every roof with a concerning build-up needs cutting open. Not every survey needs to end with the most invasive option.
Good surveying is not about doing the maximum amount of investigation possible. It is about doing the right amount of investigation, based on evidence, risk and judgement.
Sometimes that means taking core samples. Sometimes it means knowing when not to.
