Common Modes of Roof Failure

Perspectives
A Quarterly Information Source from Benchmark, Inc.
Volume 55 February 2006

Common Modes of Roof Failure

by Ryan Joyner

We all wish our roofs would never fail. If our roofs could last until retirement, that would be great - let the next person worry about it.

Needless to say, roofs are a big investment, and an integral part of any real estate asset. So being aware of some common modes of roof failure is worthwhile. The following summary explains some of these common failure modes, by roof type.

Built-Up Membranes

Blistering is the most common problem for built-up roofing membranes, followed by splitting, ridging/wrinkling and slippage of the membrane. Blisters are formed as a result of a void created between the roofing felt plies or between the membrane and the substrate. Voids can result from moisture in or on the felts, or failure to broom out entrapped air of displaced bitumen prior to the hardening of the asphalt.

Splitting of the membrane tends to be a result of poor design or application. Movement of unanchored insulation boards is the most common cause of splitting.

Ridging / wrinkling of the membrane is caused by expansion of the membrane at the insulation joints. Slippage is a minor problem in terms of incidence and is defined as a lateral movement between roofing felt plies. Slippage is common on roofs with 1/2 to 1 inch slope where added anchorage procedures are overlooked.

Modified Bitumen Membranes

Defective lap seams are the most common problem with modified bitumen membranes. Less common modes of failure include: shrinkage, checking, blistering, delamination, slippage and splitting.

Defective lap seams in asphalt applications are generally a result of underheating of the asphalt. In torch applied applications, the effect can be the same with either underheating or overheating of the membrane. Shrinkage is typical with polyester reinforced membranes. When the sheet is not allowed to relax prior to installation, the membrane will shrink typically at the end laps of the membrane.

Delamination of a modified membrane is a manufacturer defect. Delamination occurs when the manufacturer installs two layers of reinforcement in the modified bitumen sheet too close together, creating a weakness in the central part of the modified bitumen membrane.

Blistering, slippage and splitting are similar to built-up membrane defects.

Single-ply Membranes

Lap seam failure is the most common problem for single-ply membranes, followed by problems involving flashings, punctures and shrinkage of the membrane (EPDM systems).

Lap seam failure is typically a result of poor application of two membranes being seamed together. Shrinkage of EPDM membranes causes problems in the seams and flashings. The flashings tend to bridge and promote failure of the securement along the base of the wall. Both elastomeric and thermoplastic membranes are susceptible to punctures.

Maintaining Roofs

Unfortunately, roofs do fail. There is no best way for a roof to fail, however, the most practical way for a roof to fail over time is by age. By “age”, I mean the roof lasting beyond the expected service life of the roof. How does a roof last beyond its expected service life? By maintaining the roof with frequent walkovers to find potential leak problems, and repairing them before they cause major problems. Walking the roofs after severe weather is also important to determine and repair any damage that may have occurred.

One common question we are often asked is, “Why do I need to maintain my roof if it’s brand new?” Well, you would maintain a new car to last as long as possible, so why not a roof, that provides protection for all your worldly possessions?

New roofs need to be maintained and inspected, because it’s important to find membrane defects, or improper installation of the membrane, before they become major problems.

You may have defects in your new roof system that allow moisture into the roof system. This moisture enters slowly enough that no leak is observed from the interior unless it rains a heavy downpour for a significant amount of time. You may have recovered an existing roof system, giving you two roof systems. When a leak occurs in your new roof, it may take time before the leak into the building is visible, allowing the top roof system to become saturated. This increases the chance of premature failure in the roof system.

Frequent inspections of roof systems can help prevent premature failure by identifying potential leak sources. Repairing defects to prevent leaks is one way to maintain your roofs and potentially extend their service life. When leaks occur, repair the leaks promptly and have moisture surveys (infrared, nuclear) conducted to identify whether moisture has infiltrated the system. This will help identify any areas of wet insulation, and allow you to replace the wet insulation before the damage is too large for repair, and replacement becomes necessary.

Overall, common modes of roof failure are a result of either poor design and / or application. If a roof is well designed, installed and maintained, the common defects noted above can be minimal, preventable and maintained to eventually extend the service life of the roof system.

An obvious lack of maintenance!