Mechanical Fastened Roofs
FMEA for Mechanically Fastened Tile Roofs
The basic methodology used for the forensic investigation into this roof is a FMEA which includes determination of all possible failure modes, the possible causes of each of these failure modes, any effects of each of these failure modes. After determining all the possible failure modes, their symptoms are determined based on literature search, scientific data, and experience. These preliminary steps are accomplished prior to the physical investigation to determine if the symptoms for any/all of the failure modes exist, and if the causes for the failure existed or currently exist. If the symptoms and causes are present, they can document the severity of the failure.
The initial stage is examination of all available documents including a literature, documentation provided by the client, and other available industry/academic information.
As the name implies, a FMEA is the process used to determine the possible failure modes for a system, including individual pieces, parts, component, sub assemblies, and assemblies. The individual failure mode's effect on the reliability/serviceability (or failure) of the end item is then analyzed, as well as determining all of the causes of the failure mode and the symptoms the failure mode exhibits. Each failure mode is the result of one or more causes and each failure mode has one or more symptoms. Symptoms are only used help determine which root cause or causes created the failure mode and to assist in determining if a failure mode exists. Symptoms are neither the cause of failure nor the appropriate point of entry into an investigation of failure. One symptom may be indicative of multiple failure modes and a symptomatic approach to failure cause skews the conclusions depending on the experience and/or bias of the investigator.
In the case of mechanically fastened tile roofs, the FMEA involves the parts (deck, nails, tin-caps, edge metals, felts/mineral roll, tile, and others), assemblies or components and the entire end item (the roof as a system). Determining the effects of each failure mode for a part or the component leads to an analysis of the criticality of that failure (will the component fail, will components failure cause system failure, and if so what are the symptoms). This approach first determines all possible modes of failure, which (if any) failures are present, all possible causes of the failure, use of symptoms to eliminate improbable causes of failure, and finally determine the base failure mode, cause, contributing factors and appropriate cure.
The FMEA approach for this roof involved all parts and components; however, for the purpose of brevity, this discussion is limited to some of the parts and components in failure. The only parts not in failure included are those with similar symptoms that may lead an inexperienced analyst to a missed diagnosis of the failure mode and cause.
The deck: Deck failure mode is included because one of its failure modes is directly related to, and caused by, the failure of other components, specifically the waterproof barrier. Wood deck failure modes include (but are not limited to) rot, lose of structural integrity (breaking) and separation from the substructure. The cause of rot as a failure mode is moisture. The existence of rot as a failure mode is the result of the failure of another component, the underlayment. The two obvious symptoms (without destructive testing) is visible evidence on the underside of the deck in the form of water staining or mold related discoloration or a weak deck that sags under load foot traffic. Absent moisture, this failure mode is not possible and therefore any surface movement under load, without moisture, is symptomatic of another failure mode.
Underlayment deterioration/failure: The failure mode of the underlayment is caused by either exposure to sunlight (UV radiation) or by moisture introduced between the deck and the substrate. The cause of the UV deterioration failure mode is exposure to sunlight because of removal/displacement of the tiles. This failure mode is the effect of a failure mode of the tiles. Moisture induced deterioration is caused by introduction of moisture between the underlayment in the deck. Again, this failure mode is caused by the failure of another component, the tile fasteners. The primary symptom of UV deterioration is readily detectable by visual inspection in areas the tiles have been removed. The symptoms of both UV and moisture induced failure include visual inspection (when possible without destructive testing) and the introduction of water into the building envelope. Other symptoms include staining from under the edge metal or on the back side of the fascia board. These symptoms clearly indicate there is moisture/water between the deck and the underlayment.
Fastener failure: There are two principal failure modes for the fasteners. The first failure mode is complete loss of bond to the deck and removal of the fastener. On this roof this failure mode was not found, and causes are not discussed. The second failure mode is loosening of the fastener. The cause of failure loosening is movement of the tile from some exterior force, which will jostle the fastener. The effects of this failure mode is creating an elongated hole in the underlayment, breaking the seal created at installation by the insertion of the fastener through fresh mineral roll and hot mop, resulting in the underlayment failure (see paragraph above). Failure the fastener which results in failure the underlayment qualifies as failure of the roofing system. The symptoms of this failure mode include all of the symptoms for underlayment failure as well as tiles that are overly loose, twisted out of original installation alignment, nail heads that can be moved by hand or are raised above original installed position, and tiles with cracks to the nail hole.
Tile failure: Basically, tile failures are broken tiles or tiles that have been disturbed and may not have returned to their original installed location/orientation. There are several causes for tile failure. Broken tiles can result from impact damage, improper installation, and the application of exterior force. Impact damage is obvious and it's symptom is a broken tile. The effect is that this tile must be replaced. Improper installation will result in a tile that is improperly supported on the tile next to it (the overlap/under-lap do not properly aligned) making this tile vulnerable to damage from point loads such as foot traffic. The probability of this failure mode will be discussed in detail below. The effect of broken tiles is they can no longer perform their intended function and they must be replaced. The effect of tiles that have been disturbed from their original installed location is fastener failure. Tiles that have been disturbed have moved the fasteners to create a failure in the underlayment. The symptoms of tile that have been disturbed is tiles that are no longer properly aligned, have chipped or broken edges, have breaks across the entire surface of the tile (not web like in appearance as is the case of most impact damage), enlarged nail holes, are excessively loose, or the under/over lap pieces are broken. Additionally, these symptoms follow a particular pattern on the roof that is related to the direction in force of the wind. The principal cause of edge chatter chips and broken under/over lap is the tiles being lifted and dropped by wind forces.
Attributing failure of the tile to foot traffic, pressure cleaning, or old age cannot be validated in any industry or academic literature. Attributing failure of the tile (resulting in failure of the fastener, hence failure of the roof) to wind forces, provided such wind forces existed, is well-documented.
In addition to industry and academic data, wind machine tests by Poly Pro, a tile industry corporation, provides video demonstrating the effect of wind forces on mechanically fastened roofs. Tiles are disturbed and moved at wind speeds below 100 mph, without being removed from the roof.
With regard to foot traffic, the Roofing Tile Institute indicates that only in the latter portion of service life may concrete tiles be too soft walk on. Additionally, the Roofing Tile Institute approves of power washing and cleaning chemicals. Hence, general tile failure as result of foot traffic must be discounted in this is particularly true if there were other exterior forces that can be reasonably documented as affecting the structure.