FMEA for Mortar Adhered Tile Roofs
The basic methodology used for the forensic investigation into a mortar tile roof is a FMEA which includes determination of possible failure modes, the possible causes of each of these failure modes, and any effects of each of these failure modes. After determining 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, then the severity of the failure and the failure's effects can be determined.
The initial stage is examination of that of available documents including a literature search, 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 to 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 mortar adhered tile roofs, the FMEA involves the parts (deck, nails/tin-caps, edge metals, felts/mineral roll, tile, mortar), supporting assemblies or components and the entire end item (the roof as a system). Determining the affects of each failure mode for a part on the component leads to an analysis of the criticality of that failure (will the component fail, will a components failure cause system failure, and if so what are the symptoms). This approach first determines 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, discussion is limited to 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 rot; loss 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, exposure to sunlight (UV radiation), by moisture introduced between the deck and the substrate, or physical damage (a breach) such as ripping/puncturing. 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 and is an exacerbation of another failure, a breach of the underlayment. The failure mode of a breach of the underlayment has several causes including (but not limited to) damage by debris, puncture by pieces of broken tile, and tearing because of uplift on the mortar patty (uplift without failure of the mortar bonds results in the tearing of the underlayment). The primary symptom of UV deterioration is readily detectable by visual inspection in areas the tiles have been removed. The majority of the symptoms of UV, moisture induced failure, and a breach failure are detectible by 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 backside of the fascia board. These symptoms clearly indicate there is moisture/water between the deck and the underlayment. These are all symptoms of failure of the underlayment.
Mortar patty failure: There are two principal failure modes for the mortar patty. The first failure mode is complete loss of bond to the underlayment. The primary cause of this failure mode is uplift on the patty, as the result of uplift forces on the tile being transferred to this bond. Less common causes relate to the materials involved and installation techniques; however, these causes still require the application of force to break the bond. The second failure mode is loss of bond to the tile. The causes of this failure mode are the same as for failure of the bond to the underlayment, and require the application of some force to cause this failure. Age related deterioration of the bond is not a factor except in very old roofs (it is common for these bonds to outlast the building, and 60+ years are the rule not the exception). The effect of this failure mode is the tile is no longer bonded to the roof. The symptoms are the tile either is out of original installed position or can be moved. Additionally, bond failure is caused by movement of the roofing system, such as deflection of the decking and support structures, intensive vibration, etc.
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 its 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 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 position because of a bond failure is localized inability of the tile to perform its function within the roofing system and degradation of the system as a whole. 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), have slid out of horizontal alignment, are 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 and 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.