Volume 39, March 2000: Core Sampling

Perspectives
A Quarterly Information Source from Benchmark, Inc.
Volume 39 March 2000

Core Sampling

by Ron Harriman

A variety of elements are considered when evaluating the condition of a roof, and several investigation techniques can be used. A technique that is often the most revealing is a core cut, or core sample. Occasionally, it is considered objectionable to have such a destructive method used on a new roof, or a roof under warranty. Certainly there are situations when cutting a roof would be inappropriate or unnecessary. However, there are also many situations where core sampling is essential.

One of the first goals in roof assessment is to determine "what". What is the roof type, composition, method of attachment, insulation type and deck? It sounds simple, but knowing the exact components is equivalent to knowing where you are on a map. A good and accurate understanding of the "what" gives you an opportunity to then determine condition and ultimately select a strategy for preventive maintenance, repair, replacement, etc.

Following is a brief case study where a few simple core cuts, had they been taken, may have proven beneficial.

Core samples, such as this one,
reveal much about a roof's composition.

A real estate company recently purchased a building. Prior to purchase, a due diligence team evaluated the building, including a visual review of the roof. The gravel surfaced, built-up roof looked fine, with no splits or blisters. A few active leaks were reported, but repairs appeared possible. After purchase, leaks increased and a more comprehensive evaluation, including core sampling, was conducted.

Core sampling revealed a 3-ply fiberglass felt with poor interply adhesion and advanced embrittlement of the asphalt bitumen. Unexpectedly, under the built-up roof was lightweight concrete over cast-in-place structural concrete.

Due to the porous nature of fiberglass felt, in combination with brittle asphalt, which readily cracks, the roof was legitimately leaking like a sieve. The lightweight concrete was soaking up the leakage while the water would find a crack in the structural concrete. An expensive, unanticipated roof replacement project was necessary.

Core sampling is simple, and whenever possible, should be included in a roof evaluation. It usually takes less than one hour to cut and patch a core. Typical core samples vary in size from 4"-12" square depending on the situation.

Patching of a core by a consultant is typically by cold applied materials such as mastic, with a reinforced fabric. Core sampling can both verify the need for conducting moisture testing and also verify the results of a moisture survey. Core sampling is many times the only way to verify if a roof has been recovered, and it is common to find different roof construction "as built" versus "as specified".

Core sampling is a normal and common technique, especially when conducting an initial roof investigation. During annual reinspections, it is generally not necessary to cut the roof, unless changes in condition warrant a closer look. It might seem archaic or old fashion, but it is a very useful diagnostic tool.

In summary, a visual assessment is appropriate in some circumstances. If you need the whole picture it is a fair question to ask, "have the construction and condition been verified with core samples?"