Low Slope Roof Life Expectancy

At first glance, the answer to this question is deceptively simple. The old rule of thumb of “20” years as the minimum period that a roof should provide trouble free performance permeates our industry. Two senior officials of government departments attending the meeting confirmed this view by informing me that as part of their asset management policy, roofs are budgeted for replacement between twenty and twenty-five years from the date of their initial construction. Architectural specifications are replete with references to “20” year roofs, and many manufacturers claim that if their products are used, they will provide service for an even longer period. I have also been given countless examples of roofs constructed 30 or 40 years ago that are still in place and watertight.

On the other hand, we have situations where designers, manufacturers, and even contractors, when confronted with unanticipated roofing failures or performance problems, cite a much shorter period and claim that twenty years is unrealistic. This seems to occur primarily as a defensive position when a product or installation has failed, or in the attempt to limit one’s liability when a roof’s service life falls well short of what the customer expected. Citing the “poorer than expected” performance of our roofs as the industry norm is often the last legal refuge under these circumstances.

The real problem is that the question of how long conventional low slope roofs typically last cannot be answered at the present time with any certainty, because we simply do not have sufficient data to make an estimate with any level of confidence.

Without quantified and validated data on how long roofs actually last, on average, each party is free to use any of a number of incomplete and unreliable surveys and reports that are most advantageous for their purpose. Just as no specific life expectancy estimates can be verified as completely accurate, neither can they be shown to be totally false. Unfortunately, in the end it boils down to statistical numbers game with each party claiming to be right.

Mean averages, probability estimates, frequency distributions and other statistics are based on complex analyses that rely on accurate and unbiased information to have any significance. If we assume that the life of roofs follow a classical “Gaussian distribution” (i.e. that they are normally distributed, and follow the well-known bell curve), some roofs will fall well before the average, and some will remain in service for many years after. However, most roofs will be close to the average, signified by p. The difficulty we have is that we do not have enough information on all roofs to be able to determine accurately the number of years, or value of p.

There are several reports, mostly produced by private organizations and government agencies, that have attempted to measure the average life span of roofs (p) under their control. However, these provide little value as there is no evidence that they are truly representative of the “population” of roofs as a whole. For example, a report published by the Ontario Ministry of Municipal Affairs and Housing in 1982, estimated the average life of roofs In their inventory, based on 36 buildings, to between 9 and 13 years. In the same report, they cite a reference that the roofs of another building owner have an average life expectancy of fifteen to twenty years. As a rule, the more pieces of information, or the larger the sample, the more valid the estimate will be. In Project Pinpoint, a 1993 survey of 277 retrofit projects showed the average age of roofs in the sample being replaced was 19 years, and 22 years in the 1994 sample of 216 projects. As you can see, there is a large variation among the averages based on which survey the data was drawn from.

A factor that adds to the confusion is that, as in any other facet of our lives, trouble free roofs usually go unreported. We have a lot of information about failures but relatively little about successful installations. Most roofs continue to perform without problems for their full expected service lives and then are quietly replaced, without anyone but the owner and a handful of contractors ever hearing about it. On the other hand, news of a roofing failure, whatever the cause, will become known almost instantaneously. If it is a large enough problem, as in the case of a widespread product failure, the entire industry becomes immediately aware, reinforcing the perception that roofs are generally problematic and not built to last.

One thing is clear, however, and that is that we currently possess the knowledge and experience to build durable roofs that will provide trouble free performance over an extended period of time. Of course, many conditions have to be met to achieve this. Roofs must be designed properly in the first-place. Many problems associated with the hectic pace of construction in the 1960’s and 1970’s have been researched to the point where we now know why they occurred. Inferior design, as in lack of positive drainage, poor detailing and many other well documented factors contributed to premature failures. Given today’s building science and engineering concepts, these deficiencies can be almost entirely eliminated or at least avoided to the extent that roofs can be designed to last more than a few short years.

Along with a better understanding of “good design principles” has come a significant increase in the level of quality of workmanship by the legitimate and professional roofing contractor. Technology transfer, and the dissemination of technical information by the CRCA and our provincial affiliates to our active members has raised the level of knowledge of our contractors significantly. This knowledge is reflected in our current high standards of good workmanship and strict membership criteria. In addition, the tremendous strides in the last decade in the field of “materials sciences” has made it possible to manufacture a host of durable and high performance roofing materials. So the question should no longer be simply one of how long conventional flat roofs last, but more importantly, how long they should last.

Given our understanding and technical expertise, there is no reason that most roofs cannot be built to last twenty years or more. This means, of course, that the consumer or building owner must be prepared to pay for the full value of that life expectancy, which includes not only the costs of the initial construction, but also those associated with the periodic inspection and maintenance required to ensure that the roof remains in a serviceable condition. If, to reduce costs, marginal products are used, inferior design and shoddy workmanship are tolerated, a durable roof cannot be built. Unfortunately, construction appears to be driven largely by cost pressures. If we look at some past problems that have occurred, and specifically at the numerous examples of “premature failures”, we will often find that workmanship has been sacrificed for expediency, or that “inferior” materials were substituted for those specified simply because they were cheaper. This may be acceptable to a building owner if they are made aware of the resulting reduction in performance and service and are given a revised and realistic estimate of the roof’s service life. The conflict in our industry always arises as a result of the owner still expecting a much longer life span than the cheapened design or materials will provide.

If, as some claim, the average roof provides uninterrupted service for as little as five, or even ten years, we had better review how we build them, and the materials we use. As a building envelope component, roofs are subjected to the extreme conditions of the harsh external environment on a daily basis, and must be built to withstand the rigours of that exposure. Roofs that provide just a few years of trouble free performance are, for the most part, simply unacceptable. How many of you would be willing to pay to replace cladding on your house, or the electrical circuitry ever,’ few years? Can we expect our clients to accept any less from the roofs over their heads?

Considering the current “state” of the art of our industry, the “20” year roof is both achievable and realistic. I firmly believe that most of the roofs we are currently building approach or exceed this period of performance in service. Unfortunately, we do not possess sufficient data to prove this conclusively. Canadian Project Pinpoint is a good start In obtaining this information, and as we continue to conduct our biannual surveys more accurate estimates will be obtained. Canadian Project Pinpoint has been designed to provide us with more information about the roofs we construct, including their service lives.

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